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PetrologyA volcanic sand grain seen under the microscope, with plane-polarized light in the upper picture,and cross polarized light in the lower picture. Scale box is 0.25 mm.Petrology (from Greek: πέτρα, petra, rock; and λόγος, logos, knowledge) is the branch ofgeology that studies rocks, and the conditions in which rocks form.Lithology was once approximately synonymous with petrography, but in current usage, lithologyfocusses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography isthe speciality that deals with microscopic details.In the oil industry, lithology, or more specifically mud logging, is the graphic representation ofgeological formations being drilled through, and drawn on a log called a mud log. As the cuttingsare circulated out of the borehole they are sampled, examined (typically under a 10x microscope)and tested chemically when needed.MethodologyPetrology utilizes the classical fields of mineralogy, petrography, optical mineralogy, andchemical analyses to describe the composition and texture of rocks. Modern petrologists alsoinclude the principles of geochemistry and geophysics through the studies of geochemical trends
and cycles and the use of thermodynamic data and experiments to better understand the originsof rocks.BranchesThere are three branches of petrology, corresponding to the three types of rocks: igneous,metamorphic, and sedimentary, and another dealing with experimental techniques: Igneous petrology focuses on the composition and texture of igneous rocks (rocks such as granite or basalt which have crystallized from molten rock or magma). Igneous rocks include volcanic and plutonic rocks. Sedimentary petrology focuses on the composition and texture of sedimentary rocks (rocks such as sandstone, shale, or limestone which consist of pieces or particles derived from other rocks or biological or chemical deposits, and are usually bound together in a matrix of finer material). Metamorphic petrology focuses on the composition and texture of metamorphic rocks (rocks such as slate, marble, gneiss, or schist which started out as sedimentary or igneous rocks but which have undergone chemical, mineralogical or textural changes due to extremes of pressure, temperature or both) Experimental petrology employs high-pressure, high-temperature apparatus to investigate the geochemistry and phase relations of natural or synthetic materials at elevated pressures and temperatures. Experiments are particularly useful for investigating rocks of the lower crust and upper mantle that rarely survive the journey to the surface in pristine condition. The work of experimental petrologists has laid a foundation on which modern understanding of igneous and metamorphic processes has been built.Igneous rockGeologic provinces of the world (USGS) Shield Platform Orogen Basin Large igneous Oceanic crust: 0–20 Ma 20–65province Extended crust Ma >65 Ma
Volcanic rock in North America. Plutonic rock in North America.Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) isone of the three main rock types, the others being sedimentary and metamorphic rock. Igneousrock is formed through the cooling and solidification of magma or lava. Igneous rock may formwith or without crystallization, either below the surface as intrusive (plutonic) rocks or on thesurface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-existing rocks in either a planets mantle or crust. Typically, the melting is caused by one or moreof three processes: an increase in temperature, a decrease in pressure, or a change incomposition. Over 700 types of igneous rocks have been described, most of them having formedbeneath the surface of Earths crust. These have diverse properties, depending on theircomposition and how they were formed. Geological significanceThe upper 16 kilometres (10 mi) of Earths crust is composed of approximately 95% igneousrocks with only a thin, widespread covering of sedimentary and metamorphic rocks.Igneous rocks are geologically important because: which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted; their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata, allowing a time sequence of events; their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics); in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.Morphology and settingIn terms of modes of occurrence, igneous rocks can be either intrusive (plutonic), extrusive(volcanic) or hypabyssal.
Intrusive igneous rocksClose-up of granite (an intrusive igneous rock) exposed in Chennai, India.Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of aplanet. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as aresult these rocks are coarse grained. The mineral grains in such rocks can generally be identifiedwith the naked eye. Intrusive rocks can also be classified according to the shape and size of theintrusive body and its relation to the other formations into which it intrudes. Typical intrusiveformations are batholiths, stocks, laccoliths, sills and dikes.The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite.When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earthssurface.Coarse grained intrusive igneous rocks which form at depth within the crust are termed asabyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.Extrusive igneous rocksBasalt (an extrusive igneous rock in this case); light coloured tracks show the direction of lavaflow.Extrusive igneous rocks are formed at the crusts surface as a result of the partial melting of rockswithin the mantle and crust. Extrusive Igneous rocks cool and solidify quicker than intrusiveigneous rocks. Since the rocks cool very quickly they are fine grained.
The melted rock, with or without suspended crystals and gas bubbles, is called magma. Magmarises because it is less dense than the rock from which it was created. When it reaches thesurface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions ofvolcanoes into air are termed subaerial whereas those occurring underneath the ocean are termedsubmarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanicactivity.The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting.Extrusive rock is produced in the following proportions:  divergent boundary: 73% convergent boundary (subduction zone): 15% hotspot: 12%.Magma which erupts from a volcano behaves according to its viscosity, determined bytemperature, composition, and crystal content. High-temperature magma, most of which isbasaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long,thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma such asandesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscositysimilar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite isusually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes withrhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limitedextent and have steep margins, because the magma is so viscous.Felsic and intermediate magmas that erupt often do so violently, with explosions driven byrelease of dissolved gases — typically water but also carbon dioxide. Explosively eruptedpyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanicash is also erupted and forms ash tuff deposits which can often cover vast areas.Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid asto prevent the formation of even small crystals after extrusion, the resulting rock may be mostlyglass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would becoarse-grained.Because the minerals are mostly fine-grained, it is much more difficult to distinguish betweenthe different types of extrusive igneous rocks than between different types of intrusive igneousrocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only bedetermined by examination of thin sections of the rock under a microscope, so only anapproximate classification can usually be made in the field.Hypabyssal igneous rocksHypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks.Hypabyssal rocks are less common than plutonic or volcanic rocks and do often form dikes, sillsor laccoliths.
ClassificationIgneous rocks are classified according to mode of occurrence, texture, mineralogy, chemicalcomposition, and the geometry of the igneous body.The classification of the many types of different igneous rocks can provide us with importantinformation about the conditions under which they formed. Two important variables used for theclassification of igneous rocks are particle size, which largely depends upon the cooling history,and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes,amphiboles, and micas are all important minerals in the formation of almost all igneous rocks,and they are basic to the classification of these rocks. All other minerals present are regarded asnonessential in almost all igneous rocks and are called accessory minerals. Types of igneousrocks with other essential minerals are very rare, and these rare rocks include those with essentialcarbonates.In a simplified classification, igneous rock types are separated on the basis of the type of feldsparpresent, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type ofiron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannotcoexist in a stable association with quartz.Igneous rocks which have crystals large enough to be seen by the naked eye are calledphaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking,phaneritic implies an intrusive origin; aphanitic an extrusive one.An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix istermed porphyry. Porphyritic texture develops when some of the crystals grow to considerablesize before the main mass of the magma crystallizes as finer-grained, uniform material.TextureGabbro specimen showing phaneritic texture; Rock Creek Canyon, eastern Sierra Nevada,California; scale bar is 2.0 cm.Main article: Rock microstructure
Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks,including the size, shape, orientation, and distribution of mineral grains and the intergrainrelationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simplelava.However, the texture is only a subordinate part of classifying volcanic rocks, as most often thereneeds to be chemical information gleaned from rocks with extremely fine-grained groundmass orfrom airfall tuffs, which may be formed from volcanic ash.Textural criteria are less critical in classifying intrusive rocks where the majority of minerals willbe visible to the naked eye or at least using a hand lens, magnifying glass or microscope.Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics.Textural terms can be used to differentiate different intrusive phases of large plutons, forinstance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes(apophyses). Mineralogical classification is used most often to classify plutonic rocks. Chemicalclassifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix,e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".Basic classification scheme for igneous rocks on their mineralogy. If the approximate volumefractions of minerals in the rock are known the rock name and silica content can be read off thediagram. This is not an exact method because the classification of igneous rocks also depends onother components than silica, yet in most cases it is a good first guess.Chemical classificationIgneous rocks can be classified according to chemical or mineralogical parameters:Chemical: total alkali-silica content (TAS diagram) for volcanic rock classification used whenmodal or mineralogic data is unavailable: acid igneous rocks containing a high silica content, greater than 63% SiO 2 (examples granite and rhyolite)
intermediate igneous rocks containing between 52 - 63% SiO2 (example andesite and dacite) basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example gabbro and basalt) ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite) alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with a molar ratio of alkali to silica greater than 1:6. (examples phonolite and trachyte) Note: the acid-basic terminology is used more broadly in older (generally British) geological literature. In current literature felsic-mafic roughly substitutes for acid-basic.Chemical classification also extends to differentiating rocks which are chemically similaraccording to the TAS diagram, for instance; Ultrapotassic; rocks containing molar K2O/Na2O >3 Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 >1 Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1An idealized mineralogy (the normative mineralogy) can be calculated from the chemicalcomposition, and the calculation is useful for rocks too fine-grained or too altered foridentification of minerals that crystallized from the melt. For instance, normative quartzclassifies a rock as silica-oversaturated; an example is rhyolite. A normative feldspathoidclassifies a rock as silica-undersaturated; an example is nephelinite.History of classificationIn 1902 a group of American petrographers proposed that all existing classifications of igneousrocks should be discarded and replaced by a "quantitative" classification based on chemicalanalysis. They showed how vague and often unscientific was much of the existing terminologyand argued that as the chemical composition of an igneous rock was its most fundamentalcharacteristic it should be elevated to prime position.Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria forthe discrimination of rock species—were relegated to the background. The completed rockanalysis is first to be interpreted in terms of the rock-forming minerals which might be expectedto be formed when the magma crystallizes, e.g., quartz feldspars, olivine, akermannite,feldspathoids, magnetite, corundum and so on, and the rocks are divided into groups strictlyaccording to the relative proportion of these minerals to one another. Mineralogical classificationFor volcanic rocks, mineralogy is important in classifying and naming lavas. The most importantcriterion is the phenocryst species, followed by the groundmass mineralogy. Often, where thegroundmass is aphanitic, chemical classification must be used to properly identify a volcanicrock.
Mineralogic contents - felsic versus mafic felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density. mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks. ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals (e.g., dunite).For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at leastvia microscope, the mineralogy is used to classify the rock. This usually occurs on ternarydiagrams, where the relative proportions of three minerals are used to classify the rock.The following table is a simple subdivision of igneous rocks according both to their compositionand mode of occurrence. CompositionMode of occurrence Felsic Intermediate Mafic UltramaficIntrusive Granite Diorite Gabbro PeridotiteExtrusive Rhyolite Andesite Basalt Komatiite Essential rock forming silicates Felsic Intermediate Mafic UltramaficCoarse Grained Granite Diorite Gabbro PeridotiteMedium Grained DiabaseFine Grained Rhyolite Andesite Basalt KomatiiteExample of classificationGranite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silicaand predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase) andphaneritic, subeuhedral texture (minerals are visible to the unaided eye and commonly some ofthem retain original crystallographic shapes).Magma originationThe Earths crust averages about 35 kilometers thick under the continents, but averages onlysome 7-10 kilometers beneath the oceans. The continental crust is composed primarily ofsedimentary rocks resting on crystalline basement formed of a great variety of metamorphic andigneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt andgabbro. Both continental and oceanic crust rest on peridotite of the mantle.
Rocks may melt in response to a decrease in pressure, to a change in composition such as anaddition of water, to an increase in temperature, or to a combination of these processes.Other mechanisms, such as melting from impact of a meteorite, are less important today, butimpacts during accretion of the Earth led to extensive melting, and the outer several hundredkilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites inlast few hundred million years have been proposed as one mechanism responsible for theextensive basalt magmatism of several large igneous provinces.DecompressionDecompression melting occurs because of a decrease in pressure.  The solidus temperatures ofmost rocks (the temperatures below which they are completely solid) increase with increasingpressure in the absence of water. Peridotite at depth in the Earths mantle may be hotter than itssolidus temperature at some shallower level. If such rock rises during the convection of solidmantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about0.3°C per kilometer. Experimental studies of appropriate peridotite samples document that thesolidus temperatures increase by 3°C to 4°C per kilometer. If the rock rises far enough, it willbegin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. Thisprocess of melting from upward movement of solid mantle is critical in the evolution of Earth.Decompression melting creates the ocean crust at mid-ocean ridges. Decompression meltingcaused by the rise of mantle plumes is responsible for creating ocean islands like the Hawaiianislands. Plume-related decompression melting also is the most common explanation for floodbasalts and oceanic plateaus (two types of large igneous provinces), although other causes suchas melting related to meteorite impact have been proposed for some of these huge volumes ofigneous rock.Effects of water and carbon dioxideThe change of rock composition most responsible for creation of magma is the addition of water.Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth ofabout 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, butnear or above about 1500°C in the absence of water.  Water is driven out of the oceaniclithosphere in subduction zones, and it causes melting in the overlying mantle. Hydrous magmasof basalt and andesite composition are produced directly and indirectly as results of dehydrationduring the subduction process. Such magmas and those derived from them build up island arcssuch as those in the Pacific ring of fire. These magmas form rocks of the calc-alkaline series, animportant part of continental crust.The addition of carbon dioxide is relatively a much less important cause of magma formationthan addition of water, but genesis of some silica-undersaturated magmas has been attributed tothe dominance of carbon dioxide over water in their mantle source regions. In the presence ofcarbon dioxide, experiments document that the peridotite solidus temperature decreases by about200°C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. Atgreater depths, carbon dioxide can have more effect: at depths to about 200 km, the temperatures
of initial melting of a carbonated peridotite composition were determined to be 450°C to 600°Clower than for the same composition with no carbon dioxide.  Magmas of rock types such asnephelinite, carbonatite, and kimberlite are among those that may be generated following aninflux of carbon dioxide into mantle at depths greater than about 70 km.Temperature increaseIncrease of temperature is the most typical mechanism for formation of magma withincontinental crust. Such temperature increases can occur because of the upward intrusion ofmagma from the mantle. Temperatures can also exceed the solidus of a crustal rock incontinental crust thickened by compression at a plate boundary. The plate boundary between theIndian and Asian continental masses provides a well-studied example, as the Tibetan Plateau justnorth of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normalcontinental crust. Studies of electrical resistivity deduced from magnetotelluric data havedetected a layer that appears to contain silicate melt and that stretches for at least 1000kilometers within the middle crust along the southern margin of the Tibetan Plateau.  Graniteand rhyolite are types of igneous rock commonly interpreted as products of melting ofcontinental crust because of increases of temperature. Temperature increases also may contributeto the melting of lithosphere dragged down in a subduction zone.Magma evolutionSchematic diagrams showing the principles behind fractional crystallisation in a magma. Whilecooling, the magma evolves in composition because different minerals crystallize from the melt.1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclasecrystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rockforms.Most magmas only entirely melt for small parts of their histories. More typically, they are mixesof melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually havedifferent densities, and so they can separate as magmas evolve.As magma cools, minerals typically crystallize from the melt at different temperatures (fractionalcrystallization). As minerals crystallize, the composition of the residual melt typically changes. Ifcrystals separate from melt, then the residual melt will differ in composition from the parentmagma. For instance, a magma of gabbroic composition can produce a residual melt of granitic
composition if early formed crystals are separated from the magma. Gabbro may have a liquidustemperature near 1200°C, and derivative granite-composition melt may have a liquidustemperature as low as about 700°C. Incompatible elements are concentrated in the last residuesof magma during fractional crystallization and in the first melts produced during partial melting:either process can form the magma that crystallizes to pegmatite, a rock type commonly enrichedin incompatible elements. Bowens reaction series is important for understanding the idealisedsequence of fractional crystallisation of a magma.Magma composition can be determined by processes other than partial melting and fractionalcrystallization. For instance, magmas commonly interact with rocks they intrude, both bymelting those rocks and by reacting with them. Magmas of different compositions can mix withone another. In rare cases, melts can separate into two immiscible melts of contrastingcompositions.There are relatively few minerals that are important in the formation of common igneous rocks,because the magma from which the minerals crystallize is rich in only certain elements: silicon,oxygen, aluminium, sodium, potassium, calcium, iron, and magnesium. These are the elementswhich combine to form the silicate minerals, which account for over ninety percent of all igneousrocks. The chemistry of igneous rocks is expressed differently for major and minor elements andfor trace elements. Contents of major and minor elements are conventionally expressed as weightpercent oxides (e.g., 51% SiO2, and 1.50% TiO2). Abundances of trace elements areconventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm).The term "trace element" typically is used for elements present in most rocks at abundances lessthan 100 ppm or so, but some trace elements may be present in some rocks at abundancesexceeding 1000 ppm. The diversity of rock compositions has been defined by a huge mass ofanalytical data—over 230,000 rock analyses can be accessed on the web through a site sponsoredby the U. S. National Science Foundation (see the External Link to EarthChem).EtymologyThe word "igneous" is derived from the Latin ignis, meaning "of fire". Volcanic rocks are namedafter Vulcan, the Roman name for the god of fire.Intrusive rocks are also called plutonic rocks, named after Pluto, the Roman god of theunderworld.Bowens reaction series Discontinuous Continuous High Series Series Plagioclase Olivine (Calcium rich) Pyroxene
Relative Biotite Plagioclase Crystallization (Black Mica) (Sodium rich) Temperature Orthoclase Muscovite (White Mica) Quartz LowWithin the field of geology, Bowens reaction series is the work of the petrologist, Norman L.Bowen who was able to explain why certain types of minerals tend to be found together whileothers are almost never associated with one another. He experimented in the early 1900s withpowdered rock material that was heated until it melted and then allowed to cool to a targettemperature whereupon he observed the types of minerals that formed in the rocks produced. Herepeated this process with progressively cooler temperatures and the results he obtained led himto formulate his reaction series which is still accepted today as the idealized progression ofminerals produced by cooling magma. Based upon Bowens work, one can infer from theminerals present in a rock the relative conditions under which the material had formed.DescriptionOlivine weathering to iddingsite within a mantle xenolith, demonstrating the principles of theGoldich dissolution seriesThe series is broken into two branches, the continuous and the discontinuous. The branch on theright is the continuous. The minerals at the top of the illustration (given aside) are first tocrystallize and so the temperature gradient can be read to be from high to low with the hightemperature minerals being on the top and the low temperature ones on the bottom. Since thesurface of the Earth is a low temperature environment compared to the zones of rock formation,the chart also easily shows the stability of minerals with the ones at bottom being most stable andthe ones at top being quickest to weather, known as the Goldich dissolution series. This isbecause minerals are most stable in the conditions closest to those under which they had formed.Put simply, the high temperature minerals, the first ones to crystallize in a mass of magma, are
most unstable at the Earths surface and quickest to weather because the surface is most differentfrom the conditions under which they were created while the low temperature minerals are muchmore stable because the conditions at the surface are much more similar to the conditions underwhich they formed.Pluton Plutonic redirects here, for the Australian gold mine see Plutonic Gold Mine A Jurassic pluton of pink monzonite intruded below and beneath a section of gray sedimentary rocks and then was subsequently uplifted and exposed, near Notch Peak, House Range, Utah.A pluton in geology is an intrusive igneous rock (called a plutonic rock) body that crystallizedfrom magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes,sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to adistinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape likethose of dikes and sills. Batholiths commonly are aggregations of plutons. Examples of plutonsinclude Cardinal Peak and Mount Kinabalu.The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartzdiorite. The term granitoid is used for a general, light colored, coarse-grained igneous rock inwhich a proper, or more specific name, is not known. Use of granitoid should be restricted to thefield wherever possible.The term originated from Pluto, the ancient Roman god of the underworld. The use of the nameand concept goes back to the beginnings of the science of geology in the late 18th century andthe then hotly debated theories of plutonism (or vulcanism), and neptunism regarding the originof basalt.Batholith
Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevadabatholith.A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneousintrusive (also called plutonic) rock that forms from cooled magma deep in the earths crust.Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite,quartz monzonite, or diorite (see also granite dome). FormationAlthough they may appear uniform, batholiths are in fact structures with complex histories andcompositions. They are composed of multiple masses, or plutons, bodies of igneous rock ofirregular dimensions (typically at least several kilometers) that can be distinguished fromadjacent igneous rock by some combination of criteria including age, composition, texture, ormappable structures. Individual plutons are crystallized from magma that traveled toward thesurface from a zone of partial melting near the base of the Earths crust.Traditionally, these plutons have been considered to form by ascent of relatively buoyant magmain large masses called plutonic diapirs. Because, the diapirs are liquefied and very hot, they tendto rise through the surrounding country rock, pushing it aside and partially melting it. Mostdiapirs do not reach the surface to form volcanoes, but instead slow down, cool and usuallysolidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; inreference to the Roman god of the underworld Pluto). It has also been proposed[who?] that plutonscommonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation ofsmaller volumes of magma that ascended as dikes. A batholith is formed when many plutons converge to form a huge expanse of granitic rock.Some batholiths are mammoth, paralleling past and present subduction zones and other heatsources for hundreds of kilometers in continental crust. One such batholith is the Sierra NevadaBatholith, which is a continuous granitic formation that makes up much of the Sierra Nevada inCalifornia. An even larger batholith, the Coast Plutonic Complex is found predominantly in the
Coast Mountains of western Canada, and extends for 1,800 kilometers and reaches intosoutheastern Alaska.Surface expression and erosionA batholith is an exposed area of mostly continuous plutonic rock that covers an area larger than100 square kilometers. Areas smaller than 100 square kilometers are called stocks. However, themajority of batholiths visible at the surface (via outcroppings) have areas far greater than 100square kilometers. These areas are exposed to the surface through the process of erosionaccelerated by continental uplift acting over many tens of millions to hundreds of millions ofyears. This process has removed several tens of square kilometers of overlying rock in manyareas, exposing the once deeply buried batholiths.Batholiths exposed at the surface are subjected to huge pressure differences between their formerhomes deep in the earth and their new homes at or near the surface. As a result, their crystalstructure expands slightly and over time. This manifests itself by a form of mass wasting calledexfoliation. This form of erosion causes convex and relatively thin sheets of rock to slough offthe exposed surfaces of batholiths (a process accelerated by frost wedging). The result: fairlyclean and rounded rock faces. A well-known result of this process is Half Dome, located inYosemite Valley.Dike (geology)Banded gneiss with dike of granite orthogneiss.
An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosedhost rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah.A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cutsdiscordantly across planar wall rock structures, such as bedding or foliation massive rock formations, like igneous/magmatic intrusions and salt diapirs.Dikes can therefore be either intrusive or sedimentary in origin.Magmatic dikesA diabase dike crosscutting horizontal limestone beds in Arizona.A small dike on the Baranof Cross-Island Trail, Alaska.
An intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness can varyDikes in the Black Canyon of the Gunnison National Park, Colorado, USA from sub-centimeter. scale to many meters, and thelateral dimensions can extend over many kilometers. A dike is an intrusion into an openingcross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this impliesthat a dike is always younger than the rocks that contain it. Dikes are usually high angle to nearvertical in orientation, but subsequent tectonic deformation may rotate the sequence of stratathrough which the dike propagates so that the latter becomes horizontal. Near horizontal, orconformable intrusions, along bedding planes between strata are called intrusive sills.Sometimes dikes appear as swarms, consisting of several to hundreds of dikes emplaced more orless contemporaneously during a single intrusive event. The worlds largest dike swarm is theMackenzie dike swarm in the Northwest Territories, Canada.Shiprock, New Mexico, the volcanic neck in the distance, with radiating dike on its south side.Photo credit: USGS Digital Data SeriesDikes often form as either radial or concentric swarms around plutonic intrusives, volcanic necksor feeder vents in volcanic cones. The latter are known as ring dikes.Dikes can vary in texture and their composition can range from diabase or basaltic to granitic orrhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vastvolumes of mantle-derived magmas through fractured lithosphere throughout Earth history.Pegmatite dikes are extremely coarse crystalline granitic rocks often associated with late-stagegranite intrusions or metamorphic segregations. Aplite dikes are fine grained or sugary texturedintrusives of granitic composition.
Sedimentary dikesClastic dike (left of notebook) in the Chinle Formation in the Island In the Sky District ofCanyonlands National Park, Utah.Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rocklayers. They can form in two ways: When a shallow unconsolidated sediment is composed of alternating coarse grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks through overlying layers and forms a dike. When a soil is under permafrost conditions the pore water is totally frozen. When cracks are formed in such rocks, they may fill up with sediments that fall in from above. The result is a vertical body of sediment that cuts through horizontal layers: a dike. Magmatic dikes radiating from West Spanish Peak
Sill (geology)Illustration showing the difference between a dike and a sill.Salisbury Crags in Edinburgh, Scotland, a sill partially exposed during the ice agesMid-Carboniferous dolerite sill cutting Lower Carboniferous shales and sandstones, HortonBluff, Minas Basin South Shore, Nova ScotiaIn geology, a sill is a tabular sheet intrusion that has intruded between older layers ofsedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation inmetamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means thatthe sill does not cut across preexisting rocks, in contrast to dikes which do cut across older rocks.Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in ahorizontal orientation, although tectonic processes can cause rotation of sills into near vertical
orientations. They can be confused with solidified lava flows; however, there are severaldifferences between them. Intruded sills will show partial melting and incorporation of thesurrounding country rock. On both the "upper" and "lower" contact surfaces of the country rockinto which the sill has intruded, evidence of heating will be observed (contact metamorphism).Lava flows will show this evidence only on the lower side of the flow. In addition, lava flowswill typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere.Because sills generally form at depth (up to many kilometers), the pressure of overlying rockprevents this from happening much, if at all. Lava flows will also typically show evidence ofweathering on their upper surface, whereas sills, if still covered by country rock, typically do not.Associated ore depositsCertain layered intrusions are a variety of sill that often contain important ore deposits.Precambrian examples include the Bushveld, Insizwa and the Great Dyke complexes of southernAfrica, the Duluth intrusive complex of the Superior District, and the Stillwater igneous complexof the United States. Phanerozoic examples are usually smaller and include the Rùm peridotitecomplex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusionsoften contain concentrations of gold, platinum, chromium and other rare elements.Transgressive sillsDespite their concordant nature, many large sills change stratigraphic level within the intrudedsequence, with each concordant part of the intrusion linked by relatively short dike-likesegments. Such sills are known as transgressive, examples include the Whin Sill and sills withinthe Karoo basin. The geometry of large sill complexes in sedimentary basins has becomeclearer with the availability of 3D seismic reflection data. Such data has shown that many sillshave an overall saucer shape and that many others are at least in part transgressive. LaccolithA laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layersof sedimentary rock. The pressure of the magma is high enough that the overlying strata areforced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.
A laccolith intruding into and deforming strataLaccolith exposed by erosion of overlying strata in MontanaPink monzonite intrudes within the grey Cambrian and Ordovician strata near Notch Peak, Utah.Laccoliths tend to form at relatively shallow depths and are typically formed by relativelyviscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Coolingunderground takes place slowly, giving time for larger crystals to form in the cooling magma.The surface rock above laccoliths often erodes away completely, leaving the core mound ofigneous rock. The term was first applied as laccolite by Grove Karl Gilbert after his study ofintrusions of diorite in the Henry Mountains of Utah in about 1875.It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower in Wyomingwas thought to be a volcanic neck, but study has revealed it to be an eroded laccolith. The rockwould have had to cool very slowly so as to form the slender pencil-shaped columns of phonoliteporphyry seen today. However, erosion has stripped away the overlying and surrounding rock,and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock maynot be the remnant of a laccolith. At other localities, such as in the Henry Mountains and otherisolated mountain ranges of the Colorado Plateau, some intrusions demonstrably have shapes oflaccoliths. The small Barber Hill syenite-stock laccolith in Charlotte, Vermont USA, has severalvolcanic trachyte dikes associated with it. Molybdenite is also visible in outcrops on this exposedlaccolith.
There are many examples of possible laccoliths on the surface of the Moon.  These igneousfeatures may be confused with impact cratering.LopolithDiagram showing the shape of a lopolith (7)A lopolith is a large igneous intrusion which is lenticular in shape with a depressed centralregion. Lopoliths are generally concordant with the intruded strata with dike or funnel-shapedfeeder bodies below the body. The term was first defined and used by Frank Fitch Grout duringthe early 1900s in describing the Duluth gabbro complex in northern Minnesota and adjacentOntario.Lopoliths typically consist of large layered intrusions that range in age from Archean to Eocene.Examples include the Duluth gabbro, the Sudbury Igneous Complex of Ontario, the Bushveldigneous complex of South Africa, the Skaergaard complex of Greenland and the Humboldtlopolith of Nevada. The Sudbury and Bushveld occurrences have been attributed to impactevents and associated crustal melting.Subvolcanic rockA subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates atmedium to shallow depths within the crust and contain intermediate grain size and oftenporphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocksinclude diabase and porphyry.Porphyry (geology)
.A piece of porphyryRhyolite porphyry. Scale bar in lower left is 1 cm.Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar orquartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals arecalled phenocrysts. In its non-geologic, traditional use, the term "porphyry" refers to the purple-red form of this stone, valued for its appearance.The term "porphyry" is from Greek and means "purple". Purple was the color of royalty, and the"Imperial Porphyry" was a deep purple igneous rock with large crystals of plagioclase. This rockwas prized for various monuments and building projects in Imperial Rome and later.Subsequently the name was given to igneous rocks with large crystals. Porphyritic now refers toa texture of igneous rocks. Its chief characteristic is a large difference between the size of thetiny matrix crystals and other much larger phenocrysts. Porphyries may be aphanites orphanerites, that is, the groundmass may have invisibly small crystals, like basalt, or theindividual crystals of the groundmass may be easily distinguished with the eye, as in granite.Most types of igneous rocks may display some degree of porphyritic texture.Granite Granite — Igneous Rock —
Granite containing potassium feldspar, plagioclase feldspar, quartz, and biotite and/or amphibole CompositionPotassium feldspar, plagioclase feldspar, and quartz;differing amounts of muscovite, biotite, and hornblende-typeamphiboles.Granite (pronounced /ˈɡrænɨt/) is a common and widely occurring type of intrusive, felsic,igneous rock. Granites usually have a medium- to coarse-grained texture. Occasionally someindividual crystals (phenocrysts) are larger than the groundmass, in which case the texture isknown as porphyritic. A granitic rock with a porphyritic texture is sometimes known as aporphyry. Granites can be pink to gray in color, depending on their chemistry and mineralogy.By definition, granite has a color index (the percentage of the rock made up of dark minerals) ofless than 25%. Outcrops of granite tend to form tors and rounded massifs. Granites sometimesoccur in circular depressions surrounded by a range of hills, formed by the metamorphic aureoleor hornfels. Granite is usually found in the continental plates of the Earths crust.Granite is nearly always massive (lacking internal structures), hard and tough, and therefore ithas gained widespread use as a construction stone. The average density of granite is between2.65 and 2.75 g/cm3, its compressive strength usually lies above 200 MPa, and its viscosity atstandard temperature and pressure is 3-6 • 1019 Pa·s.The word granite comes from the Latin granum, a grain, in reference to the coarse-grainedstructure of such a crystalline rock.Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous rocks.Petrographic examination is required for identification of specific types of granitoids. Mineralogy
Orbicular granite near the town of Caldera, northern ChileGranite is classified according to the QAPF diagram for coarse grained plutonic rocks and isnamed according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline)and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modernpetrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoidor nearly devoid of plagioclase, the rock is referred to as alkali granite. When a granitoidcontains less than 10% orthoclase, it is called tonalite; pyroxene and amphibole are common intonalite. A granite containing both muscovite and biotite micas is called a binary or two-micagranite. Two-mica granites are typically high in potassium and low in plagioclase, and areusually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite.Granite has poor primary permeability but strong secondary permeability.Chemical compositionA worldwide average of the chemical composition of granite, by weight percent: The Stawamus Chief is a granite monolith in British Columbia SiO2 — 72.04% Al2O3 — 14.42% K2O — 4.12% Na2O — 3.69% CaO — 1.82% FeO — 1.68%
Fe2O3 — 1.22% MgO — 0.71% TiO2 — 0.30% P2O5 — 0.12% MnO — 0.05%Based on 2485 analysesOccurrenceGranite is currently known only on Earth, where it forms a major part of continental crust.Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and inbatholiths that are often associated with orogenic mountain ranges. Small dikes of graniticcomposition called aplites are often associated with the margins of granitic intrusions. In somelocations, very coarse-grained pegmatite masses occur with granite.Granite has been intruded into the crust of the Earth during all geologic periods, although muchof it is of Precambrian age. Granitic rock is widely distributed throughout the continental crustand is the most abundant basement rock that underlies the relatively thin sedimentary veneer ofthe continents.OriginClose-up of granite exposed in Chennai, India.Granite is an igneous rock and is formed from magma. Granitic magma has many potentialorigins but it must intrude other rocks. Most granite intrusions are emplaced at depth within thecrust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust.The origin of granite is contentious and has led to varied schemes of classification. Classificationschemes are regional and include French, British, and American systems.Geochemical origins
Various granites (cut and polished surfaces)Granitoids are a ubiquitous component of the crust. They have crystallized from magmas thathave compositions at or near a eutectic point (or a temperature minimum on a cotectic curve).Magmas will evolve to the eutectic because of igneous differentiation, or because they representlow degrees of partial melting. Fractional crystallisation serves to reduce a melt in iron,magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon - alkalifeldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite.Close-up of granite from Yosemite National Park, valley of the Merced RiverThis process operates regardless of the origin of the parental magma to the granite, andregardless of its chemistry. However, the composition and origin of the magma whichdifferentiates into granite, leaves certain geochemical and mineral evidence as to what thegranites parental rock was. The final mineralogy, texture and chemical composition of a graniteis often distinctive as to its origin. For instance, a granite which is formed from melted sedimentsmay have more alkali feldspar, whereas a granite derived from melted basalt may be richer inplagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes arebased.Chappell & White classification systemThe letter-based Chappell & White classification system was proposed initially to divide granitesinto I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite.Both of these types of granite are formed by melting of high grade metamorphic rocks, eitherother granite or intrusive mafic rocks, or buried sediment, respectively.M-type or mantle derived granite was proposed later, to cover those granites which were clearlysourced from crystallized mafic magmas, generally sourced from the mantle. These are rare,because it is difficult to turn basalt into granite via fractional crystallisation.
A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiarmineralogy and geochemistry. These granites are formed by melting of the lower crust underconditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examplesof volcanic equivalents of A-type granite.GranitizationAn old, and largely discounted theory, granitization states that granite is formed in place byextreme metasomatism by fluids bringing in elements e.g. potassium and removing others e.g.calcium to transform the metamorphic rock into a granite. This was supposed to occur across amigrating front. The production of granite by metamorphic heat is difficult, but is observed tooccur in certain amphibolite and granulite terrains. In-situ granitisation or melting bymetamorphism is difficult to recognise except where leucosome and melanosome textures arepresent in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and isa magma, so these rocks are seen as a transitional between the two, but are not technicallygranite as they do not actually intrude into other rocks. In all cases, melting of solid rock requireshigh temperature, and also water or other volatiles which act as a catalyst by lowering the solidustemperature of the rock.Ascent and emplacementRoche Rock, CornwallThe Cheesewring, a granite tor on the southern edge of Bodmin Moor, Cornwall
The ascent and emplacement of large volumes of granite within the upper continental crust is asource of much debate amongst geologists. There is a lack of field evidence for any proposedmechanisms, so hypotheses are predominantly based upon experimental data. There are twomajor hypotheses for the ascent of magma through the crust: Stokes Diapir Fracture PropagationOf these two mechanisms, Stokes diapir was favoured for many years in the absence of areasonable alternative. The basic idea is that magma will rise through the crust as a single massthrough buoyancy. As it rises it heats the wall rocks, causing them to behave as a power-lawfluid and thus flow around the pluton allowing it to pass rapidly and without major heat loss. This is entirely feasible in the warm, ductile lower crust where rocks are easily deformed, butruns into problems in the upper crust which is far colder and more brittle. Rocks there do notdeform so easily: for magma to rise as a pluton it would expend far too much energy in heatingwall rocks, thus cooling and solidifying before reaching higher levels within the crust.Nowadays fracture propagation is the mechanism preferred by many geologists as it largelyeliminates the major problems of moving a huge mass of magma through cold brittle crust.Magma rises instead in small channels along self-propagating dykes which form along new orpre-existing fault systems and networks of active shear zones (Clemens, 1998).  As thesenarrow conduits open, the first magma to enter solidifies and provides a form of insulation forlater magma.Granitic magma must make room for itself or be intruded into other rocks in order to form anintrusion, and several mechanisms have been proposed to explain how large batholiths have beenemplaced: Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust Assimilation, where the granite melts its way up into the crust and removes overlying material in this way Inflation, where the granite body inflates under pressure and is injected into positionMost geologists today accept that a combination of these phenomena can be used to explaingranite intrusions, and that not all granites can be explained entirely by one or anothermechanism.Granodiorite
A sample of granodiorite from Massif Central, FrancePhotomicrograph of thin section of granodiorite from Slovakia (in crossed polarised light)Granodiorite (pronounced /ˌɡrænɵˈdaɪ.ɵraɪt/ or /ˌɡreɪnɵˈdaɪ.ɵraɪt/) is an intrusive igneous rocksimilar to granite, but containing more plagioclase than potassium feldspar. Officially, it isdefined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65%of the feldspar is plagioclase. It usually contains abundant biotite mica and hornblende, giving ita darker appearance than true granite. Mica may be present in well-formed hexagonal crystals,and hornblende may appear as needle-like crystals. GeologyOn average the upper continental crust has the same composition as granodiorite.Granodiorite is a plutonic igneous rock, formed by an intrusion of silica-rich magma, whichcools in batholiths or stocks below the Earths surface. It is usually only exposed at the surfaceafter uplift and erosion have occurred. The volcanic equivalent of granodiorite is dacite.SyeniteFrom Wikipedia, the free encyclopediaJump to: navigation, search
Syeniteleucocratic variety of nepheline syenite from Sweden (särnaite).Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite butwith the quartz either absent or present in relatively small amounts (<5%).The feldspar component of syenite is predominantly alkaline in character (usually orthoclase) .Plagioclase feldspars may be present in small quantities, less than 10%.When present, ferromagnesian minerals are usually hornblende amphibole, rarely pyroxene orbiotite. Biotite is rare, because in a syenite magma most aluminium is used in producing feldspar.Syenites are usually peralkaline and peraluminous, with high proportions of alkali elements andaluminium.Syenites are formed from alkaline igneous activity, generally formed in thick continental crustalareas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a graniticor igneous protolith to a fairly low degree of partial melting. This is required because potassiumis an incompatible element and tends to enter a melt first, whereas higher degrees of partialmelting will liberate more calcium and sodium, which produce plagioclase, and hence a granite,adamellite or tonalite.At very low degrees of partial melting a silica undersaturated melt is produced, forming anepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline oranalcime.
Syenite is not a common rock, some of the more important occurrences being in New England,Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, and Norway.EtymologyThe term syenite was originally applied to hornblende granite like that of Syene in Egypt, fromwhich the name is derived.EpisyeniteEpisyenite (or epi-syenite) is a term used in petrology to describe to the result of alteration of aSiO2 rich rock to a more SiO2 depleted rock.The process which results in SiO2 depletion can be termed episyenitization. This process is onlyreferring to the macroscopic result of relative SiO2 depletion in a rock. The actual physicalprocess leading to this SiO2 depletion may vary in a given metamorphic environment. Diffusionof chemical components in a stagnant fluid, related to differences in chemical potential orpressure as well as advection of a SiO2- undersaturated fluid may lead to the dissolution of quartzfrom the un-altered rock, thus depleting it of this component.Nepheline syeniteNepheline syenite from SwedenNephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkalifeldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are notunlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusiveequivalent.PetrologyNepheline syenites are silica-undersaturated and some are peralkaline (terms discussed inigneous rock). Nepheline is a feldspathoid, a solid-solution mineral, that does not coexist withquartz; rather, nepheline would react with quartz to produce alkali feldspar.
They are distinguished from ordinary syenites not only by the presence of nepheline but also bythe occurrence of many other minerals rich in alkalis and in rare earths and other incompatibleelements. Alkali feldspar dominates, commonly represented by orthoclase and the exsolvedlamellar albite, form perthite. In some rocks the potash feldspar, in others the soda feldsparpredominates. Fresh clear microcline is very characteristic of some types of nepheline syenite.Sodalite, colorless and transparent in thin section, but frequently pale blue in the handspecimens, is the principal feldspathoid mineral in addition to nepheline. Reddish-brown to blacktriclinic aenigmatite occurs also in these rocks. Extremely iron-rich olivine is rare, but is presentin some nepheline syenite. Other minerals common in minor amounts include sodium-richpyroxene, biotite, titanite, zircon, iron oxides, apatite, fluorite, melanite garnet, and zircon.Cancrinite occurs in several nepheline-syenites. A great number of interesting and rare mineralshave been recorded from nepheline syenites and the pegmatite veins which intersect them.GenesisSilica-undersaturated igneous rocks typically are formed by low degrees of partial melting in theEarths mantle. Carbon dioxide may dominate over water in source regions. Magmas of suchrocks are formed in a variety of environments, including continental rifts, ocean islands, andsupra-subduction positions in subduction zones. Nepheline syenite and phonolite may be derivedby crystal fractionation from more mafic silica-undersaturated mantle-derived melts, or as partialmelts of such rocks. Igneous rocks with nepheline in their normative mineralogy commonly areassociated with other unusual igneous rocks such as carbonatite.DistributionNepheline syenites and phonolites occur in Canada, Norway, Greenland, Sweden, the UralMountains, the Pyrenees, Italy, Brazil, China, the Transvaal region, and Magnet Cove igneouscomplex of Arkansas, as well as on oceanic islands.Phonolite lavas formed in the East African rift in particularly large quantity, and the volumethere may exceed the volume of all other phonolite occurrences combined, as discussed byBarker (1983).Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex,possibly formed from partial melting of the wall rocks to that large ultramafic layered intrusion.Nepheline syenites are rare; there is only one occurrence in Great Britain and one in France andPortugal. They are known also in Bohemia and in several places in Norway, Sweden andFinland. In the Americas these rocks have been found in Texas, Arkansas and Massachusetts,also in Ontario, British Columbia and Brazil. South Africa, Madagascar, India, Tasmania, Timorand Turkestan are other localities for the rocks of this series.Rocks of this class also occur in Brazil (Serra de Tingua) containing sodalite and often muchaugite, in the western Sahara and Cape Verde Islands; also at Zwarte Koppies in the Transvaal,
Madagascar, São Paulo in Brazil, Paisano Pass in West Texas and Montreal, Canada. The rock ofSalem, Massachusetts, United States, is a mica-foyaite rich in albite and aegirine: it accompaniesgranite and essexite. Litchfieldite is another well-marked type of nepheline-syenite, in whichalbite is the dominant feldspar. It is named after Litchfield, Maine, United States, where it occursin scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similarnepheline-syenite is known from Hastings County, Ontario, and contains hardly any orthoclase,but only albite feldspar. Nepheline is very abundant and there is also cancrinite, sodalite,scapolite, calcite, biotite and hornblende. The lujaurites are distinguished from the rocks abovedescribed by their dark color, which is due to the abundance of minerals such as augite, aegirine,arfvedsonite and other kinds of amphibole. Typical examples are known near Lujaur on theWhite Sea, where they occur with umptekites and other very peculiar rocks. Other localities forthis group are at Julianehaab in Greenland with sodalite-syenite; at their margins they containpseudomorphs after leucite. The lujaurites frequently have a parallel-banding or gneissosestructure. Sodalite-syenites in which sodalite very largely or completely takes the place ofnepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsoniteand eudialyte.Cancrinite syenite, with a large percentage of cancrinite, has been described from Dalekarlia,Sweden and from Finland. We may also mention urtite from Lujaur Urt on the White Sea, whichconsists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (fromJacupiranga in Brazil) is a blackish rock composed of titaniferous augite, magnetite, ilmenite,perofskite and nepheline, with secondary biotite.NomenclatureThere is a wide variety of silica-undersaturated and peralkaline igneous rocks, including manyinformal place-name varieties named after the locations in which they were first discovered. Inmany cases these are plain nepheline syenites containing one or more rare minerals ormineraloids, which do not warrant a new formal classification. These include;Foyaite: foyaites are named after Foya in the Serra de Monchique, in southern Portugal. Theseare K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usually pyroxene-,hornblende- and biotite.Laurdalite: The laurdalites, from Laurdal in Norway, are grey or pinkish, and in many waysclosely resemble the laurvikites of southern Norway, with which they occur. They containanorthoclase feldspars, biotite or greenish augite, much apatite and in some cases, olivine.Ditroite: Ditroite derives is name from Ditrau, Transylvania, Romania. It is essentially amicrocline, sodalite and cancrinite variety of nepheline syenite. It contains also orthoclase,nepheline, biotite, aegirine, acmite.Chemical compositionThe chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly richin alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica
varying from 50 to 56%, while lime, magnesia[disambiguation needed] and iron are never present ingreat quantity, though somewhat more variable than the other components. A worldwide averageof the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressedas weight percent oxides. SiO2 — 54.99% TiO2 — 0.60% Al2O3 — 20.96% Fe2O3 — 2.25% FeO — 2.05% MnO — 0.15% MgO — 0.77% CaO — 2.31% Na2O — 8.23% K2O — 5.58% H2O — 1.47% P2O5 — 0.13%The normative mineralogy of this average composition contains about 22 percent nepheline and66 percent feldspar.MonzonitePhotomicrograph of thin section of monzonite (in cross polarised light)
The QAPF diagram, by which a monzonite is definedPhotomicrograph of thin section of monzonite (in plane polarised light)An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosedhost rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah.Monzonite is an intermediate igneous intrusive rock composed of approximately equal amountsof sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende,biotite and other minerals. Quartz a minor constituent or is absent; with greater than 10% quartzthe rock is termed a quartz monzonite.If the rock has more orthoclase or potassium feldspar it grades into a syenite. With an increase ofcalcic plagioclase and mafic minerals the rock type becomes a diorite. The volcanic equivalent isthe latite.Tonalite
A piece of tonalite on red granite gneiss from Tjörn, SwedenTonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture.Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkalifeldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes arecommon accessory minerals.In older references tonalite is sometimes used as a synonym for quartz diorite. However thecurrent IUGS classification defines tonalite as having greater than 20% quartz and quartz dioritewith from 5 to 20% quartz.The name is derived from the type locality of tonalites, adjacent to the Tonale Line, a majorstructural lineament and mountain pass, Tonale Pass, in the Italian and Austrian Alps.Trondhjemite is an orthoclase-deficient variety of tonalite with minor biotite as the only maficmineral, named after Norways third largest city, Trondheim. Igneous rocks by composition Intermediate- Ultramafic Mafic Intermediate FelsicType Felsic < 45% SiO2 < 52% SiO2 52–63% SiO2 >69 % SiO2 63–69% SiO2 Volcanic Komatiite Basalt Andesite Dacite Rhyolite rocks: Kimberlite, Diabase Aplite— Subvolcanic Lamproite (Dolerite) Diorite Granodiorite Pegmatite rocks: Peridotite Gabbro GranitePlutonic rocks:Diorite
DioriteDiorite classification on QAPF diagramDiorite (pronounced /ˈdaɪəraɪt/) is a grey to dark grey intermediate intrusive igneous rockcomposed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/orpyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite,sphene, magnetite, ilmenite and sulfides occur as accessory minerals. It can also be black orbluish-grey, and frequently has a greenish cast. Varieties deficient in hornblende and other darkminerals are called leucodiorite. When olivine and more iron-rich augite are present, the rockgrades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makesthe rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassiumfeldspar) is present at greater than ten percent the rock type grades into monzodiorite orgranodiorite. Diorite has a medium grain size texture, occasionally with porphyry.Diorites may be associated with either granite or gabbro intrusions, into which they may subtlymerge. Diorite results from partial melting of a mafic rock above a subduction zone. It iscommonly produced in volcanic arcs, and in cordilleran mountain building such as in the AndesMountains as large batholiths. The extrusive volcanic equivalent rock type is andesite.Occurrence
DioriteDiorite is a relatively rare rock; source localities include Leicestershire; UK  (one name formicrodiorite - Markfieldite - exists due to the rock being found in the village of Markfield),Sondrio, Italy; Thuringia and Saxony in Germany; Finland; Romania; Northeastern Turkey;central Sweden; Scotland; the Darrans range of New Zealand; the Andes Mountains; the Isle ofGuernsey; Basin and Range province and Minnesota in the USA; Idahet in EgyptAn orbicular variety found in Corsica is called corsite.GabbroGabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.Close-up of gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.
Photomicrograph of a thin section of gabbro.Gabbro (pronounced /ˈɡæbroʊ/) refers to a large group of dark, coarse-grained, intrusive maficigneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when moltenmagma is trapped beneath the Earths surface and cools into a crystalline mass.The vast majority of the Earths surface is underlain by gabbro within the oceanic crust, producedby basalt magmatism at mid-ocean ridges. PetrologyA gabbro landscape on the main ridge of the Cuillin, Isle of Skye, Scotland.Gabbro as a xenolith in a granite, eastern Sierra Nevada, Rock Creek Canyon, California.Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, andolivine (olivine gabbro when olivine is present in a large amount).
The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If theamount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock isthen a norite. Quartz gabbros are also known to occur and are probably derived from magma thatwas over-saturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid mineral nepheline. (Silicasaturation of a rock can be evaluated by normative mineralogy). Gabbros contain minor amounts,typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finergrained equivalents of gabbro are called diabase, although the vernacular term microgabbro isoften used when extra descriptiveness is desired. Gabbro may be extremely coarse grained topegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro,although these may exhibit acicular crystal habits.Gabbro is usually equigranular in texture, although it may be porphyritic at times, especiallywhen plagioclase oikocrysts have grown earlier than the groundmass minerals.DistributionGabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene andplagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene andplagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes asparts of zones III and IV (sheeted dyke zone to massive gabbro zone). Long belts of gabbroicintrusions are typically formed at proto-rift zones and around ancient rift zone margins, intrudinginto the rift flanks. Mantle plume hypotheses may rely on identifying mafic and ultramaficintrusions and coeval basalt volcanism.NoriteNorite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclaselabradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbrowithout thin section study under the petrographic microscope. It occurs with gabbro and othermafic to ultramafic rocks in layered intrusions which are often associated with platinumorebodies such as in the Bushveld Igneous Complex in South Africa, the Skaergaard igneouscomplex of Greenland, and the Stillwater igneous complex in Montana, USA. Norite is also thebasal igneous rock of the Sudbury Basin complex in Ontario which is the site of a meteoriteimpact and the worlds second largest nickel mining region. Norite is a common rock type of theApollo samples. On a smaller scale, norite can be found in small localized intrusions such as theGombak Norite in Bukit Gombak, Singapore.The name Norite is derived from the Norwegian name for Norway: Norge.
AnorthositeAnorthosite from PolandLunar anorthosite from Apollo 15 landing siteAnorthosite (pronounced /ænˈɔrθəsaɪt/) is a phaneritic, intrusive igneous rock characterized by apredominance of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%).Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.Anorthosite on Earth can be divided into two types: Proterozoic anorthosite (also known asmassif or massif-type anorthosite) and Archean anorthosite. These two types of anorthosite havedifferent modes of occurrence, appear to be restricted to different periods in Earths history, andare thought to have had different origins.Lunar anorthosites constitute the light-coloured areas of the Moons surface and have been thesubject of much research.Proterozoic anorthosite
AgeAlthough a few Proterozoic anorthosite bodies were emplaced either late in the Archean Eon, orearly in the Phanerozoic Eon, the vast majority of Proterozoic anorthosites were emplaced, astheir name suggests, during the Proterozoic Eon (ca. 2,500-542 Ma).Mode of occurrenceAnorthosite from southern FinlandAnorthosite plutons occur in a wide range of sizes. Some smaller plutons, exemplified by manyanorthosite bodies in the U.S. and Harris in Scotland, cover only a few dozen square kilometres.Larger plutons, like the Mt. Lister Anorthosite, in northern Labrador, Canada, cover severalthousands of square kilometres.Many Proterozoic anorthosites occur in spatial association with other highly distinctive,contemporaneous rock types (the so-called anorthosite suite or anorthosite-mangerite-charnockite complex). These rock types include iron-rich diorite, gabbro, and norite; leucocraticmafic rocks such as leucotroctolite and leuconorite; and iron-rich felsic rocks, includingmonzonite and rapakivi granite. Importantly, large volumes of ultramafic rocks are not found inassociation with Proterozoic anorthosites.Occurrences of Proterozoic anorthosites are commonly referred to as massifs. However, there issome question as to what name would best describe any occurrence of anorthosite together withthe rock types mentioned above. Early works used the term complex The term plutonic suitehas been applied to some large occurrences in northern Labrador, Canada; however, it has beensuggested (in 2004-2005) that batholith would be a better term. Batholith is used to describesuch occurrences for the remainder of this article.The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds ofsquare kilometres) to nearly 20,000 km2 (7,700 sq mi), in the instance of the Nain Plutonic Suitein northern Labrador, Canada.Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the AppalachianMountains, eastern Canada, across southern Scandinavia and eastern Europe. Mapped onto thePangaean continental configuration of that eon, these occurrences are all contained in a single
straight belt, and must all have been emplaced intracratonally. The conditions and constraints ofthis pattern of origin and distribution are not clear. However, see the Origins section below.Anorthosites are also common in layered intrusions. Anorthosite in these layered intrusions canform as cumulate layers in the upper parts of the intrusive complex or as later-stage intrusionsinto the layered intrusion complex. Physical characteristicsSince they are primarily composed of plagioclase feldspar, most of Proterozoic anorthositesappear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue,or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces. Thefeldspar variety labradorite is commonly present in anorthosites. Mineralogically, labradorite is acompositional term for any calcium-rich plagioclase feldspar containing between 50–70molecular percent anorthite (An 50–70), regardless of whether it shows labradorescence. Themafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, morerarely, amphibole. Oxides, such as magnetite or ilmenite, are also common.Most anorthosite plutons are very coarse grained; that is, the individual plagioclase crystals andthe accompanying mafic mineral are more than a few centimetres long. Less commonly,plagioclase crystals are megacrystic, or larger than one metre long. However, most Proterozoicanorthosites are deformed, and such large plagioclase crystals have recrystallized to form smallercrystals, leaving only the outline of the larger crystals behind.While many Proterozoic anorthosite plutons appear to have no large-scale relict igneousstructures (having instead post-emplacement deformational structures), some do have igneouslayering, which may be defined by crystal size, mafic content, or chemical characteristics. Suchlayering clearly has origins with a rheologically liquid-state magma.Chemical and isotopic characteristicsThe composition of plagioclase feldspar in Proterozoic anorthosites is most commonly betweenAn40 and An60 (40-60% anorthite). This compositional range is intermediate, and is one of thecharacteristics which distinguish Proterozoic anorthosites from Archean anorthosites. Maficminerals in Proterozoic anorthosites have a wide range of composition, but are not generallyhighly magnesian.The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has beenexamined in some detail by researchers with the aim of arriving at a plausible genetic theory.However, there is still little agreement on just what the results mean for anorthosite genesis; seethe Origins section below. A very short list of results, including results for rocks thought to berelated to Proterozoic anorthosites. Some research has focused on neodymium (Nd) and strontium (Sr) isotopic determinations foranorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic
determinations are of use in gauging the viability of prospective sources for magmas that gaverise to anorthosites. Some results are detailed below in the Origins section.Origins of Proterozoic anorthositesThe origins of Proterozoic anorthosites have been a subject of theoretical debate for manydecades. A brief synopsis of this problem is as follows. The problem begins with the generationof magma, the necessary precursor of any igneous rock.Magma generated by small amounts of partial melting of the mantle is generally of basalticcomposition. Under normal conditions, the composition of basaltic magma requires it tocrystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magmacrystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content(90–100% plagioclase), and are not found in association with contemporaneous ultramafic rocks.This is now known as the anorthosite problem. Proposed solutions to the anorthosite problemhave been diverse, with many of the proposals drawing on different geological subdisciplines.It was suggested early in the history of anorthosite debate that a special type of magma,anorthositic magma, had been generated at depth, and emplaced into the crust. However, thesolidus of an anorthositic magma is too high for it to exist as a liquid for very long at normalambient crustal temperatures, so this appears to be unlikely. The presence of water vapour hasbeen shown to lower the solidus temperature of anorthositic magma to more reasonable values,but most anorthosites are relatively dry. It may be postulated, then, that water vapour be drivenoff by subsequent metamorphism of the anorthosite, but some anorthosites are undeformed,thereby invalidating the suggestion.The discovery, in the late 1970s, of anorthositic dykes in the Nain Plutonic Suite, suggested thatthe possibility of anorthositic magmas existing at crustal temperatures needed to be reexamined.However, the dykes were later shown to be more complex than was originally thought. Insummary, though liquid-state processes clearly operate in some anorthosite plutons, the plutonsare probably not derived from anorthositic magmas.Many researchers have argued that anorthosites are the products of basaltic magma, and thatmechanical removal of mafic minerals has occurred. Since the mafic minerals are not found withthe anorthosites, these minerals must have been left at either a deeper level or the base of thecrust. A typical theory is as follows: partial melting of the mantle generates a basaltic magma,which does not immediately ascend into the crust. Instead, the basaltic magma forms a largemagma chamber at the base of the crust and fractionates large amounts of mafic minerals, whichsink to the bottom of the chamber. The cocrystallizing plagioclase crystals float, and eventuallyare emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals formultramafic cumulates which stay at the base of the crust.This theory has many appealing features, of which one is the capacity to explain the chemicalcomposition of high-alimuna orthopyroxene megacrysts (HAOM). This is detailed below in thesection devoted to the HAOM. However, on its own, this hypothesis cannot coherently explainthe origins of anorthosites, because it does not fit with, among other things, some important
isotopic measurements made on anorthositic rocks in the Nain Plutonic Suite. The Nd and Srisotopic data shows the magma which produced the anorthosites cannot have been derived onlyfrom the mantle. Instead, the magma that gave rise to the Nain Plutonic Suite anorthosites musthave had a significant crustal component. This discovery led to a slightly more complicatedversion of the previous hypothesis: Large amounts of basaltic magma form a magma chamber atthe base of the crust, and, while crystallizing, assimilating large amounts of crust.This small addendum explains both the isotopic characteristics and certain other chemicalniceties of Proterozoic anorthosite. However, at least one researcher has cogently argued, on thebasis of geochemical data, that the mantles role in production of anorthosites must actually bevery limited: the mantle provides only the impetus (heat) for crustal melting, and a small amountof partial melt in the form of basaltic magma. Thus anorthosites are, in this view, derived almostentirely from lower crustal melts. High-alumina orthopyroxene megacrystsThe high-alumina orthopyroxene megacrysts (HAOM) have, like Proterozoic anorthosites, beenthe subject of great debate, although a tentative consensus about their origin appears to haveemerged. The peculiar characteristic worthy of such debate is reflected in their name. Normalorthopyroxene has chemical composition (Fe,Mg)2 Si2O6, whereas the HAOM have anomalouslylarge amounts of aluminium (up to about 9%) in their atomic structure.Because the solubility of aluminium in orthopyroxene increases with increasing pressure, manyresearchers, have suggested that the HAOM crystallized at depth, near the base of the Earthscrust. The maximum amounts of aluminium correspond to a 30–35 km (19–22 mi) depth.Other researchers consider the chemical compositions of the HAOM to be the product of rapidcrystallization at moderate or low pressures. Archaean anorthositeSmaller amounts of anorthosite were emplaced during the Archaean eon (ca 3,800-2,400 Ma),although most have been dated between 3,200 and 2,800 Ma. They are distinct texturally andmineralogically from Proterozoic anorthosite bodies. Their most characteristic feature is thepresence of equant megacrysts of plagioclase surrounded by a fine-grained mafic groundmass.Diabase
DiabaseDiabase (pronounced /ˈdaɪ.əbeɪs/) or Dolerite is a mafic, holocrystalline, subvolcanic rockequivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabaserefers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabaserefers to altered material.  Diabase dikes and sills are typically shallow intrusive bodies andoften exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark maficglass).PetrologyDiabase normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals(62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3%up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%). Accessory and alterationminerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, andcalcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is alsotermed interstitial. The feldspar is high in anorthite (as opposed to albite), the calciumendmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.Diabase/dolerite
The Candlestick, Tasman Peninsula, Tasmania, is composed of Jurassic Dolerite. Tasmania hasthe worlds largest areas of dolerite.In non-North American usage dolerite is preferred due to the various conflicting uses of diabase.Dolerite (Greek: doleros, meaning "deceptive") was the name given by Haüy in his 1822 Traitéde minéralogie. In continental Europe diabase was reserved by Brongniart for pre-Tertiary (pre-Cenozoic) material, with dolerite used for more recent rock. The use of diabase in this sensewas abandoned in Britain in favor of dolerite for rocks of all ages by Allport (1874), thoughsome British geologists continued to use diabase to describe slightly altered dolerite, in whichpyroxene has been altered to amphibole.LocationsA diabase dike crosscutting horizontal limestone beds in Arizona
Diabase is usually found in smaller relatively shallow intrusive bodies such as dikes and sills.Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundredsof individual dikes or sills radiating from a single volcanic center.The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near NewYork City, is an example of a diabase sill. The dike complexes of the British Tertiary VolcanicProvince which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullionregion of Ireland, and extends across northern England contains many examples of diabase dikeswarms. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includesdolerite. It is also abundant in large parts of Curaçao, an island off the coast of Venezuela.In Western Australia a 200 km long dolerite dike, the Norseman–Wiluna Belt is associatedwith the non-alluvial gold mining area between Norseman and Kalgoolie, which includes thelargest gold mine in Australia, the Super Pit gold mine. West of the Norseman–Wiluna Belt isthe Yalgoo–Singleton Belt, where complex dolerite dike swarms obscure the volcaniclasticsediments.The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup ofGondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dikeswarms. These include the Karoo dolerites of South Africa, the Ferrar Dolerites of Antarctica,and the largest of these, indeed the most extensive of all dolerite formations worldwide, arefound in Tasmania. Here, the volume of magma which intruded into a thin veneer of Permianand Triassic rocks from multiple feeder sites, over a period of perhaps a million years, may haveexceeded 40,000 cubic kilometres. In Tasmania alone dolerite dominates the landscape.Ring dikes are large, near vertical dikes showing above ground as circular outcrops up to 30 kmin diameter, with a depth from hundreds of metres to several kilometres. Thicker dikes are madeup of plutonic rocks, rather than hypabyssal and are centred around deep intrusions. The centralpart may be a block sunken into underlying magma, the ring dikes forming in the fracture zonearound the sunken block.Peridotite Peridotite — Igneous Rock — Peridotite xenolith from San Carlos, southwestern United
States. The rock is typical olivine-rich peridotite, cut by a centimeter-thick layer of greenish-black pyroxenite. Compositionolivine, pyroxeneA peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivineand pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high inmagnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron.Peridotite is derived from the Earths mantle, either as solid blocks and fragments, or as crystalsaccumulated from magmas that formed in the mantle. The compositions of peridotites from theselayered igneous complexes vary widely, reflecting the relative proportions of pyroxenes,chromite, plagioclase, and amphibole.Peridotite is the dominant rock of the upper part of the Earths mantle. The compositions ofperidotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest,because they provide samples of the Earths Mantle roots of continents brought up from depthsfrom about 30 km or so to depths at least as great as about 200 km. Some of the nodules preserveisotope ratios of osmium and other elements that record processes over three billion years ago,and so they are of special interest to paleogeologists because they provide clues to thecomposition of the Earths early mantle and the complexities of the processes that were involved.The word peridotite comes from the gemstone peridot, which consists of pale green olivine. Types of peridotite Dunite: more than 90% olivine, typically with Mg/Fe ratio of about 9:1. Wehrlite: mostly composed of olivine plus clinopyroxene. Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low proportions of basaltic ingredients (because garnet and clinopyroxene are minor). Lherzolite: mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). Partial fusion of lherzolite and extraction of the melt fraction can leave a solid residue of harzburgite.
Classification diagram for peridotite and pyroxenite, based on proportions of olivine andpyroxene. The pale green area encompasses the most common compositions of peridotite in theupper part of the Earths mantle (partly adapted from Bodinier and Godard (2004)).CompositionPeridotites are rich in magnesium, reflecting the high proportions of magnesium-rich olivine.The compositions of peridotites from layered igneous complexes vary widely, reflecting therelative proportions of pyroxenes, chromite, plagioclase, and amphibole. Minor minerals andmineral groups in peridotite include plagioclase, spinel (commonly the mineral chromite), garnet(especially the mineral pyrope), amphibole, and phlogopite. In peridotite, plagioclase is stable atrelatively low pressures (crustal depths), aluminous spinel at higher pressures (to depths of 60km or so), and garnet at yet higher pressures.Pyroxenites are related ultramafic rocks, which are composed largely of orthopyroxene and/orclinopyroxene; minerals that may be present in lesser abundance include olivine, garnet,plagioclase, amphibole, and spinel.Distribution and locationOlivine in a peridotite weathering to iddingsite within a mantle xenolithPeridotite is the dominant rock of the Earths mantle above a depth of about 400 km; below thatdepth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist ofup to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick,consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssalperidotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usuallysubducted back into the mantle in subduction zones. However, pieces can be emplaced into oroverthrust on continental crust by a process called obduction, rather than carried down into themantle; the emplacement may occur during orogenies, as during collisions of one continent withanother or with an island arc. The pieces of oceanic plates emplaced within continental crust arereferred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks suchas gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses ofperidotite have been emplaced into mountain belts as solid masses but do not appear to be relatedto ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites."
Peridotites also occur as fragments (xenoliths) carried up by magmas from the mantle. Amongthe rocks that commonly include peridotite xenoliths are basalt and kimberlite. Certain volcanicrocks, sometimes called komatiites, are so rich in olivine and pyroxene that they also can betermed peridotite. Small pieces of peridotite have even been found in lunar breccias.The rocks of the peridotite family are uncommon at the surface and are highly unstable, becauseolivine reacts quickly with water at typical temperatures of the upper crust and at the Earthssurface. Many, if not most, surface outcrops have been at least partly altered to serpentinite, aprocess in which the pyroxenes and olivines are converted to green serpentine. This hydrationreaction involves considerable increase in volume with concurrent deformation of the originaltextures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctiveplant communities grow in soils developed on serpentinite, because of the unusual compositionof the underlying rock. One mineral in the serpentine group, chrysotile, is a type of asbestos.Morphology and textureSome peridotites are layered or are themselves layers; others are massive. Many layeredperidotites occur near the base of bodies of stratified gabbroic complexes. Other layeredperidotites occur isolated, but possibly once composed part of major gabbroic complexes. Bothlayered and massive peridotites can have any of three principal textures: (1) rather well formedcrystals of olivine separated by other minerals. This probably reflects the original deposition ofolivine sediment from magma. (2) Equigranular crystals with straight grain boundariesintersecting at about 120°. This may result from slow cooling whereby recrystallization leads to aminimization of surface energy. (3) Long crystals with ragged curvilinear boundaries. Thisprobably results from internal deformation.Many peridotite occurrences have characteristic textures. For example, peridotites with well-formed olivine crystals occur mainly as layers in gabbroic complexes. "Alpine" peridotitesgenerally have irregular crystals that occur as more or less serpentinized lenses bounded by faultsin belts of folded mountains such as the Alpines, the Pacific coast ranges, and in the Appalachianpiedmont. Peridotite nodules with irregular equigranular textures are often found in alkalinebasalts and in kimberlite pipes. Some peridotites rich in amphibole have a concentric layeredstructure and form parts of plutons called Alaskan-type zoned ultramafic complexes.OriginPeridotites have two primary modes of origin, as mantle rocks formed during the accretion anddifferentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenesfrom basaltic or ultramafic magmas; these magmas are ultimately derived from the upper mantleby partial melting of mantle peridotites.Mantle peridotites are sampled as alpine-type massifs in collisional mountain ranges or asxenoliths in basalt or kimberlite. In all cases these rocks are pyrometamorphic (that is,metamorphosed in the presence of molten rock) and represent either fertile mantle (lherzolite) orpartially depleted mantle (harzburgite, dunite). Alpine peridotites may be either of the ophiolite
association and representing the uppermost mantle below ocean basins, or masses ofsubcontinental mantle emplaced along thrust faults in mountain belts.Layered peridotites are igneous sediments and form by mechanical accumulation of denseolivine crystals. Some peridotite forms by precipitation and collection of cumulate olivine andpyroxene from mantle-derived magmas, such as those of basalt composition. Peridotitesassociated with Alaskan-type ultramafic complexes are cumulates that probably formed in theroot zones of volcanoes. Cumulate peridotites are also formed in komatiite lava flows.Mantle lherzolites may be the principal source rock for basaltic magmas, whereas mantleharzburgites probably form both from the crystalline residue left after basaltic magma migratesout of lherzolite and from a crystalline accumulation of early solidification products of somebasaltic magmas within the mantle.Associated rocksKomatiites are the rare volcanic equivalent of peridotite.Eclogite, a rock similar to basalt in composition, is composed primarily of sodic clinopyroxeneand garnet. Eclogite is associated with peridotite in some xenolith occurrences; it also occurswith peridotite in rocks metamorphosed at high pressures during processes related to subduction.PyroxeniteA sample of the orthopyroxenite meteorite ALH84001Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxenegroup, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified (seediagram below) into clinopyroxenites, orthopyroxenites, and the websterites which contain bothpyroxenes. Closely allied to this group are the hornblendites, consisting essentially of hornblendeand other amphiboles.They are essentially of igneous origin, though some pyroxenites are included in the metamorphicLewisian complex of Scotland. The pyroxene-rich rocks which result from the contact
metamorphism of impure limestones are described as pyroxene hornfelses (calc-silicatehornfelses).Intrusive and mantle pyroxenitesThe igneous pyroxenites are closely allied to the gabbros and norites, from which they differ bythe absence of feldspar, and to the peridotites, which are distinguished from them by containingmore than 40% olivine. This connection is indicated also by their mode of occurrence, for theyusually accompany masses of gabbro and peridotite and seldom are found by themselves.They are often very coarse-grained, containing individual crystals which may be several inchesin length. The principal accessory minerals, in addition to olivine and feldspar, are chromite andother spinels, garnet, magnetite, rutile, and scapolite.Pyroxenites can be formed as cumulates in ultramafic intrusions by accumulation of pyroxenecrystals at the base of the lava chamber. Here they are generally associated with gabbro andanorthite cumulate layers and are typically high up in the intrusion. They may be accompaniedby magnetite layers, ilmenite layers, but rarely chromite cumulates.Pyroxenites are also found as layers within masses of peridotite. These layers most commonlyhave been interpreted as products of reaction between ascending magmas and peridotite of theupper mantle. The layers typically are a few centimeters to a meter or so in thickness.Pyroxenites that occur as xenoliths in basalt and in kimberlite have been interpreted as fragmentsof such layers. Although some mantle pyroxenites contain garnet, they are not eclogites, asclinopyroxene in them is less sodic than omphacite and the pyroxenite compositions typically areunlike that of basalt. It has been proposed that large volumes of pyroxenite form in the uppermantle as a result of reaction between peridotite and magma derived from partial melting ofeclogite, and that such pyroxenite volumes are important sources of basalt magma (e.g., Sobolevand others, 2007).Pyroxenite lavasPurely pyroxene-bearing volcanic rocks are rare, restricted to spinifex textured sills, lava tubesand thick flows in the Archaean greenstone belts. Here, the pyroxenite lavas are created by in-situ crystallisation and accumulation of pyroxene on the floor of a lava flow, creating thedistinctive spinifex texture, but also occasionally mesocumulate and orthocumulate segregations.This is in essence similar to the formation of olivine spinifex textures in komatiite lava flows, thechemistry of the magma differing only to favor crystallisation of pyroxene.A type locality is the Gullewa Greenstone Belt, in the Murchison region of Western Australia,and the Duketon Belt near Laverton, where pyroxene spinifex lavas are closely associated withgold deposits.Distribution
They frequently occur in the form of dikes or segregations in gabbro and peridotite: in Shetland,Cortland on the Hudson river, North Carolina (websterite), Baltimore, New Zealand, and inSaxony.Classification diagram for peridotite and pyroxenite, based on proportions of olivine andpyroxene. The pale green area encompasses the most common compositions of peridotite in theupper part of the Earths mantleThe pyroxenites are often subject serpentinization under low temperature retrogrademetanorphism and weathering. The rocks are often completely replaced by serpentines, whichsometimes preserve the original structures of the primary minerals, such as the lamination ofhypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende isdeveloped and various types of amphibolite and hornblende-schist are produced. Occasionallyrocks rich in pyroxene are found as basic facies of nepheline syenite; a good example is providedby the melanite pyroxenites associated with the borolanite variety found in the Loch Borralanigneous complex of Scotland.DuniteSmall volcanic bomb of (black) basanite with (green) duniteDunite (pronounced /ˈdʌnaɪt/ or /ˈdjuːnaɪt/) is an igneous, plutonic rock, of ultramaficcomposition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope.Dunite is the olivine-rich end-member of the peridotite group of mantle-derived rocks. Duniteand other peridotite rocks are considered the major constituents of the Earths mantle above a
depth of about 400 kilometers. Dunite is rarely found within continental rocks, but where it isfound, it typically occurs at the base of ophiolite sequences where slabs of mantle rock from asubduction zone have been thrust onto continental crust by obduction during continental orisland arc collisions (orogeny). It is also found in alpine peridotite massifs that represent sliversof sub-continental mantle exposed during collisional orogeny. Dunite typically undergoesretrograde metamorphism in near-surface environments and is altered to serpentinite andsoapstone.Dunite may represent the refractory residue left after the extraction of basaltic magmas in theupper mantle. This is the type of dunite found in the lowermost parts of ophiolites, alpineperidotite massifs, and xenoliths. Dunite may also form by the accumulation of olivine crystalson the floor of large basaltic or picritic magma chambers. These "cumulate" dunites typicallyoccur in thick layers in layered intrusions, associated with cumulate layers of wehrlite, olivinepyroxenite, harzburgite, and even chromitite (a cumulate rock consisting largely of chromite).Small layered intrusions may be of any geologic age, for example, the Triassic Palisades Sill inNew York and the larger Eocene Skaergaard complex in Greenland. The largest layered maficintrusions are tens of kilometers in size and almost all are Proterozoic in age, e.g., the Stillwaterigneous complex (Montana), the Muskox intrusion (Canada), and the Great Dyke (Zimbabwe).Cumulate dunite may also be found in ophiolite complexes, associated with layers of wehrlite,pyroxenite, and gabbro.Dunite was named by the Austrian geologist, Ferdinand von Hochstetter in 1859 after DunMountain near Nelson, New Zealand. Dun Mountain was given its name because of the duncolour of the underlying ultramafic rocks. This color results from surface weathering thatoxidizes the iron in olivine in temperate climates (weathering in tropical climates creates a deepred soil). Dun Mountain is separated from its sister massif, Red Mountain, at the southern end ofSouth Island, New Zealand, by the Alpine Fault, an approximately 600 km long right lateralstrike slip fault similar to the San Andreas fault in California.A massive exposure of dunite in the United States can be found as the Twin Sisters Peaks, nearMt. Baker, in the northern Cascade Mountains of Washington State.Dunite could be used to sequester CO2 and help mitigate global climate change via acceleratedrock weathering. This would involve spreading large quantities of finely ground dunite intropical regions known near sources of dunite. One significant environmental side effect wouldbe a significant increase the pH of nearby rivers.Hornblendite
Hornblendite from PolandHornblendite is a plutonic rock consisting mainly of the amphibole hornblende. Hornblenderich ultramafic rocks are rare and when hornblende is the dominant mineral phase they areclassified as hornblendites with qualifiers such as garnet hornblendite identifying a secondabundant contained mineral.Metamorphic rocks composed dominantly of amphiboles are referred to as amphibolitesKimberliteKimberlite from U.S.A.
QEMSCAN mineral map of kimberlite from South AfricaKimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. Itis named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.7g) diamond in 1871 spawned a diamond rush, eventually creating the Big Hole.Kimberlite occurs in the Earths crust in vertical structures known as kimberlite pipes. Kimberlitepipes are the most important source of mined diamonds today. The consensus on kimberlites isthat they are formed deep within the mantle. Formation occurs at depths between 150 and 450kilometres (93 and 280 mi), from anomalously enriched exotic mantle compositions, and areerupted rapidly and violently, often with considerable carbon dioxide and other volatilecomponents. It is this depth of melting and generation which makes kimberlites prone to hostingdiamond xenocrysts.Kimberlite has attracted more attention than its relative volume might suggest that it deserves.This is largely because it serves as a carrier of diamonds and garnet peridotite mantle xenoliths tothe Earths surface. Its probable derivation from depths greater than any other igneous rock type,and the extreme magma composition that it reflects in terms of low silica content and high levelsof incompatible trace element enrichment, make an understanding of kimberlite petrogenesisimportant. In this regard, the study of kimberlite has the potential to provide information on thecomposition of the deep mantle, and melting processes occurring at or near the interface betweenthe cratonic continental lithosphere and the underlying convecting asthenospheric mantle. Morphology and volcanologyKimberlites occur as carrot-shaped, vertical intrusions termed pipes. This classic carrot shape isformed due to a complex intrusive process of kimberlitic magma which inherits a largeproportion of both CO2 and H2O in the system, which produces a deep explosive boiling stagethat causes a significant amount of vertical flaring (Bergman, 1987). Kimberlite classification isbased on the recognition of differing rock facies. These differing facies are associated with a
particular style of magmatic activity, namely crater, diatreme and hypabyssal rocks (Clement andSkinner 1985, and Clement, 1982).The morphology of kimberlite pipes, and the classical carrot shape, is the result of explosivediatreme volcanism from very deep mantle-derived sources. These volcanic explosions producevertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlitepipes is varied but generally includes a sheeted dyke complex of tabular, vertically dippingfeeder dykes in the root of the pipe which extends down to the mantle. Within 1.5–2 km (0.93–1.2 mi) of the surface, the highly pressured magma explodes upwards and expands to form aconical to cylindrical diatreme, which erupts to the surface. The surface expression is rarelypreserved, but is usually similar to a maar volcano. The diameter of a kimberlite pipe at thesurface is typically a few hundred meters to a kilometer (up to 0.6 mile).Two Jurassic kimberlite dikes exist in Pennsylvania. One, the Gates-Adah Dike, outcrops on theMonongahela River on the border of Fayette and Greene Counties. The other, the Dixonville-Tanoma Dike in central Indiana County, does not outcrop at the surface and was discovered byminers.PetrologyBoth the location and origin of kimberlitic magmas are areas of contention. Their extremeenrichment and geochemistry has led to a large amount of speculation about their origin, withmodels placing their source within the sub-continental lithospheric mantle (SCLM) or even asdeep as the transition zone. The mechanism of enrichment has also been the topic of interest withmodels including partial melting, assimilation of subducted sediment or derivation from aprimary magma source.Historically, kimberlites have been subdivided into two distinct varieties termed basaltic andmicaceous based primarily on petrographic observations (Wagner, 1914). This was later revisedby Smith (1983) who re-named these divisions Group I and Group II based on the isotopicaffinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) later proposed thatthese group I and II kimberlites display such distinct differences, that they may not be as closelyrelated as once thought. He showed that Group II kimberlites actually show closer affinities tolamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites asorangeites to prevent confusion.Group I kimberlitesGroup-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primarymineral assemblage of forsteritic olivine, magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poorchromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic(0.5–10 mm, 0.020–0.39 in) to megacrystic (10–200 mm, 0.39–7.9 in) phenocrysts of olivine,pyrope, chromian diopside, magnesian ilmenite and phlogopite, in a fine to medium grainedgroundmass.
The groundmass mineralogy, which more closely resembles a true composition of the igneousrock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.Group II kimberlitesGroup-II kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles(dominantly H2O). The distinctive characteristic of orangeites is phlogopite macrocrysts andmicrophenocrysts, together with groundmass micas that vary in composition from phlogopite to"tetraferriphlogopite" (anomalously Fe-rich phlogopite). Resorbed olivine macrocrysts andeuhedral primary crystals of groundmass olivine are common but not essential constituents.Characteristic primary phases in the groundmass include: zoned pyroxenes (cores of diopsiderimmed by Ti-aegirine); spinel-group minerals (magnesian chromite to titaniferous magnetite);Sr- and REE-rich perovskite; Sr-rich apatite; REE-rich phosphates (monazite, daqingshanite);potassian barian hollandite group minerals; Nb-bearing rutile and Mn-bearing ilmenite.Kimberlitic indicator mineralsKimberlites are peculiar igneous rocks because they contain a variety of mineral species withpeculiar chemical compositions. These minerals such as potassic richterite, chromian diopside (apyroxene), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium, aregenerally absent from most other igneous rocks, making them particularly useful as indicators forkimberlites.These indicator minerals are generally sought in stream sediments in modern alluvial material.Their presence may indicate the presence of a kimberlite within the erosional watershed whichproduced the alluvium.Volcanic rock.Ignimbrite is a deposit of a pyroclastic flow.Volcanic rock is an igneous rock of volcanic origin.
TexturePhotomicrograph of a volcanic lithic fragment (sand grain); upper picture is plane-polarizedlight, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.Volcanic rocks are usually fine-grained or aphanitic to glass in texture. They often contain clastsof other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and areidentifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shapedphenocrysts embedded in a very fine grained matrix.Volcanic rocks often have a vesicular texture caused by voids left by volatiles escaping from themolten lava. Pumice is an example of explosive volcanic eruption. It is so vesicular that it floatsin water.Naming
Vesicular olivine basalt from La Palma (green phenocrysts are olivine).Volcanic rocks are named according to both their chemical composition and texture. Basalt is avery common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silicacontent. Rhyolite has silica content similar to that of granite while basalt is compositionallyequal to gabbro. Intermediate volcanic rocks include andesite, dacite, trachyte, and latite.Pyroclastic rocks are the product of explosive volcanism. They are often felsic (high in silica).Pyroclastic rocks are often the result of volcanic debris, such as ash, bombs and tephra, and othervolcanic ejecta. Examples of pyroclastic rocks are tuff and ignimbrite.Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks are alsoconsidered to be volcanic.Composition of volcanic rocksʻAʻā next to pāhoehoe lava at the Craters of the Moon National Monument and Preserve, Idaho,United States.The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (todifferentiate them from igneous rocks that form from magma below the surface, called igneousplutonic rocks).The lavas of different volcanoes, when cooled and hardened, differ much in their appearance andcomposition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassysubstance called obsidian. When filled with bubbles of gas, the same lava may form the spongymineral pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock calledrhyolite.The lavas, having cooled rapidly in contact with the air or water, are mostly finely crystalline orhave at least fine-grained ground-mass representing that part of the viscous semi-crystalline lavaflow that was still liquid at the moment of eruption. At this time they were exposed only toatmospheric pressure, and the steam and other gases, which they contained in great quantity werefree to escape; many important modifications arise from this, the most striking being the frequentpresence of numerous steam cavities (vesicular structure) often drawn out to elongated shapessubsequently filled up with minerals by infiltration (amygdaloidal structure).
As crystallization was going on while the mass was still creeping forward under the surface ofthe Earth, the latest formed minerals (in the ground-mass) are commonly arranged in subparallelwinding lines that follow the direction of movement (fluxion or fluidal structure)—and largerearly minerals that previously crystallized may show the same arrangement. Most lavas fallconsiderably below their original temperatures before emitted. In their behavior, they present aclose analogy to hot solutions of salts in water, which, when they approach the saturationtemperature, first deposit a crop of large, well-formed crystals (labile stage) and subsequentlyprecipitate clouds of smaller less perfect crystalline particles (metastable stage).In igneous rocks the first generation of crystals generally forms before the lava has emerged tothe surface, that is to say, during the ascent from the subterranean depths to the crater of thevolcano. It has frequently been verified by observation that freshly emitted lavas contain largecrystals borne along in a molten, liquid mass. The large, well-formed, early crystals(phenocrysts) are said to be porphyritic; the smaller crystals of the surrounding matrix or ground-mass belong to the post-effusion stage. More rarely lavas are completely fused at the moment ofejection; they may then cool to form a non-porphyritic, finely crystalline rock, or if more rapidlychilled may in large part be non-crystalline or glassy (vitreous rocks such as obsidian, tachylyte,pitchstone).A common feature of glassy rocks is the presence of rounded bodies (spherulites), consisting offine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixedwith quartz or tridymite; similar bodies are often produced artificially in glasses that are allowedto cool slowly. Rarely these spherulites are hollow or consist of concentric shells with spacesbetween (lithophysae). Perlitic structure, also common in glasses, consists of the presence ofconcentric rounded cracks owing to contraction on cooling.Volcanic rocks, Porto Moniz, MadeiraThe phenocrysts or porphyritic minerals are not only larger than those of the ground-mass; as thematrix was still liquid when they formed they were free to take perfect crystalline shapes,without interference by the pressure of adjacent crystals. They seem to have grown rapidly, asthey are often filled with enclosures of glassy or finely crystalline material like that of theground-mass . Microscopic examination of the phenocrysts often reveals that they have had acomplex history. Very frequently they show layers of different composition, indicated byvariations in color or other optical properties; thus augite may be green in the center surrounded
by various shades of brown; or they may be pale green centrally and darker green with strongpleochoism (aegirine) at the periphery.In the feldspars the center is usually richer in calcium than the surrounding layers, and successivezones may often be noted, each less calsic than those within it. Phenocrysts of quartz (and ofother minerals), instead of sharp, perfect crystalline faces, may show rounded corroded surfaces,with the points blunted and irregular tongue-like projections of the matrix into the substance ofthe crystal. It is clear that after the mineral had crystallized it was partly again dissolved orcorroded at some period before the matrix solidified.Corroded phenocrysts of biotite and hornblende are very common in some lavas; they aresurrounded by black rims of magnetite mixed with pale green augite. The hornblende or biotitesubstance has proved unstable at a certain stage of consolidation, and has been replaced by aparamorph of augite and magnetite, which may partially or completely substitute for the originalcrystal but still retains its characteristic outlines.FelsiteFelsite covered with dendritic pyrolusite formations.Felsite (also called felstone ) is a very fine grained volcanic rock that may or may not containlarger crystals. Felsite is a field term for a light colored rock that typically requires petrographicexamination or chemical analysis for more precise definition. Color is generally white throughlight gray, or red to tan and may include any color except dark gray, green or black (the colors oftraprock). The mass of the rock consists of a fine-grained matrix of felsic materials,particularly quartz, sodium and potassium feldspar, and may be termed a quartz felsite or quartzporphyry if the quartz phenocrysts are present. This rock is typically of volcanic origin, and maybe found in association with obsidian and rhyolite. In some cases, it is sufficiently fine-grainedfor use in making stone tools.Rhyolite Rhyolite
— Igneous Rock — CompositionFelsic: igneous quartz and alkali feldspar (orthoclase,sanidine and sodic plagioclase), biotite and hornblende.This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellitesystem, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silica-rich) composition (typically >69% SiO2 — see the TAS classification). It may have any texture from glassy to aphanitic toporphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio> 1:2 — see the QAPF diagram). Biotite and hornblende are common accessory minerals.Rocks from the Bishop tuff, uncompressed with pumice on left; compressed with fiamme onright.GeologyRhyolite can be considered as the extrusive equivalent to the plutonic granite rock, andconsequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high contentof silica and low iron and magnesium contents, rhyolite melts are highly polymerized and formhighly viscous lavas. They can also occur as breccias or in volcanic plugs and dikes. Rhyolitesthat cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian.Slower cooling forms microscopic crystals in the lava and results in textures such as flow
foliations, spherulitic, nodular, and lithophysal structures. Some rhyolite is highly vesicularpumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallouttephra/tuff or of ignimbrites.HistoryDuring the second millennium BC, rhyolite was quarried extensively in what is now easternPennsylvania in the United States. Among the leading quarries was the Carbaugh Run RhyoliteQuarry Site in Adams County, where as many as fifty small quarry pits are known. NameTop stone is obsidian (vitrophyre), below that is pumice and in lower right corner is rhyolite(light color)A sample of Rhyolite from the Conical Hill dome at the head Lyttelton Harbour, BanksPeninsula, New ZealandThe name rhyolite was introduced into science by the German traveler and geologist Ferdinandvon Richthofen after his explorations in the Rocky Mountains in the 1860s.Obsidian. Obsidian
Obsidian from Lake County, Oregon GeneralCategory Volcanic glass 70–75% SiO2,Chemical formula plus MgO, Fe3O4 Identification Black, gray, dark green, red, yellow,Color pinkFracture ConchoidalMohs scale ~ 5 to 5.5hardnessLuster VitreousSpecific gravity ~ 2.5Optical properties TranslucentObsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly without crystal growth.Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidianflows, where the chemical composition (high silica content) induces a high viscosity andpolymerization degree of the lava. The inhibition of atomic diffusion through this highly viscousand polymerized lava explains the lack of crystal growth. Because of this lack of crystalstructure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use asprojectile points and blades, and its modern use as surgical scalpel blades.
Origin and propertiesPlinys Natural History features volcanic glass called "Obsidianus", so named from itsresemblance to a stone found in Ethiopia by one Obsius. Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition,its composition is too complex to comprise a single mineral. It is sometimes classified as amineraloid. Though obsidian is dark in color similar to mafic rocks such as basalt, obsidianscomposition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70%or more. Crystalline rocks with obsidians composition include granite and rhyolite. Becauseobsidian is metastable at the Earths surface (over time the glass becomes fine-grained mineralcrystals), no obsidian has been found that is older than Cretaceous age. This breakdown ofobsidian is accelerated by the presence of water. Obsidian has low water content when fresh,typically less than 1% water by weight,  but becomes progressively hydrated when exposed togroundwater, forming perlite. Tektites were once thought by many to be obsidian produced bylunar volcanic eruptions, though few scientists now adhere to this hypothesis.Pure obsidian is usually dark in appearance, though the color varies depending on the presence ofimpurities. Iron and magnesium typically give the obsidian a dark green to brown to black color.Very few samples are nearly colorless. In some stones, the inclusion of small, white, radiallyclustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern(snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow,aligned along layers created as the molten rock was flowing before being cooled. These bubblescan produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent,rainbow-like sheen (rainbow obsidian).Glass Mountain, a large Counterclockwise from top:obsidian flow at Medicine Snowflake obsidian, pumice andLake Volcano obsidian Rainbow obsidian rhyolite (light color)OccurrenceObsidian can be found in locations which have experienced rhyolitic eruptions. It can be found inArgentina, Armenia, Canada, Chile, Greece, Guatemala, Iceland, Italy, Japan, Kenya, Mexico,New Zealand, Peru, Scotland and United States. Obsidian flows which may be hiked on arefound within the calderas of Newberry Volcano and Medicine Lake Volcano in the CascadeRange of western North America, and at Inyo Craters east of the Sierra Nevada in California.Yellowstone National Park has a mountainside containing obsidian located between MammothHot Springs and the Norris Geyser Basin, and deposits can be found in many other western U.S.
states including Arizona, Colorado, New Mexico, Texas, Utah, Washington, Oregon andIdaho. Obsidian can also be found in the eastern U.S. state of Virginia.Obsidian arrowhead.Historical useObsidian was valued in Stone Age cultures because, like flint, it could be fractured to producesharp blades or arrowheads. Like all glass and some other types of naturally occurring rocks,obsidian breaks with a characteristic conchoidal fracture. It was also polished to create earlymirrors.Modern archaeologists have developed a relative dating system, obsidian hydration dating, tocalculate the age of obsidian artifacts.Middle EastIn Ubaid in the 5th millennium BC, blades were manufactured from obsidian mined in todaysTurkey.Obsidian talus at Obsidian Dome, California.AmericasLithic analysis can be instrumental in understanding prehispanic groups in Mesoamerica. Acareful analysis of obsidian in a culture or place can be of considerable use to reconstruct
commerce, production, distribution and thereby understand economic, social and politicalaspects of a civilization. This is the case in Yaxchilán, a Maya city where even warfareimplications have been studied linked with obsidian use and its debris.  Another example is thearcheological recovery at coastal Chumash sites in California indicating considerable trade withthe distant site of Casa Diablo, California in the Sierra Nevada Mountains.Pre-Columbian Mesoamericans use of obsidian was extensive and sophisticated; includingcarved and worked obsidian for tools and decorative objects. Mesoamericans also made a type ofsword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon wascapable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade withthe ragged cut of a serrated weapon.Native American people traded obsidian throughout the Americas. Each volcano and in somecases each volcanic eruption produces a distinguishable type of obsidian, making it possible forarchaeologists to trace the origins of a particular artifact. Similar tracing techniques have allowedobsidian to be identified in Greece also as coming from Melos, Nisyros or Yiali, islands in theAegean Sea. Obsidian cores and blades were traded great distances inland from the coast. [citationneeded]In Chile obsidian tools from Chaitén Volcano have been found as far away as in Chan-Chan400 km north of the volcano and also in sites 400 km south of it.PitchstonePitchstone ridge: An Sgurr, Isle of Eigg, ScotlandPitchstone is a dull black glassy volcanic rock formed when viscous lava or magma coolsswiftly. It is similar to but coarser than obsidian. It is a volcanic glass with a conchoidal fracture(like glass), a resinous lustre, and a variable composition. Its colour may be mottled, streaked, oruniform brown, red, green, gray, or black. It is an extrusive rock that is very resistant to erosion.The ridge of An Sgurr on the Isle of Eigg was originally formed as a lava flow in a valley.Pumice
Specimen of highly porous pumice from Teide volcano on Tenerife, Canary Islands. Density ofspecimen approx 0.25 g/cm³. Scale is in centimeters.Pumice (pronounced /ˈpʌməs/ ) is a textural term for a volcanic rock that is a solidified frothylava typically created when super-heated, highly pressurized rock is violently ejected from avolcano. It can be formed when lava and water are mixed. This unusual formation is due to thesimultaneous actions of rapid cooling and rapid depressurization. The depressurization createsbubbles by lowering the solubility of gases (including water and CO2) dissolved in the lava, sothat they rapidly exsolve (like the bubbles of CO2 that appear when a carbonated drink isopened). The simultaneous cooling then freezes the bubbles in the matrix.PropertiesIllustrates the porous nature in detail
Rocks from the Bishop tuff, uncompressed with pumice on left; compressed with fiamme onright.A 10 centimeter (6 inch) piece of pumice supported by a rolled-up U.S. 20-dollar billdemonstrates its very low density.Pumice is composed of highly microvesicular glass pyroclastic with very thin, translucent bubblewalls of extrusive igneous rock. It is commonly, but not exclusively of silicic or felsic tointermediate in composition (e.g., rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte),but basaltic and other compositions are known. Pumice is commonly pale in color, ranging fromwhite, cream, blue or grey, to green-brown or black. It forms when volcanic gases exsolvingfrom viscous magma nucleate bubbles which cannot readily decouple from the viscous magmaprior to chilling to glass. Pumice is a common product of explosive eruptions (plinian andignimbrite-forming) and commonly forms zones in upper parts of silicic lavas. Pumice has anaverage porosity of 90%, and initially floats on water.Scoria differs from pumice in being denser, with larger vesicles and thicker vesicle walls; it sinksrapidly. The difference is the result of the lower viscosity of the magma that forms scoria. Whenlarger amounts of gas are present, the result is a finer-grained variety of pumice known aspumicite. Pumice is considered a glass because it has no crystal structure. Pumice varies indensity according to the thickness of the solid material between the bubbles; many samples floatin water. After the explosion of Krakatoa, rafts of pumice drifted through the Pacific Ocean forup to 20 years, with tree trunks floating among them.  In fact, pumice rafts disperse and supportseveral marine species. In 1979, 1984 and 2006, underwater volcanic eruptions near Tongacreated large pumice rafts, some as large as 30 kilometres that floated hundreds of kilometres toFiji.There are two main forms of vesicles. Most pumice contains tubular microvesicles that canimpart a silky or fibrous fabric. The elongation of the microvesicles occurs due to ductileelongation in the volcanic conduit or, in the case of pumiceous lavas, during flow. The otherform of vesicles are subspherical to spherical and result from high vapor pressure duringeruption.Scoria
Scoria of various hues exists on Mount Tarawera, from its 1886 eruption.ScoriaHolocene scoria-producing volcano near Veyo, Utah
Tuff moai with red scoria pukao on its headFresh scoria sometimes has a blue sheen to its surface.Photomicrograph of a volcanic lithic fragment (sand grain) derived from scoria; upper picture isplane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25millimeter.Scoria is a volcanic rock containing many holes or vesicules. It is most generally dark in color(generally dark brown, black or red), and basaltic or andesitic in composition. Scoria is relativelylight as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all
scoria has a specific gravity greater than 1, and sinks in water. The holes or vesicules form whengases that were dissolved in the magma come out of solution as it erupts, creating bubbles in themolten rock, some of which are frozen in place as the rock chills and solidifies. Scoria may formas part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks andbombs), for instance in Strombolian eruptions that form steep-sided scoria cones. Most scoria iscomposed of glassy fragments, and may contain phenocrysts.The word scoria comes from the Greek σκωρία, skōria, rust. An old name for scoria is cinder.ComparisonsScoria differs from pumice, another vesicular volcanic rock, in having larger vesicles and thickervesicle walls, and hence is denser. The difference is probably the result of lower magmaviscosity, allowing rapid volatile diffusion, bubble growth, coalescence, and bursting.FormationAs rising magma encounters lower pressures, dissolved gases are able to exsolve and formvesicles. Some of the vesicles are trapped when the magma chills and solidifies. Vesicles areusually small, spheroidal and do not impinge upon one another; instead they open into oneanother with little distortion.Volcanic cones of scoria can be left behind after eruptions, usually forming mountains with acrater at the summit. An example is Mount Wellington, Auckland in New Zealand, which likethe Three Kings in the south of the same city has been extensively quarried. Quincan, a uniqueform of Scoria, is quarried at Mount Quincan in Far North Queensland, Australia.TrachyteA sample of trachyteTrachyte is an igneous, volcanic rock with an aphanitic to porphyritic texture. The mineralassemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or afeldspathoid such as nepheline may also be present. (See the QAPF diagram). Biotite,clinopyroxene and olivine are common accessory minerals.
Chemically, trachyte contains less SiO2 than rhyolite and more (Na2O plus K2O) than dacite.These chemical differences are consistent with the position of trachyte in the TAS classification,and they account for the feldspar-rich mineralogy of the rock type.Trachytes usually consist mainly of sanidine feldspar. Very often they have minute irregularsteam cavities which make the broken surfaces of specimens of these rocks rough and irregular,and from this character they have derived their name. It was first given to certain rocks of thisclass from Auvergne, and was long used in a much wider sense than that defined above, in fact itincluded quartz-trachytes (now known as liparites and rhyolites) and oligoclase-trachytes, whichare now more properly assigned to andesites. The trachytes are often described as being thevolcanic equivalents of the plutonic syenites. Their dominant mineral, sanidine feldspar, verycommonly occurs in two generations, i.e. both as large well-shaped porphyritic crystals and insmaller imperfect rods or laths forming a finely crystalline groundmass. With this there ispractically always a smaller amount of plagioclase, usually oligoclase; but the potash felspar(sanidine) often contains a considerable proportion of the sodium feldspar (albite), and has ratherthe characteristics of anorthoclase or cryptoperthite than of pure sanidine. Rhomb porphyry is anexample with usually large porphyritic rhomb shaped phenocrysts embedded in a very finegrained matrix.Quartz is typically rare in trachyte, but tridymite (which likewise consists of silica) is by nomeans uncommon. It is rarely in crystals large enough to, be visible without the aid of themicroscope, but in thin sections it may appear as small hexagonal plates, which overlap and formdense aggregates, like a mosaic or like the tiles on a roof. They often cover the surfaces of thelarger feldspars or line the steam cavities of the rock, where they may be mingled withamorphous opal or fibrous chalcedony. In the older trachytes, secondary quartz is not rare, andprobably sometimes results from the recrystallization of tridymite.Of the mafic minerals present, augite is the most common. It is usually of pale green color, andits small crystals are often very perfect in form. Brown hornblende and biotite occur also, and areusually surrounded by black corrosion borders composed of magnetite and pyroxene; Sometimesthe replacement is complete and no bornblende or biotite is left, though the outlines of the clusterof magnetite and augite may clearly indicate from which of these minerals it was derived.Olivine is unusual, though found in some trachytes, like those of the Arso in Isthia. Basicvarieties of plagioclase, such as labradorite, are known also as phenocrysts in some Italiantrachytes. Dark brown varieties of augite and rhombic pyroxene (hypersthene or bronzite) havebeen observed but are not common. Apatite, zircon and magnetite are practically always presentas accessory minerals.The trachytes being very rich in potash feldspar, necessarily contain considerable amounts ofalkali; in this character they approach the phonolites. Occasionally minerals of the feldspathoidgroup, such as nepheline, sodalite and leucite, occur, and rocks of this kind are known asphonolitic trachytes. The sodium-bearing amphiboles and pyroxenes so characteristic of thephonolites may also be found in some trachytes; thus aegirine or aegirine augite formsoutgrowths on diopside crystals, and riebeckite may be present in spongy growths among thefeldspars of the groundmass (as in the trachyte of Berkum on the Rhine). Trachytic rocks aretypically porphyritic, and some of the best known examples, such as the trachyte of Drachenfels
on the Rhine, show this character excellently, having large sanidine crystals of tabular form aninch or two in length scattered through their fine-grained groundmass. In many trachytes,however, the phenocrysts are few and small, and the groundmass comparatively coarse. Theferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in handspecimens of these rocks. Two types of groundmass are generally recognized: the trachytic,composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consistingof small, squarish or rectangular prisms of the same mineral. Sometimes granular augite orspongy riebeckite occurs in the groundmass, but as a rule this part of the rock is highlyfeldspathic. Glassy forms of trachyte (obsidian) occur, as in Iceland, and pumiceous varieties areknown (in Teneriffe and elsewhere), but these rocks as contrasted with the rhyolites have aremarkably strong tendency to crystallize, and are rarely to any considerable extent vitreous.A polished opal on trachyteTrachytes are well represented among the Tertiary and recent volcanic rocks of Europe. InBritain they occur in Skye as lava flows and as dikes or intrusions, but they are much morecommon on the continent of Europe, as in the Rhine district and the Eifel, also in Auvergne,Bohemia and the Euganean Hills. In the neighborhoord of Rome, Naples and the island of Ischiatrachytic lavas and tuffs are of common occurrence. In the United States trachytes are lessfrequent, being known in South Dakota (Black Hills). In Iceland, the Azores, Teneriffe andAscension there are recent trachytic lavas, and rocks of this kind occur also in New South Wales(Cambewarra range), East Africa, Madagascar, Aden and in many other districts.Among the older volcanic rocks trachytes also are not scarce, though they have often beendescribed under the names orthophyre and orthoclase-porphyry, while trachyte was reserved forTertiary and recent rocks of similar composition. In England there are Permian trachytes in theExeter district, and Carboniferous trachytes are found in many parts of the central valley ofScotland. The latter differ in no essential respect from their modern representatives in Italy andthe Rhine valley, but their augite and biotite are often repiaced by chlorite and other secondaryproducts. Permian trachytes occur also in Thuringia and the Saar district in Germany.Closely allied to the trachytes are the keratophyres, which occur mainly in Palaeozoic strata inthe Harz (Germany), in the Southern Uplands of Scotland, in Cornwall, etc. They are usuallyporphyritic and fluidal; and consist mainly of alkali feldspar (anorthoclase principally, but alsoalbite and orthoclase), with a small quantity of chlorite and iron oxides.
PhonoliteAegirine phonolite. Dark prismatic minerals are aegirine phenocrysts.Phonolite is a rare igneous, volcanic (extrusive) rock of intermediate (between felsic and mafic)composition, with aphanitic to porphyritic texture.The name phonolite comes from the Greek meaning (more or less) "sounding stone" because ofthe metallic sound it produces if an unfractured plate is hit, hence the English name clinckstone.GenesisPhonolite is unusual in that it forms from a highly silica undersaturated melt by low degrees ofpartial melting (less than 10%) of highly aluminous lower crustal rocks such as tonalite,monzonite and metamorphic rocks. Melting of such rocks to a very low degree promotes theliberation of aluminium, potassium, sodium and calcium via melting of feldspar, with someinvolvement of mafic minerals. The melt formed is silica undersaturated (ie; quartz is absentfrom the melts or solidified rocks), with feldspathoid species dominating over feldspar species inthe melt.Phonolite occurrences are associated with a few geological processes and tectonic events, whichcan lead to the melting of appropriate precursor lithologies. These include intracontinentalhotspot volcanism, such as may form above mantle plumes covered by thick continental crust. A-type granites and alkaline igneous provinces are usually associated with phonolites. Phonolitesmay also be produced by low degree partial melting of underplates of granitic material incollisional orogenic belts.MineralogyPhonolites, as they are products of low degree partial melts, are silica undersaturated, and havefeldspathoids in their normative mineralogy.Mineral assemblages in phonolite occurrences are usually abundant feldspathoids (nepheline,sodalite, hauyne, leucite and analcite) and alkali feldspar (sanidine, anorthoclase or orthoclase),
and rare sodic plagioclase. Biotite, sodium rich amphiboles and pyroxenes along with iron richolivine are common minor minerals. Accessory phases include titanite, apatite, corundum.zircon, magnetite and ilmenite. Phonolites are silica under-saturated, as illustrated by theposition of phonolite in the TAS classification and QAPF diagrams.Phonolite is a fine-grained equivalent of nepheline syenite, and the genesis of such magmas isdiscussed in the treatment of that rock type.OccurrenceNepheline syenites and phonolites occur widely distributed throughout the world  in Canada,Norway, Greenland, Sweden, the Ural Mountains, the Pyrenees, Italy, Brazil,the Transvaalregion, and Magnet Cove igneous complex of Arkansas, as well as on oceanic islands (eg;Canary Islands).Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex,possibly formed from partial melting of the wall rocks adjacent to that large ultramafic layeredintrusion.Examples Devils Tower, a striking example of columnar jointed phonolite. Dunedin, New Zealand Hoodoo Mountain, northwestern British Columbia, Canada. Jebel Nefusa, Libya Teide, a stratovolcano on the island of Tenerife. Mont Gerbier de Jonc South East FranceLatite Latite from Boxberg, High-Eifel, Germany
Photomicrograph of thin section of latite (in plane polarised light) Photomicrograph of thin section of latite (in cross polarised light) Latite is an igneous, volcanic (extrusive) rock, with aphanitic-aphyric to aphyric-porphyritic texture. Its mineral assemblage is usually alkali feldspar and plagioclase (in a ratio < 1:4). Quartz is absent in a feldspathoid-bearing latite, and olivine is absent in a quartz-bearing latite. Biotite, hornblende, pyroxene and scarce olivine or quartz are common accessory minerals.Rhomb porphyries are an unusual variety with gray-white porphyritic rhomb shaped phenocrysts embedded in a very fine grained red-brown matrix. The composition of rhomb porphyry places it in the trachyte - latite classification of the QAPF diagram.Quartz latiteA quartz latite is a latite with a phenocryst modal composition containing 5-20% quartz. Above20% quartz, the rock would be classified as a rhyolite.PegmatitePegmatite with blue corundum crystals
Pegmatite containing lepidolite, tourmaline, and quartz from the White Elephant Mine in theBlack Hills, South Dakota.A pegmatite is a very coarse-grained, intrusive igneous rock composed of interlocking grainsusually larger than 2.5 cm in size; such rocks are referred to as pegmatitic.Most pegmatites are composed of quartz, feldspar and mica; in essence a granite. Rarerintermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar,pyroxene and other minerals are known, found in recrystallised zones and apophyses associatedwith large layered intrusions.Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size.Individual crystals over 10 meters across have been found, and the worlds largest crystal wasfound within a pegmatite.Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size andperfection. Feldspar within a pegmatite may display exaggerated and perfect twinning,exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture isknown, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often showsgigantic perthitic texture visible to the naked eye.PetrologyCrystal growth rates in pegmatite must be incredibly fast to allow gigantic crystals to growwithin the confines and pressures of the Earths crust. For this reason, the consensus onpegmatitic growth mechanisms involves a combination of the following processes; Low rates of nucleation of crystals coupled with high diffusivity to force growth of a few large crystals instead of many smaller crystals High vapor and water pressure, to assist in the enhancement of conditions of diffusivity High concentrations of fluxing elements such as boron and lithium which lower the temperature of solidification within the magma or vapor Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to occur only within greenschist metamorphic terranes
Despite this consensus on likely chemical, thermal and compositional conditions required topromote pegmatite growth there are three main theories behind pegmatite formation; 1. Metamorphic; pegmatite fluids are created by devolatilisation (dewatering) of metamorphic rocks, particularly felsic gneiss, to liberate the right constituents and water, at the right temperature 2. Magmatic; pegmatites tend to occur in the aureoles of granites in most cases, and are usually granitic in character, often closely matching the compositions of nearby granites. Pegmatites thus represent exsolved granitic material which crystallises in the country rocks 3. Metasomatic; pegmatite, in a few cases, could be explained by the action of hot alteration fluids upon a rock mass, with bulk chemical and textural change.Metasomatism is currently not well favored as a mechanism for pegmatite formation and it islikely that metamorphism and magmatism are both contributors toward the conditions necessaryfor pegmatite genesis.MineralogyPegmatitic granite, Rock Creek Canyon, eastern Sierra Nevada, California. Note pink potassiumfeldspars and cumulate-filled chamber.The mineralogy of a pegmatite is in all cases dominated by some form of feldspar, often withmica and usually with quartz, being altogether "granitic" in character. Beyond that, pegmatitemay include most minerals associated with granite and granite-associated hydrothermal systems,granite-associated mineralisation styles, for example greisens, and somewhat with skarnassociated mineralisation.It is however impossible to quantify the mineralogy of pegmatite in simple terms because of theirvaried mineralogy and difficulty in estimating the modal abundance of mineral species which areof only a trace amount. This is because of the difficulty in counting and sampling mineral grainsin a rock which may have crystals from centimeters to meters across.Garnet, commonly almandine or spessartine, is a common mineral within pegmatites intrudingmafic and carbonate-bearing sequences. Pegmatites associated with granitic domes within the
Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red, orange and brownalmandine garnet.Tantalum and niobium minerals (columbite, tantalite, niobite) are found in association withspodumene, lepidolite, tourmaline, cassiterite in the massive Greenbushes Pegmatite in theYilgarn Craton of Western Australia, considered a typical metamorphic pegmatite unassociatedwith granite.GeochemistryPegmatite is difficult to sample representatively due to the large size of the constituent mineralcrystals. Often, bulk samples of some 50–60 kg of rock must be crushed to obtain a meaningfuland repeatable result. Hence, pegmatite is often characterised by sampling the individualminerals which comprise the pegmatite, and comparisons are made according to mineralchemistry.Geochemically, pegmatites typically have major element compositions approximating "granite",however, when found in association with granitic plutons it is likely that a pegmatite dike willhave a different trace element composition with greater enrichment in large-ion lithophile(incompatible) elements, boron, beryllium, aluminium, potassium and lithium, uranium, thorium,cesium, et cetera.Occasionally, enrichment in the unusual trace elements will result in crystallisation of equallyunusual and rare minerals such as beryl, tourmaline, columbite, tantalite, zinnwaldite and soforth. In most cases, there is no particular genetic significance to the presence of rare mineralogywithin a pegmatite, however it is possible to see some causative and genetic links between, say,tourmaline-bearing granite dikes and tourmaline-bearing pegmatites within the area of influenceof a composite granite intrusion (Mount Isa Inlier, Queensland, Australia).Sedimentary rockMiddle Triassic marginal marine sequence of siltstones (below) and limestones (above), VirginFormation, southwestern Utah, USASedimentary rock is a type of rock that is formed by sedimentation of material at the Earthssurface and within bodies of water. Sedimentation is the collective name for processes that cause
mineral and/or organic particles (detritus) to settle and accumulate or minerals to precipitatefrom a solution. Particles that form a sedimentary rock by accumulating are called sediment.Before being deposited, sediment was formed by weathering and erosion in a source area, andthen transported to the place of deposition by water, wind, mass movement or glaciers which arecalled agents of denudation.The sedimentary rock cover of the continents of the Earths crust is extensive, but the totalcontribution of sedimentary rocks is estimated to be only 5% of the total volume of the crust.Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous andmetamorphic rocks.Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. The studyof sedimentary rocks and rock strata provides information about the subsurface that is useful forcivil engineering, for example in the construction of roads, houses, tunnels, canals or otherconstructions. Sedimentary rocks are also important sources of natural resources like coal, fossilfuels, drinking water or ores.The study of the sequence of sedimentary rock strata is the main source for scientific knowledgeabout the Earths history, including palaeogeography, paleoclimatology and the history of life.The scientific discipline that studies the properties and origin of sedimentary rocks is calledsedimentology. Sedimentology is both part of geology and physical geography and overlapspartly with other disciplines in the Earth sciences, such as pedology, geomorphology,geochemistry or structural geology.Genetic classification schemesBased on the processes responsible for their formation, sedimentary rocks can be subdivided intofour groups: clastic sedimentary rocks, biochemical (or biogenic) sedimentary rocks, chemicalsedimentary rocks and a fourth category for "other" sedimentary rocks formed by impacts,volcanism, and other minor processes.Clastic sedimentary rocksMain article: Clastic rock
Claystone deposited in Glacial Lake Missoula, Montana, USA. Note very fine and flat bedding, commonfor distal lacustrine deposition.Clastic sedimentary rocks are composed of silicate minerals and rock fragments that weretransported by moving fluids (as bed load, suspended load, or by sediment gravity flows) andwere deposited when these fluids came to rest. Clastic rocks are composed largely of quartz,feldspar, rock (lithic) fragments, clay minerals, and mica; numerous other minerals may bepresent as accessories and may be important locally.Clastic sediment, and thus clastic sedimentary rocks, are subdivided according to the dominantparticle size (diameter). Most geologists use the Udden-Wentworth grain size scale and divideunconsolidated sediment into three fractions: gravel (>2 mm diameter), sand (1/16 to 2 mmdiameter), and mud (clay is <1/256 mm and silt is between 1/16 and 1/256 mm). Theclassification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias aremade mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly ofmud. This tripartite subdivision is mirrored by the broad categories of rudites, arenites, andlutites, respectively, in older literature.Subdivision of these three broad categories is based on differences in clast shape (conglomeratesand breccias), composition (sandstones), grain size and/or texture (mudrocks). Conglomerates and brecciasConglomerates are dominantly composed of rounded gravel and breccias are composed ofdominantly angular gravel.SandstonesSandstone classification schemes vary widely, but most geologists have adopted the Dottscheme, which uses the relative abundance of quartz, feldspar, and lithic framework grains andthe abundance of muddy matrix between these larger grains. Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. For naming purposes, the abundance of framework grains is normalized to quartz, feldspar, and lithic fragments formed from other rocks. These are the three most abundant components of sandstones; all other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grains
Abundance of muddy matrix between sand grains When sand-sized particles are deposited, the space between the sand grains either remains open or is filled with mud (silt and/or clay sized particle). "Clean" sandstones with open pore space (that may later be filled with cement) are called arenites Muddy sandstones with abundant (>10%) muddy matrix are called wackes.Six sandstone names are possible using descriptors for grain composition (quartz-, feldspathic-,and lithic-) and amount of matrix (wacke or arenite). For example, a quartz arenite would becomposed of mostly (>90%) quartz grains and have little/no clayey matrix between the grains, alithic wacke would have abundant lithic grains (<90% quartz, remainder would have more lithicsthan feldspar) and abundant muddy matrix, etc.Although the Dott classification scheme  is widely used by sedimentologists, common nameslike greywacke, arkose, and quartz sandstone are still widely used by nonspecialists and inpopular literature.MudrocksLower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weatheringand chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents.Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. Theserelatively fine-grained particles are commonly transported as suspended particles by turbulentflow in water or air, and deposited as the flow calms and the particles settle out of suspension.Most authors presently use the term "mudrock" to refer to all rocks composed dominantly ofmud. Mudrocks can be divided into siltstones (composed dominantly of silt-sizedparticles), mudstones (subequal mixture of silt- and clay-sized particles), and claystones(composed mostly of clay-sized particles). Most authors use "shale" is a term for a fissilemudrock (regardless of grain size), although some older literature uses the term "shale" as asynonym for mudrock.Biochemical sedimentary rocks
Outcrop of Ordovician oil shale (kukersite), northern EstoniaBiochemical sedimentary rocks are created when organisms use materials dissolved in air orwater to build their tissue. Examples include: Most types of limestone are formed from the calcareous skeletons of organisms such as corals, mollusks, and foraminifera. Coal which forms as plants remove carbon from the atmosphere and combine with other elements to build their tissue. Deposits of chert formed from the accumulation of siliceous skeletons from microscopic organisms such as radiolaria and diatoms.Chemical sedimentary rocksChemical sedimentary rock forms when mineral constituents in solution become supersaturatedand inorganically precipitate. Common chemical sedimentary rocks include oolitic limestone androcks composed of evaporite minerals such as halite (rock salt), sylvite, barite and gypsum."Other" sedimentary rocksThis fourth miscellaneous category includes rocks formed by Pyroclastic flows, impact breccias,volcanic breccias, and other relatively uncommon processes.Compositional classification schemesAlternately, sedimentary rocks can be subdivided into compositional groups based on theirmineralogy: Siliciclastic sedimentary rocks, as described above, are dominantly composed of silicate minerals. The sediment that makes up these rocks was transported as bed load, suspended load, or by sediment gravity flows. Siliciclastic sedimentary rocks are subdivided into conglomerates and breccias, sandstone, and mudrocks.
Carbonate sedimentary rocks are composed of calcite (rhombohedral CaCO3), aragonite (orthorhombic CaCO3), dolomite (CaMg(CO3)2), and other carbonate minerals based on the CO32- ion. Common examples include limestone and dolostone. Evaporite sedimentary rocks are composed of minerals formed from the evaporation of water. The most common evaporite minerals are carbonates (calcite and others based on CO32-), chlorides (halite and others built on Cl-), and sulfates (gypsum and others built on SO42-). Evaporite rocks commonly include abundant halite (rock salt), gypsum, and anhydrite. Organic-rich sedimentary rocks have significant amounts of organic material, generally in excess of 5% total organic carbon. Common examples include coal, oil shale, and other sedimentary rocks that act as reservoirs for liquid hydrocarbons and natural gas. Siliceous sedimentary rocks are almost entirely composed of silica (SiO2), typically as chert, opal, chalcedony or other microcrystalline forms. Iron-rich sedimentary rocks are composed of >15% iron; the most common forms are banded iron formations and ironstones Phosphatic sedimentary rocks are composed of phosphate minerals and contain more than 6.5% phosphorus; examples include deposits of phosphate nodules, bone beds, and phosphatic mudrocksCreation of sedimentary rocksSediment transport and depositionCross-bedding and scour in a fine sandstone; the Logan Formation (Mississippian) of Jackson County,Ohio.Sedimentary rocks are formed when sediment is deposited out of air, ice, wind, gravity, or waterflows carrying the particles in suspension. This sediment is often formed when weathering anderosion break down a rock into loose material in a source area. The material is then transportedfrom the source area to the deposition area. The type of sediment transported depends on thegeology of the hinterland (the source area of the sediment). However, some sedimentary rocks,like evaporites, are composed of material that formed at the place of deposition. The nature of asedimentary rock therefore not only depends on sediment supply, but also on the sedimentarydepositional environment in which it formed.
DiagenesisPressure solution at work in a clastic rock. While material dissolves at places where grains are in contact,material crystallizes from the solution (as cement) in open pore spaces. This means there is a net flow ofmaterial from areas under high stress to those under low stress. As a result, the rock becomes morecompact and harder. Loose sand can become sandstone in this way.Main article: diagenesisThe term diagenesis is used to describe all the chemical, physical, and biological changes,including cementation, undergone by a sediment after its initial deposition, exclusive of surfaceweathering. Some of these processes cause the sediment to consolidate: a compact, solidsubstance forms out of loose material. Young sedimentary rocks, especially those of Quaternaryage (the most recent period of the geologic time scale) are often still unconsolidated. Assediment deposition builds up, the overburden (or lithostatic) pressure rises and a process knownas lithification takes place.Sedimentary rocks are often saturated with seawater or groundwater, in which minerals candissolve or from which minerals can precipitate. Precipitating minerals reduce the pore space in arock, a process called cementation. Due to the decrease in pore space, the original connate fluidsare expelled. The precipitated minerals form a cement and make the rock more compact andcompetent. In this way, loose clasts in a sedimentary rock can become "glued" together.When sedimentation continues, an older rock layer becomes buried deeper as a result. Thelithostatic pressure in the rock increases due to the weight of the overlying sediment. This causescompaction, a process in which grains mechanical reorganize. Compaction is, for example, animportant diagenetic process in clay, which can initially consist of 60% water. Duringcompaction, this interstitial water is pressed out of pore spaces. Compacation can also be theresult of dissolution of grains by pressure solution. The dissolved material precipitates again inopen pore spaces, which means there is a nett flow of material into the pores. However, in somecases a certain mineral dissolves and not precipitate again. This process is called leaching andincreases pore space in the rock.Some biochemical processes, like the activity of bacteria, can affect minerals in a rock and aretherefore seen as part of diagenesis. Fungi and plants (by their roots) and various other organismsthat live beneath the surface can also influence diagenesis.
Burial of rocks due to ongoing sedimentation leads to increased pressure and temperature, whichstimulates certain chemical reactions. An example is the reactions by which organic materialbecomes lignite or coal. When temperature and pressure increase still further, the realm ofdiagenesis makes way for metamorphism, the process that forms metamorphic rock.PropertiesA piece of a banded iron formation, a type of rock that consists of alternating layers with iron(III) oxide(red) and iron(II) oxide (grey). BIFs were mostly formed during the Precambrian, when the atmospherewasnt yet rich in oxygen. Moories Group, Barberton Greenstone Belt, South Africa.ColorThe color of a sedimentary rock is often mostly determined by iron, an element with two majoroxides: iron(II) oxide and iron(III) oxide. Iron(II) oxide only forms under anoxic circumstancesand gives the rock a grey or greenish colour. Iron(III) oxide is often in the form of the mineralhematite and gives the rock a reddish to brownish colour. In arid continental climates rocks arein direct contact with the atmosphere, and oxidation is an important process, giving the rock ared or orange colour. Thick sequences of red sedimentary rocks formed in arid climates arecalled red beds. However, a red colour does not necessarily mean the rock formed in acontinental environment or arid climate. The presence of organic material can colour a rock black or grey. Organic material is in natureformed from dead organisms, mostly plants. Normally, such material eventually decays byoxidation or bacterial activity. Under anoxic circumstances, however, organic material cannotdecay and becomes a dark sediment, rich in organic material. This, can for example, occur at thebottom of deep seas and lakes. There is little water current in such environments, so oxygen fromsurface water is not brought down, and the deposited sediment is normally a fine dark clay. Darkrocks rich in organic material are therefore often shales.Texture
Diagram showing the difference between well-sorted (left) and poorly sorted (right) clastic rocks.The size, form and orientation of clasts or minerals in a rock is called its texture. The texture is asmall-scale property of a rock, but determined many of its large-scale properties, such as thedensity, porosity or permeabililty.Clastic rocks have a clastic texture, which means they consist of clasts. The 3D orientation ofthese clasts is called the fabric of the rock. Between the clasts the rock can be composed of amatrix or a cement (the latter can consist of crystals of one or more precipitated minerals). Thesize and form of clasts can be used to determine the velocity and direction of current in thesedimentary environment where the rock was formed; fine, calcareous mud only settles in quietwater, while gravel and larger clasts are only deposited by rapidly moving water.  The grainsize of a rock is usually expressed with the Wentworth scale, though alternative scales are usedsometimes. The grain size can be expressed as a diameter or a volume, and is always an averagevalue - a rock is composed of clasts with different sizes. The statistical distribution of grain sizesis different for different rock types and is described in a property called the sorting of the rock.When all clasts are more or less of the same size, the rock is called well-sorted, when there is alarge spread in grain size, the rock is called poorly sorted. Diagram showing the influence of rounding and sphericity.The form of clasts can reflect the origin of the rock.Coquina, a rock composed of clasts of broken shells, can only form in energetic water. The formof a clast can be described by using four parameters:  Surface texture describes the amount of small-scale relief of the surface of a grain that is too small to influence the general shape. rounding describes the general smoothness of the shape of a grain. Sphericity describes the degree to which the grain approaches a sphere.
Grain form describes the three dimensional shape of the grain.Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals. Todescribe such a texture only the average size of the crystals and the fabric are necessary.MineralogyMost sedimentary rocks contain either quartz (especially siliciclastic rocks) or calcite (especiallycarbonate rocks). In contrast with igneous and metamorphic rocks, a sedimentary rocks usuallycontains very few different major minerals. However, the origin of the minerals in a sedimentaryrock is often more complex than those in an igneous rock. Minerals in a sedimentary rock canhave formed by precipitation during sedimentation or diagenesis. In the second case, the mineralprecipitate can have grown over an older generation of cement.  A complex diagenetic historycan be studied by optical mineralogy, using a petrographic microscope.Carbonate rocks dominantly consist of carbonate minerals like calcite, aragonite or dolomite.Both cement and clasts (including fossils and ooids) of a carbonate rock can consist of carbonateminerals. The mineralogy of a clastic rock is determined by the supplied material from the sourcearea, the manner of transport to the place of deposition and the stability of a particular mineral.The stability of the major rock forming minerals (their resistance to weathering) is expressed byBowens reaction series. In this series, quartz is most stable, followed by feldspar, micas, andother less stable minerals that are only present when little weathering has occurred.  Theamount of weathering depends mainly on the distance to the source area, the local climate andthe time it took for the sediment to be transported there. In most sedimentary rocks, mica,feldspar and less stable minerals have reacted to clay minerals like kaolinite, illite or smectite.FossilsFossil-rich layers in a sedimentary rock, Año Nuevo State Reserve, California.Main articles: fossil and fossilisationSedimentary rocks are the only type of rock that can contain fossils, the remains or imprints ofdead organisms. In nature, dead organisms are usually quickly removed by scavengers, bacteria,rotting and erosion. In some exceptional circumstances a carcass is fossilized because these
natural processes are unable to work. The chance of fossilisation is higher when thesedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (wherelittle bacterial activity exists) or when the organism had a particularly hard skeleton. Larger,well-preserved fossils are relatively rare. Most sedimentary rocks contains fossils, though withmany the fact only becomes apparent when studied under a microscope (microfossils) or with aloupe.Burrows in a turbidite, made by crustaceans. San Vincente Formation (early Eocene) of the Ainsa Basin,southern foreland of the Pyrenees.Fossils can both be the direct remains or imprints of organisms and their skeletons. Mostcommonly preserved are the harder parts of organisms such as bones, shells, woody tissue ofplants. Soft tissue has a much smaller chance of being preserved and fossilized and soft tissue ofanimals older than 40 million years is very rare.  Imprints of organisms made while still aliveare called trace fossils. Examples are burrows, foot prints, etc.Being part of a sedimentary rock, fossils undergo the same diagenetic processes as the rock. Ashell consisting of calcite can for example dissolve, while a cement of silica then fills the cavity.In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels,vascular tissue or other soft tissues. This preserves the form of the organism but changes thechemical composition, a process called permineralisation. The most common minerals inpermineralisation cements are carbonates (especially calcite), forms of amorphous silica(chalcedony, flint, chert) and pyrite. In the case of silica cements, the process is calledlithification.At high pressure and temperature, the organic material of a dead organism undergoes chemicalreactions in which volatiles like water and carbon dioxide are expulsed. The fossil, in the end,consists of a thin layer of pure carbon or its mineralized form, graphite. This form of fossilisationis called carbonisation. It is particularly important for plant fossils.  The same process isresponsible for the formation of fossil fuels like lignite or coal.Primary sedimentary structures
Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, ShetlandIslands.A flute cast, a type of sole marking, from the Book Cliffs of Utah.Ripple marks formed by a current in a sandstone that was later tilted. Location: Haßberge, Bavaria.
Structures in sedimentary rocks can be divided in primary structures (formed during deposition)and secondary structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field. Sedimentary structures can tell somethingabout the sedimentary environment or can serve to tell which side originally faced up wheretectonics have tilted or overturned sedimentary layers.Sedimentary rocks are laid down in layers called beds or strata. A bed is defined as a layer ofrock that has a uniform lithology and texture. Beds form by the deposition of layers of sedimenton top of each other. The sequence of beds that characterizes sedimentary rocks is calledbedding. Single beds can be a couple of centimetres to several meters thick. Finer, lesspronounced layers are called laminae and the structure it forms in a rock is called lamination.Laminae are usually less than a few centimetres thick. Though bedding and lamination areoften originally horizontal in nature, this is not always the case. In some environments, beds aredeposited at a (usually small) angle. Sometimes multiple sets of layers with different orientationsexist in the same rock, a structure called cross-bedding. Cross-bedding forms when small-scale erosion occurs during deposition, cutting off part of the beds. Newer beds then form at anangle to older ones.The opposite of cross-bedding is parallel lamination, where all sedimentary layering isparallel. With laminations, differences are generally caused by cyclic changes in the sedimentsupply, caused for example by seasonal changes in rainfall, temperature or biochemical activity.Laminae that represent seasonal changes (like tree rings) are called varves. Some rocks have nolamination at all, their structural character is called massive bedding.Graded bedding is a structure where beds with a smaller grain size occur on top of beds withlarger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clastsin suspension settle first, then smaller clasts. Though graded bedding can form in many differentenvironments, it is characteristic for turbidity currents.The bedform (the surface of a particular bed) can be indicative for a particular sedimentaryenvironment too. Examples of bed forms include dunes and ripple marks. Sole markings, such astool marks and flute casts, are groves dug into a sedimentary layer that are preserved. These areoften elongated structures and can be used to establish the direction of the flow duringdeposition.Ripple marks also form in flowing water. There are two types: asymmetric wave ripples andsymmetric current ripples. Environments where the current is in one direction, such as rivers,produce asymmetric ripples. The longer flank of such ripples is oriented opposite to the directionof the current. Wave ripples occur in environments where currents occur in all directions, suchas tidal flats.Another type of bed form are mud cracks, caused by the dehydration of sediment thatoccasionally comes above the water surface. Such structures are commonly found at tidal flats orpoint bars along rivers.Secondary sedimentary structures
Secondary sedimentary structures are structures in sedimentary rocks which formed afterdeposition. Such structures form by chemical, physical and biological processes inside thesediment. They can be indicators for circumstances after deposition. Some can be used as way upcriteria.Organic presence in a sediment can leave more traces than just fossils. Preserved tracks andburrows are examples of trace fossils (also called ichnofossils). Some trace fossils such aspaw prints of dinosaurs or early humans can capture human imagination, but such traces arerelatively rare. Most trace fossils are burrows of molluscs or arthropods. This burrowing is calledbioturbation by sedimentologists. It can be a valuable indicator of the biological and ecologicalenvironment after the sediment was deposited. On the other hand, the burrowing activity oforganisms can destroy other (primary) structures in the sediment, making a reconstruction moredifficult.Chert concretions in chalk, Middle Lefkara Formation (upper Paleocene to middle Eocene), Cyprus.Secondary structures can also have been formed by diagenesis or the formation of a soil(pedogenesis) when a sediment is exposed above the water level. An example of a diageneticstructure common in carbonate rocks is a stylolite. Stylolites are irregular planes were materialwas dissolved into the pore fluids in the rock. The result of precipitation of a certain chemicalspecies can be colouring and staining of the rock, or the formation of concretions. Concretionsare roughly concentric bodies with a different composition from the host rock. Their formationcan be the result of localized precipitation due to small differences in composition or porosity ofthe host rock, such as around fossils, inside burrows or around plant roots.  In carbonate rockssuch as limestone or chalk, chert or flint concretions are common, while terrestrial sandstonescan have iron concretions. Calcite concretions in clay are called septarian concretions.After deposition, physical processes can deform the sediment, forming a third class of secondarystructures. Density contrasts between different sedimentary layers, such as between sand andclay, can result in flame structures or load casts, formed by inverted diapirism. The diapirismcauses the denser upper layer to sink into the other layer. Sometimes, density contrast can resultor grow when one of the lithologies dehydrates. Clay can be easily compressed as a result ofdehydration, while sand retains the same volume and becomes relatively less dense. On the otherhand, when the pore fluid pressure in a sand layer surpasses a critical point the sand can flowthrough overlying clay layers, forming discordant bodies of sedimentary rock called sedimentarydykes (the same process can form mud volcanoes on the surface).
A sedimentary dyke can also be formed in a cold climate where the soil is permanently frozenduring a large part of the year. Frost weathering can form cracks in the soil that fill with rubblefrom above. Such structures can be used as climate indicators as well as way up structures. Density contrasts can also cause small-scale faulting, even while sedimentation goes on (syn-sedimentary faulting). Such faulting can also occur when large masses of non-lithifiedsediment are deposited on a slope, such as at the front side of a delta or the continental slope.Instabilities in such sediments can result in slumping. The resulting structures in the rock aresyn-sedimentary folds and faults, which can be difficult to distinguish from folds and faultsformed by tectonic forces in lithified rocks.Sedimentary environmentsThe setting in which a sedimentary rock forms is called the sedimentary environment. Everyenvironment has a characteristic combination of geologic processes and circumstances. The typeof sediment that is deposited is not only dependent on the sediment that is transported to a place,but also on the environment itself.A marine environment means the rock was formed in a sea or ocean. Often, a distinction is madebetween deep and shallow marine environments. Deep marine usually refers to environmentsmore than 200 m below the water surface. Shallow marine environments exist adjacent tocoastlines and can extend out to the boundaries of the continental shelf. The water in suchenvironments has a generally higher energy than that in deep environments, because of waveactivity. This means coarser sediment particles can be transported and the deposited sedimentcan be coarser than in deep environments. When the available sediment is transported from thecontinent, an alternation of sand, clay and silt is deposited. When the continent is far away, theamount of such sediment brought in may be small, and biochemical processes dominate the typeof rock that forms. Especially in warm climates, shallow marine environments far offshoremainly see deposition of carbonate rocks. The shallow, warm water is an ideal habitat for manysmall organisms that build carbonate skeletons. When these organisms die their skeletons sink tothe bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warmshallow marine environments also are ideal environments for coral reefs, where the sedimentconsists mainly of the calcareous skeletons of larger organisms. In deep marine environments, the water current over the sea bottom is small. Only fine particlescan be transported to such places. Typically sediments depositing on the ocean floor are fine clayor small skeletons of micro-organisms. At 4 km depth, the solubility of carbonates increasesdramatically (the depth zone where this happens is called the lysocline). Calcareous sedimentthat sinks below the lysocline dissolve, so no limestone can be formed below this depth.Skeletons of micro-organisms formed of silica (such as radiolarians) still deposit though. Anexample of a rock formed out of silica skeletons is radiolarite. When the bottom of the sea has asmall inclination, for example at the continental slopes, the sedimentary cover can becomeunstable, causing turbidity currents. Turbidity currents are sudden disturbances of the normallyquite deep marine environment and can cause the geologically speaking instantaneous depositionof large amounts of sediment, such as sand and silt. The rock sequence formed by a turbiditycurrent is called a turbidite.
The coast is an environment dominated by wave action. At the beach, dominantly coarsesediment like sand or gravel is deposited, often mingled with shell fragments. Tidal flats andshoals are places that sometimes dry out because of the tide. They are often cross-cut by gullies,where the current is strong and the grain size of the deposited sediment is larger. Where along acoast (either the coast of a sea or a lake) rivers enter the body of water, deltas can form. Theseare large accumulations of sediment transported from the continent to places in front of themouth of the river. Deltas are dominantly composed of clastic sediment.A sedimentary rock formed on the land has a continental sedimentary environment. Examples ofcontinental environments are lagoons, lakes, swamps, floodplains and alluvial fans. In the quietwater of swamps, lakes and lagoons, fine sediment is deposited, mingled with organic materialfrom dead plants and animals. In rivers, the energy of the water is much higher and thetransported material consists of clastic sediment. Besides transport by water, sediment can incontinental environments also be transported by wind or glaciers. Sediment transported by windis called aeolian and is always very well sorted, while sediment transported by a glacier is calledglacial and is characterized by very poor sorting. Sedimentary faciesSedimentary environments usually exist alongside each other in certain natural successions. Abeach, where sand and gravel is deposited, is usually bounded by a deeper marine environment alittle offshore, where finer sediments are deposited at the same time. Behind the beach, there canbe dunes (where the dominant deposition is well sorted sand) or a lagoon (where fine clay andorganic material is deposited). Every sedimentary environment has its own characteristicdeposits. The typical rock formed in a certain environment is called its sedimentary facies. Whensedimentary strata accumulate through time, the environment can shift, forming a change infacies in the subsurface at one location. On the other hand, when a rock layer with a certain ageis followed laterally, the lithology (the type of rock) and facies eventually change.Shifting sedimentary facies in the case of transgression (above) and regression of the sea (below).Facies can be distinguished in a number of ways: the most common ways are by the lithology(for example: limestone, siltstone or sandstone) or by fossil content. Coral for example only lives
in warm and shallow marine environments and fossils of coral are thus typical for shallowmarine facies. Facies determined by lithology are called lithofacies; facies determined by fossilsare biofacies.Sedimentary environments can shift their geographical positions through time. Coastlines canshift in the direction of the sea when the sea level drops, when the surface rises due to tectonicforces in the Earths crust or when a river forms a large delta. In the subsurface, such geographicshifts of sedimentary environments of the past are recorded in shifts in sedimentary facies. Thismeans that sedimentary facies can change either parallel or perpendicular to an imaginary layerof rock with a fixed age, a phenomenon described by Walthers facies rule.The situation in which coastlines move in the direction of the continent is called transgression. Inthe case of transgression, deeper marine facies are deposited over shallower facies, a successioncalled onlap. Regression is the situation in which a coastline moves in the direction of the sea.With regression, shallower facies are deposited on top of deeper facies, a situation calledofflap.The facies of all rocks of a certain age can be plotted on a map to give an overview of thepalaeogeography. A sequence of maps for different ages can give an insight in the developmentof the regional geography.Sedimentary basinsMain article: sedimentary basinPlaces where large-scale sedimentation takes place are called sedimentary basins. The amount ofsediment that can be deposited in a basin depends on the depth of the basin, the so calledaccommodation space. Depth, shape and size of a basin depend on tectonics, movements withinthe Earths lithosphere. Where the lithosphere moves upward (tectonic uplift), land eventuallyrises above sea level, so that and erosion removes material, and the area becomes a source fornew sediment. Where the lithosphere moves downward (tectonic subsidence), a basin forms andsedimentation can take place. When the lithosphere keeps subsiding, new accommodation spacekeeps being created.A type of basin formed by the moving apart of two pieces of a continent is called a rift basin.Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere isstretched and thinned, so that the hot asthenosphere rises and heats the overlying rift basin. Apartfrom continental sediments, rift basins normally also have part of their infill consisting ofvolcanic deposits. When the basin grows due to continued stretching of the lithosphere, the riftgrows and the sea can enter, forming marine deposits.When a piece of lithosphere that was heated and stretched cools again, its density rises, causingisostatic subsidence. If this subsidence continues long enough the basin is called a sag basin.Examples of sag basins are the regions along passive continental margins, but sag basins can alsobe found in the interior of continents. In sag basins, the extra weight of the newly deposited
sediments is enough to keep the subsidence going in a vicious circle. The total thickness of thesedimentary infill in a sag basins can thus exceed 10 km.A third type of basin exists along convergent plate boundaries - places where one tectonic platemoves under another into the asthenosphere. The subducting plate bends and forms a fore-arcbasin in front of the overriding plate—an elongated, deep asymmetric basin. Fore-arc basins arefilled with deep marine deposits and thick sequences of turbidites. Such infill is called flysch.When the convergent movement of the two plates results in continental collision, the basinbecomes shallower and develops into a foreland basin. At the same time, tectonic uplift forms amountain belt in the overriding plate, from which large amounts of material are eroded andtransported to the basin. Such erosional material of a growing mountain chain is called molasseand has either a shallow marine or a continental facies.At the same time, the growing weight of the mountain belt can cause isostatic subsidence in thearea of the overriding plate on the other side to the mountain belt. The basin type resulting fromthis subsidence is called a back-arc basin and is usually filled by shallow marine deposits andmolasse.Cyclic alternation of competent and less competent beds in the Blue Lias at Lyme Regis, southernEngland.Influence of astronomical cyclesIn many cases facies changes and other lithological features in sequences of sedimentary rockhave a cyclic nature. This cyclic nature was caused by cyclic changes in sediment supply and thesedimentary environment. Most of these cyclic changes are caused by astronomic cycles. Shortastronomic cycles can be the difference between the tides or the spring tide every two weeks. Ona larger time-scale, cyclic changes in climate and sea level are caused by Milankovitch cycles:cyclic changes in the orientation and/or position of the Earths rotational axis and orbit aroundthe Sun. There are a number of Milankovitch cycles known, lasting between 10,000 and 200,000years.Relatively small changes in the orientation of the Earths axis or length of the seasons can be amajor influence on the Earths climate. An example are the ice ages of the past 2.6 million years(the Quaternary period), which are assumed to have been caused by astronomic cycles. Climate change can influence the global sea level (and thus the amount of accommodation space
in sedimentary basins) and sediment supply from a certain region. Eventually, small changes inastronomic parameters can cause large changes in sedimentary environment and sedimentation.Sedimentation ratesThe rate at which sediment is deposited differs depending on the location. A channel in a tidalflat can see the deposition of a few metres of sediment in one day, while on the deep ocean flooreach year only a few millimetres of sediment accumulate. A distinction can be made betweennormal sedimentation and sedimentation caused by catastrophic processes. The latter categoryincludes all kinds of sudden exceptional processes like mass movements, rock slides or flooding.Catastrophic processes can see the sudden deposition of a large amount of sediment at once. Insome sedimentary environments, most of the total column of sedimentary rock was formed bycatastrophic processes, even though the environment is usually a quiet place. Other sedimentaryenvironments are dominated by normal, ongoing sedimentation.In some sedimentary environments, sedimentation only occurs in some places. In a desert, forexample, the wind deposits siliciclastic material (sand or silt) in some spots, or catastrophicflooding of a wadi may cause sudden deposis of large quantities of detrital material, but in mostplaces eolian erosion dominates. The amount of sedimentary rock that forms is not onlydependent on the amount of supplied material, but also on how well the material consolidates.Erosion removes most deposited sediment shortly after deposition. StratigraphyThe Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah thatmakes up much of the famous prominent rock formations in protected areas such as Capitol ReefNational Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the NavajoSandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone,slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layeredCutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.That new rock layers are above older rock layers is stated in the principle of superposition. Thereare usually some gaps in the sequence called unconformities. These represent periods where no
new sediments were laid down, or when earlier sedimentary layers raised above sea level anderoded away.Sedimentary rocks contain important information about the history of the Earth. They containfossils, the preserved remains of ancient plants and animals. Coal is considered a type ofsedimentary rock. The composition of sediments provides us with clues as to the original rock.Differences between successive layers indicate changes to the environment over time.Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, theyform at temperatures and pressures that do not destroy fossil remains.Conglomerate (geology) conglomerate — Sedimentary Rock — Boulder of conglomerate with cobble-sized clasts. Rock hammer for scale.Carmelo Formation (Conglomerate) at Point Lobos
Rock climbing hold on a Conglomerate rock in Margalef, Spain.A conglomerate (pronounced /kəŋˈɡlɒmərɨt/) is a rock consisting of individual clasts within afiner-grained matrix that have become cemented together. Conglomerates are sedimentary rocksconsisting of rounded fragments and are thus differentiated from breccias, which consist ofangular clasts. Both conglomerates and breccias are characterized by clasts larger than sand(>2 mm).ClassificationIn addition to the factors described in this section, conglomerates are classified in terms of boththeir rounding and sorting.TextureParaconglomerates consist of a matrix-supported rock that contains at least 15% sand-sized orsmaller grains (<2 mm), the rest being larger grains of varying sizes. Orthoconglomerates consist of a clast-supported rock with less than 15% matrix of sand andfiner particles.Metamorphic alteration transforms conglomerate into metaconglomerate.A conglomerate at the base of the Cambrian in the Black Hills, South Dakota.
Section of polymict conglomerate from offshore rock core, Alaska, approximate depth 10,000 ft.Clast compositionConglomerates are classified for the lithologies of the clasts Monomict - clasts with only a single lithology Oligomict - clasts of only a few different lithologies Polymict - clasts of many different lithologies Intraformational - clasts derived from the same formation in which they are found Extraformational - clasts derived older rocks than the formation in which they are foundClast sizeConglomerates are also classified by the dominant clast size. Granule conglomerate 2–4 mm Pebble conglomerate 4–64 mm Cobble conglomerate 64–256 mm Boulder conglomerate >256 mmSedimentary environmentsConglomerates are deposited in a variety of sedimentary environments.Deepwater marineIn turbidites, the basal part of a bed is typically coarse-grained and sometimes conglomeratic. Inthis setting, conglomerates are normally very well sorted, well-rounded and often with a strongA-axis type imbrication of the clasts.Shallow marine
Conglomerates are normally present at the base of sequences laid down during marinetrangressions above an unconformity, and are known as basal conglomerates. They represent theposition of the shoreline at a particular time and will be diachronous.FluvialConglomerates deposited in fluvial environments are typically well-rounded and well-sorted.Clasts of this size are carried as bedload and only at times of high flow-rate. The maximum clastsize decreases as the clasts are transported further due to attrition, so conglomerates are morecharacteristic of immature river systems. In the sediments deposited by mature rivers,conglomerates are generally confined to the basal part of a channel fill where they are known aspebble lags. Conglomerates deposited in a fluvial environment often have an AB-plane typeimbrication.AlluvialAlluvial deposits are formed in areas of high relief and are typically coarse-grained. At mountainfronts individual alluvial fans merge together to form braidplains and these two environments areassociated with the thickest deposits of conglomerates. The bulk of conglomerates deposited inthis setting are clast-supported with a strong AB-plane imbrication. Some matrix-supportedconglomerates are present, a result of debris-flow deposition on some alluvial fans.GlacialGlaciers carry a lot of coarse-grained material and many glacial deposits are conglomeratic.Tillites, the sediments deposited directly by a glacier, are typically poorly-sorted, matrix-supported conglomerates. The matrix is generally fine-grained, consisting of finely milled rockfragments. Waterlain deposits associated with glaciers are often conglomeratic, formingstructures such as eskers.ExamplesA spectacular example of conglomerate can be seen at Montserrat, near Barcelona. Here erosionhas created vertical channels giving the characteristic jagged shapes for which the mountain isnamed (Montserrat literally means "jagged mountain"). The rock is strong enough to be used as abuilding material - see Montserrat abbey front at full resolution for detail of the rock structure.Another spectacular example of conglomerate, the Crestone Conglomerate may be viewed in andnear the town of Crestone, at the foot of the Sangre de Cristo Range in Colorados San LuisValley. The Crestone Conglomerate is a metamorphic rock stratum and consists of tiny to quitelarge rocks that appear to have been tumbled in an ancient river. Some of the rocks have hues ofred and green.Conglomerate may also be seen in the domed hills of Kata Tjuta, in Australias NorthernTerritory.
In the nineteenth century a thick layer of Pottsville conglomerate was recognized to underlieanthracite coal measures in Pennsylvania.FanglomerateFanglomerateWhen a series of conglomerates accumulates into an alluvial fan, in rapidly eroding (e.g. desert)environments, the resulting rock unit is often called a fanglomerate. These form the basis of anumber of large oil fields, e.g. the Tiffany and Brae fields in the North Sea. These fanglomerateswere actually deposited into a deep marine environment but against a rapidly moving fault line,which supplied an intermittent stream of debris into the conglomerate pile. The sediment fans areseveral kilometers deep at the fault line and the sedimentation moved focus repeatedly, asdifferent sectors of the fault moved.Sandstone Sandstone — Sedimentary Rock — Prepared sample of sandstone CompositionTypically quartz and/or feldspar (on earth); lithic fragmentsare also common. Other minerals may be found inparticularly immature sandstone.
Alcove in the Navajo SandstoneThis article is about the geological rock type. For other uses, see Sandstone (disambiguation).Sandstone (sometimes known as arenite) is a sedimentary rock composed mainly of sand-sizedminerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these arethe most common minerals in the Earths crust. Like sand, sandstone may be any color, but themost common colors are tan, brown, yellow, red, gray and pink, white. Since sandstone bedsoften form highly visible cliffs and other topographic features, certain colors of sandstone havebeen strongly identified with certain regions.Some sandstones are resistant to weathering, yet are easy to work. This makes sandstone acommon building and paving material. However, some that have been used in the past, such asthe Collyhurst sandstone used in North West England, have been found less resistant,necessitating repair and replacement in older buildings.  Because of the hardness of theindividual grains, uniformity of grain size and friability of their structure, some types ofsandstone are excellent materials from which to make grindstones, for sharpening blades andother implements. Non-friable sandstone can be used to make grindstones for grinding grain,e.g., gritstone.Rock formations that are primarily composed of sandstone usually allow percolation of waterand other fluids and are porous enough to store large quantities, making them valuable aquifersand petroleum reservoirs. Fine-grained aquifers, such as sandstones, are more apt to filter outpollutants from the surface than are rocks with cracks and crevices, such as limestone or otherrocks fractured by seismic activity.Origins
Sand from Coral Pink Sand Dunes State Park, Utah. These are grains of quartz with a hematitecoating providing the orange color. Scale bar is 1.0 mm.Millet-Seed sandstone macro (size: ~4 cm or ~1.6 in).Sandstones are clastic in origin (as opposed to either organic, like chalk and coal, or chemical,like gypsum and jasper). They are formed from cemented grains that may either be fragmentsof a pre-existing rock or be mono-minerallic crystals. The cements binding these grains togetherare typically calcite, clays and silica. Grain sizes in sands are defined (in geology) within therange of 0.0625 mm to 2 mm (0.002-0.079 inches). Clays and sediments with smaller grain sizesnot visible with the naked eye, including siltstones and shales, are typically called argillaceoussediments; rocks with larger grain sizes, including breccias and conglomerates are termedrudaceous sediments.Red sandstone interior of Lower Antelope Canyon, Arizona, worn smooth by erosion from flashflooding over thousands of years.
The formation of sandstone involves two principal stages. First, a layer or layers of sandaccumulates as the result of sedimentation, either from water (as in a river, lake, or sea) or fromair (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension; i.e.,ceasing to be rolled or bounced along the bottom of a body of water (e.g., seas or rivers) orground surface (e.g., in a desert or erg). Finally, once it has accumulated, the sand becomessandstone when it is compacted by pressure of overlying deposits and cemented by theprecipitation of minerals within the pore spaces between sand grains.The most common cementing materials are silica and calcium carbonate, which are often derivedeither from dissolution or from alteration of the sand after it was buried. Colors will usually betan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand).A predominant additional colorant in the southwestern United States is iron oxide, which impartsreddish tints ranging from pink to dark red (terracotta), with additional manganese imparting apurplish hue. Red sandstones are also seen in the Southwest and West of England and Wales, aswell as central Europe and Mongolia. The regularity of the latter favors use as a source formasonry, either as a primary building material or as a facing stone, over other construction.The environment where it is deposited is crucial in determining the characteristics of theresulting sandstone, which, in finer detail, include its grain size, sorting and composition and, inmore general detail, include the rock geometry and sedimentary structures. Principalenvironments of deposition may be split between terrestrial and marine, as illustrated by thefollowing broad groupings: Terrestrial environmentsSandstone near Stadtroda, Germany. 1. Rivers (levees, point bars, channel sands) 2. Alluvial fans 3. Glacial outwash 4. Lakes 5. Deserts (sand dunes and ergs) Marine environments
1. Deltas 2. Beach and shoreface sands 3. Tidal flats 4. Offshore bars and sand waves 5. Storm deposits (tempestites) 6. Turbidites (submarine channels and fans)TypesSandstone composed mainly of quartz grainsPhotomicrograph of a volcanic sand grain; upper picture is plane-polarized light, bottom pictureis cross-polarized light, scale box at left-center is 0.25 millimeter. This type of grain would be amain component of a lithic sandstone.Arkose
Arkosic sand in the Llano Uplift, Texas, USA with granite outcrops.Arkose (pronounced /ˈɑrkoʊz/) is a detrital sedimentary rock, specifically a type of sandstonecontaining at least 25% feldspar., Arkosic sand is sand that is similarly rich in feldspar, andthus the potential precursor of arkose. The other mineral components may vary, but quartz iscommonly dominant, and some mica is often present. Apart from the mineral content, rockfragments may also be a significant component. Arkose usually contains small amounts of calcitecement, which causes it to fizz slightly in dilute hydrochloric acid; sometimes the cement alsocontains iron oxide. Arkose is typically grey to reddish in colour. The sand grains making up anarkose may range from fine to very coarse, but tends toward the coarser end of the scale. Fossilsare rare in arkose, due to the depositional processes that form it, although bedding is frequentlyvisible.Arkose sandstone found in SlovakiaArkose is generally formed from the weathering of feldspar-rich igneous or metamorphic, mostcommonly granitic rocks, which are primarily composed of quartz and feldspar. These sedimentsmust be deposited rapidly and/or in a cold or arid environment such that the feldspar does notundergo significant chemical weathering and decomposition; therefore arkose is designated atexturally immature sedimentary rock. Arkose is often associated with conglomerate depositssourced from granitic terrain and is often found above unconformities over such granitic terrain.The famous central Australian monolith Uluru (Ayers Rock) is composed of lateNeoproterozoic/Cambrian arkose, deposited in the Amadeus Basin.Sandstones fall into several major groups based on their mineralogy and texture. Below is apartial list of common sandstone types.
quartz arenites are made up almost entirely of quartz grains, usually well sorted and rounded. These pure quartz sands result from extensive weathering that occurred before and during transport and removed everything but quartz, the most stable mineral. They are common in beach environments. arkoses are more than 25 percent feldspar. The grains tend to be poorly rounded and less well sorted than those of pure quartz sandstones. These feldspar-rich sandstones come from rapidly eroding granitic and metamorphic terrains where chemical weathering is subordinate to physical weathering. lithic sandstones contain many lithic fragments derived from fine-grained rocks, mostly shales, volcanic rocks, and fine-grained metamorphic rocks. graywacke is a heterogeneous mixture of lithic fragments and angular grains of quartz and feldspar, and/or grains surrounded by a fine-grained clay matrix. Much of this matrix is formed by relatively soft fragments, such as shale and some volcanic rocks, that are chemically altered and physically compacted after deep burial of the sandstone formation. Eolianite is a term used for a rock which is composed of sand grains that show signs of significant transportation by wind. These have usually been deposited in desert environments. They are commonly extremely well sorted and rich in quartz. Oolite is more a limestone than a sandstone, but is made of sand-sized carbonate ooids, and is common in saline beaches with gentle wave action.Sandstone composition is (generally) based on the make up of the framework, or sand-sizedgrains in the sandstone. This is typically done by point-counting a thin section of the sandstoneusing a method like the Gazzi-Dickinson Method. The composition of a sandstone can haveimportant information regarding the genesis of the sediment when used with QFL diagrams.Shale Shale — Sedimentary Rock — Shale Composition
Clay minerals and quartzShale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clayminerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.The ratio of clay to other minerals is variable.  Shale is characterized by breaks along thinlaminae or parallel layering or bedding less than one centimeter in thickness, called fissility.Mudstones, on the other hand, are similar in composition but do not show the fissility.Historical mining terminologyBefore the mid 19th century, the terms slate, shale and schist were not sharply distinguished. Inthe context of underground coal mining, shale was frequently referred to as slate well into the20th century.TextureShale typically exhibits varying degrees of fissility breaking into thin layers, often splintery andusually parallel to the otherwise indistinguishable bedding plane because of parallel orientationof clay mineral flakes. Non-fissile rocks of similar composition but made of particles smallerthan 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystone (less than 1/3silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and thereforegrittier are siltstones. Shale is the most common sedimentary rock.Sample of drill cuttings of shale while drilling an oil well in Louisiana. Sand grain = 2 mm. in dia.Composition and colorShales are typically composed of variable amounts of clay minerals and quartz grains and thetypical color is gray. Addition of variable amounts of minor constituents alters the color of therock. Black shale results from the presence of greater than one percent carbonaceous materialand indicates a reducing environment. Black shale can also be referred to as black metal. Red, brown and green colors are indicative of ferric oxide (hematite - reds), iron hydroxide
(goethite - browns and limonite - yellow), or micaceous minerals (chlorite, biotite and illite -greens).Clays are the major constituent of shales and other mudrocks. The clay minerals represented arelargely kaolinite, montmorillonite and illite. Clay minerals of Late Tertiary mudstones areexpandable smectites whereas in older rocks especially in mid to early Paleozoic shales illitespredominate. The transformation of smectite to illite produces silica, sodium, calcium,magnesium, iron and water. These released elements form authigenic quartz, chert, calcite,dolomite, ankerite, hematite and albite, all trace to minor (except quartz) minerals found inshales and other mudrocks.Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks.However, this amounts to less than one percent by mass in an average shale. Black shales whichform in anoxic conditions contain reduced free carbon along with ferrous iron (Fe 2+) and sulfur(S2-). Pyrite and amorphous iron sulfide along with carbon produce the black coloration.FormationLimey shale overlaid by limestone, Cumberland Plateau, TennesseeThe process in the rock cycle which forms shale is compaction. The fine particles that composeshale can remain suspended in water long after the larger and denser particles of sand havedeposited. Shales are typically deposited in very slow moving water and are often found in lakesand lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They canalso be deposited on the continental shelf, in relatively deep, quiet water.Black shales are dark, as a result of being especially rich in unoxidized carbon. Common insome Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducingenvironments, such as in stagnant water columns. Some black shales contain abundant heavymetals such as molybdenum, uranium, vanadium, and zinc.  The enriched values are ofcontroversial origin, having been alternatively attributed to input from hydrothermal fluidsduring or after sedimentation or to slow accumulation from sea water over long periods ofsedimentation.
Splitting the shale with a large knife to reveal fossils.Fossils, animal tracks/burrows and even raindrop impact craters are sometimes preserved onshale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite, orvarious carbonate minerals.Shales that are subject to heat and pressure of metamorphism alter into a hard, fissile,metamorphic rock known as slate. With continued increase in metamorphic grade the sequence isphyllite, then schist and finally to gneiss.Weathering shale at a road cut in southeastern KentuckyWhen shale is hit against other rock it can emit sparks of various colors like blue, green,purple,yellow, and white. dependent on the type of rock it is hit against.Limestone Limestone — Sedimentary Rock —
Limestone in Waitomo District, New Zealand CompositionCalcium carbonate: inorganic crystalline calcite and/ororganic calcareous material.Limestone is a sedimentary rock composed largely of the minerals calcite and/or aragonite,which are different crystal forms of calcium carbonate (CaCO3).Like most other sedimentary rocks, limestones are composed of grains; however, most grains inlimestone are skeletal fragments of marine organisms such as coral or foraminifera. Othercarbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts. Somelimestones do not consist of grains at all and are formed completely by the chemical precipitationof calcite or aragonite. i.e. travertine.The solubility of limestone in water and weak acid solutions leads to karst landscapes. Regionsoverlying limestone bedrock tend to have fewer visible groundwater sources (ponds andstreams), as surface water easily drains downward through joints in the limestone. Whiledraining, water and organic acid from the soil slowly (over thousands or millions of years)enlarges these cracks; dissolving the calcium-carbonate and carrying it away in solution. Mostcave systems are through limestone bedrock.Description
Limestone quarry at Cedar Creek, Virginia, USA.La Zaplaz formations in Piatra Craiului MountainsLimestone often contains variable amounts of silica in the form of chert (aka chalcedony, flint,jasper, etc.) or siliceous skeletal fragment (sponge spicules, diatoms, radiolarians), as well asvarying amounts of clay, silt and sand sized terrestrial detritus carried in by rivers. The primarysource of the calcite in limestone is most commonly marine organisms. These organisms secreteshells made of aragonite or calcite and leave these shells behind after the organism dies. Some ofthese organisms can construct mounds of rock known as reefs, building upon past generations.Below about 3,000 meters, water pressure and temperature causes the dissolution of calcite toincrease non-linearly so that limestone typically does not form in deeper waters (see lysocline).Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater thatprecipitates the material in caves). This produces speleothems such as stalagmites and stalactites.Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by itsgranular appearance.
Limestone makes up about 10% of the total volume of all sedimentary rocks.  Limestonesmay also form in both lacustrine and evaporite depositional environments.Calcite can be either dissolved by groundwater or precipitated by groundwater, depending onseveral factors including the water temperature, pH, and dissolved ion concentrations. Calciteexhibits an unusual characteristic called retrograde solubility in which it becomes less soluble inwater as the temperature increases.When conditions are right for precipitation, calcite forms mineral coatings that cement theexisting rock grains together or it can fill fractures.Karst topography and caves develop in carbonate rocks due to their solubility in dilute acidicgroundwater. Cooling groundwater or mixing of different groundwaters will also createconditions suitable for cave formation.Coastal limestones are often eroded by organisms which bore into the rock by various means.This process is known as bioerosion. It is most common in the tropics, and it is knownthroughout the fossil record (see Taylor and Wilson, 2003).Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, manylimestones exhibit different colors, especially on weathered surfaces. Limestone may becrystalline, clastic, granular, or massive, depending on the method of formation. Crystals ofcalcite, quartz, dolomite or barite may line small cavities in the rock. Folk and Dunhamclassifications are used to describe limestones more precisely.Travertine is a banded, compact variety of limestone formed along streams, particularly wherethere are waterfalls and around hot or cold springs. Calcium carbonate is deposited whereevaporation of the water leaves a solution that is supersaturated with chemical constituents ofcalcite. Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is apoorly consolidated limestone composed of pieces of coral or shells.During regional metamorphism that occurs during the mountain building process (orogeny)limestone recrystallizes into marble.Limestone is a parent material of Mollisol soil group.TypesMain article: List of types of limestoneLimestone landscapeMain article: Karst topography
The Cudgel of Hercules, a tall limestone rock and Pieskowa Skała Castle in the background.Limestone is partially soluble, especially in acid, and therefore forms many erosional landforms.These include limestone pavements, pot holes, cenotes, caves and gorges. Such erosionlandscapes are known as karsts. Limestone is less resistant than most igneous rocks, but moreresistant than most other sedimentary rocks. Limestone is therefore usually associated with hillsand downland and occurs in regions with other sedimentary rocks, typically clays.Bands of limestone emerge from the Earths surface in often spectacular rocky outcrops andislands. Examples include the Burren in Co. Clare, Ireland; the Verdon Gorge in France; MalhamCove in North Yorkshire and the Isle of Wight, England; on Fårö near the Swedish island ofGotland, the Niagara Escarpment in Canada/United States, Notch Peak in Utah, the Ha Long BayNational Park in Vietnam and the hills around the Lijiang River and Guilin city in China.The Florida Keys, islands off the south coast of Florida, are composed mainly of ooliticlimestone (the Lower Keys) and the carbonate skeletons of coral reefs (the Upper Keys), whichthrived in the area during interglacial periods when sea level was higher than at present.Unique habitats are found on alvars, extremely level expanses of limestone with thin soilmantles. The largest such expanse in Europe is the Stora Alvaret on the island of Öland, Sweden.Another area with large quantities of limestone is the island of Gotland, Sweden. Huge quarriesin northwestern Europe, such as those of Mount Saint Peter (Belgium/Netherlands), extend formore than a hundred kilometers.The worlds largest limestone quarry is at Michigan Limestone and Chemical Company inRogers City, Michigan.
The Great Pyramid of Giza. One of the Seven Wonders of the Ancient World, the structure is madeentirely from limestone.Courthouse built of limestone in Manhattan, Kansas
Gallery A stratigraphic section of Thin-section view of a Ordovician limestone Middle Jurassic limestoneLimestone Photo and Etched exposed in central in southern Utah. Thecropping out at section of a sample of Tennessee, U.S. The less- round grains are ooids; theSão Pedro de fossiliferous limestone resistant and thinner beds largest is 1.2 mm inMoel beach, from the Kope are composed of shale. diameter. This limestone isMarinha Grande, Formation near Vertical lines are drill holes an oosparite.Portugal. Cincinnati, Oh for explosives used during road construction.Karst topographyThe Kravice waterfall on the Trebižat river in Bosnia and Herzegovina has karst geology.A karst landscape in Minerve, Hérault, France.
The karst hills of The Burren on the west coast of IrelandKarst topography is a landscape shaped by the dissolution of a layer or layers of solublebedrock, usually carbonate rock such as limestone or dolomite.Due to subterranean drainage, there may be very limited surface water, even to the absence of allrivers and lakes. Many karst regions display distinctive surface features, with sinkholes ordolines being the most common. However, distinctive karst surface features may be completelyabsent where the soluble rock is mantled, such as by glacial debris, or confined by asuperimposed non-soluble rock strata. Some karst regions include thousands of caves, eventhough evidence of caves that are big enough for human exploration is not a requiredcharacteristic of karst.Various karst landforms have been found on all continents except Antarctica (see below: Notablekarst areas).BackgroundKarst topography is characterized by subterranean limestone caverns, carved by groundwater.The geographer Jovan Cvijić (1865–1927) was born in western Serbia and studied widely in theDinaric Kras region. His publication of Das Karstphänomen (1893) established that rockdissolution was the key process and that it created most types of dolines, "the diagnostic karstlandforms". The Dinaric Kras thus became the type area for dissolutional landforms andaquifers; the regional name kras, Germanicised as "karst", is now applied to modern and paleo-dissolutional phenomena worldwide. Cvijić related the complex behaviour of karstic aquifers todevelopment of solutional conduit networks and linked it to a cycle of landform evolution. AfterCvijić, two main kinds of karstic areas exist: holokarst i.e. karst developed at whole as it isDinaric region along eastern Adriatic coast comprises deep in the inland of Balkan Peninsula andmerokarst developed imperfectly with some karstic forms as it is in eastern Serbia. He isrecognized as "the father of karst geomorphology".The international community has settled on karst, the German name for Kras, a region inSlovenia partially extending into Italy, where it is called "Carso" and where the first scientificresearch of a karst topography was made. The name has an Indo-European origin (from karrameaning "stone"), and in antiquity it was called "Carusardius" in Latin. The Slovene form
grast is attested since 1177, and the Croatian kras since 1230.. "Krš" - "Krsh"meaning in Serbo-Croatian "barren land" which is typical feature in the Northern Dinariclimestone mountains could also be a origin to the word Karst.ChemistryKarst lake (Doberdò del Lago, Italy), from underground water springing into a depression. This lake hasno surface inlet or outlet.Karst landforms are generally the result of mildly acidic water acting on soluble bedrock such aslimestone or dolostone. The carbonic acid that causes these features is formed as rain passesthrough the atmosphere picking up CO2, which dissolves in the water. Once the rain reaches theground, it may pass through soil that may provide further CO2 to form a weak carbonic acidsolution: H2O + CO2 → H2CO3 (the acid).Recent studies of sulfates, in karst waters, suggests sulfuric acid and hydrosulfuric acid may alsoplay an important role in karst formation. One such study, in the Frasassi Caves of Italy, showedthe oxidation of sulfuric acid is one of the leading corrosion factors in karst formation. As waterseeps into karst caves it brings in oxygen which reacts with H 2S to form sulfuric acid (H2SO4)and a hydronium (H3O). The H3O reacts with the limestone causing increased erosion within theformation. 2O2 + H2S → H2SO4 H2SO4 + 2H2O → 2H3O + SO22- H2O + CaCO3 → Ca2+ + HCO- + H2OAs a result of this reaction the mineral gypsum forms as a replacement mineral since it providesmany similar structures to the dissolution and redeposition of calcium carbonate.This mildly acidic water begins to dissolve the surface along fractures or bedding planes in thelimestone bedrock. Over time, these fractures enlarge as the bedrock continues to dissolve.Openings in the rock increase in size, and an underground drainage system begins to develop,
allowing more water to pass through the area, and accelerating the formation of undergroundkarst features.MorphologyLimestone pavement in Dent de Crolles, FranceThe karstification of a landscape may result in a variety of large or small scale features both onthe surface and beneath. On exposed surfaces, small features may include flutes, runnels, clintsand grikes, collectively called karren or lapiez. Medium-sized surface features may includesinkholes or cenotes (closed basins), vertical shafts, foibe (inverted funnel shaped sinkholes),disappearing streams, and reappearing springs. Large-scale features may include limestonepavements, poljes and blind valleys. Mature karst landscapes, where more bedrock has beenremoved than remains, may result in karst towers, or haystack/eggbox landscapes. Beneath thesurface, complex underground drainage systems (such as karst aquifers) and extensive caves andcavern systems may form.
The Witchs Finger stalagmite in Carlsbad Caverns, USAErosion along limestone shores, notably in the tropics, produces karst topography that includes asharp makatea surface above the normal reach of the sea and undercuts that are mostly the resultof biological activity or bioerosion at or a little above mean sea level. Some of the most dramaticof these formations can be seen in Thailands Phangnga Bay and Halong Bay in Vietnam.Calcium carbonate dissolved into water may precipitate out where the water discharges some ofits dissolved carbon dioxide. Rivers which emerge from springs may produce tufa terraces,consisting of layers of calcite deposited over extended periods of time. In caves, a variety offeatures collectively called speleothems are formed by deposition of calcium carbonate and otherdissolved minerals.HydrologyA karst spring in the Jura mountains near Ouhans in eastern France at the source of the river LoueFarming in karst areas must take into account the lack of surface water. The soils may be fertileenough, and rainfall may be adequate, but rainwater quickly moves through the crevices into theground, sometimes leaving the surface soil parched between rains.A karst fenster is where an underground stream emerges onto the surface between layers of rock,cascades some feet, and then disappears back down, often into a sinkhole. Rivers in karst areasmay disappear underground a number of times and spring up again in different places, usuallyunder a different name (like Ljubljanica, the river of seven names). An example of this is thePopo Agie River in Fremont County, Wyoming. At a site simply named "The Sinks" in SinksCanyon State Park, the river flows into a cave in a formation known as the Madison Limestone,and then rises again a half-mile down the canyon in a placid pool. A Turlach is a unique type ofseasonal lake found in Irish karst areas which are formed through the annual welling-up of waterfrom the underground water system.
Water supplies from wells in karst topography may be unsafe, as the water may have rununimpeded from a sinkhole in a cattle pasture, through a cave and to the well, bypassing thenormal filtering that occurs in a porous aquifer. Karst formations are cavernous and thereforehave high rates of permeability, resulting in reduced opportunity for contaminants to be filteredout.Groundwater in karst areas is just as easily polluted as surface streams. Sinkholes have oftenbeen used as farmstead or community trash dumps. Overloaded or malfunctioning septic tanks inkarst landscapes may dump raw sewage directly into underground channels.The karst topography itself also poses difficulties for human inhabitants. Sinkholes can developgradually as surface openings enlarge, but quite often progressive erosion is unseen and the roofof an underground cavern suddenly collapses. Such events have swallowed homes, cattle, cars,and farm machinery.The Driftless Area National Wildlife Refuge in Iowa protects Discus macclintocki, a species ofice age snail surviving in air chilled by flowing over buried karst ice formations.PseudokarstPseudokarsts are similar in form or appearance to karst features, but are created by differentmechanisms. Examples include lava caves and granite tors—for example, Labertouche Cave inVictoria, Australia and paleocollapse features.Notable karst areasAfrica PolandMadagascar Kraków-Częstochowa Upland (Jura Krakowsko-Częstochowska) Anjajavy Forest, western Madagascar Holy Cross Mountains (Góry Ankarana Reserve, Madagascar Świętokrzyskie) with the Jaskinia Raj (Raj Madagascar dry deciduous forests, western Cave) Madagascar Tatra Mountains including the Jaskinia Tsingy de Bemaraha Strict Nature Reserve, Wielka Śnieżna (Great Snowy Cave)— the Madagascar longest cave in Poland RomaniaAsia Apuseni Mountains, Romania Bucegi Mountains, Romania Serbia Dinaric Alps region
merokarst of eastern Serbia Scotland Assynt, southeast Skye and near Kentallen in Scotland, United Kingdom Slovakia Slovak Paradise, Slovak Karst and Muránska planina, Slovakia Slovenia Region of Inner Carniola, Goriška, Upper Carniola and Lower Carniola Kras (German: Karst), a plateau in southwestern Slovenia and northeastern ItalyPhong Nha Cave in Phong Nha-Ke Bang, Vietnam SpainChina Area around Guilin and Yangshuo Jiuzhaigou Valley and Huanglong National Park, (UNESCO World Heritage Site) South China Karst, World Heritage Site Stone Forest Zhangjiajie National Forest park, forming part of the Wulingyuan scenic area, World Heritage Site El Torcal (Antequera - Spain)Dunnieh mountains, North Lebanon
Chalk Chalk — Sedimentary Rock — The Needles, situated on the Isle of Wight, are part of the extensive Southern England Chalk Formation. Compositioncalcite (calcium carbonate)Chalk (pronounced /ˈtʃɔːk/) is a soft, white, porous sedimentary rock, a form of limestonecomposed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under relativelydeep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shedfrom micro-organisms called coccolithophores. It is common to find chert or flint nodulesembedded in chalk. Chalk can also refer to other compounds including magnesium silicate andcalcium sulfate.Chalk is resistant to weathering and slumping compared to the clays with which it is usuallyassociated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known aschalk downland, usually form where bands of chalk reach the surface at an angle, so forming ascarp slope. Because chalk is porous it can hold a large volume of ground water, providing anatural reservoir that releases water slowly through dry seasons.Deposits
The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. Itforms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of theCap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostlyunderlain by chalk deposits, which contain artificial caves used for wine storage. Some of thehighest chalk cliffs in the world occur at Møns Klint in Denmark.FormationNinety million years ago the chalk downland of Northern Europe was ooze at the bottom of agreat sea. Protozoans such as foraminifera lived on the marine debris that showered down fromthe upper layers of the ocean. Their bodies were made of chalk extracted from the rich sea-water.As they died a deep layer built up and slowly became consolidated into rock. At a later date thesea-bed became dry land, as earth movements thrust it upward.CompositionChalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It isnormally formed underwater, commonly on the sea bed, then consolidated and compressedduring diagenesis into the form commonly seen today. During diagenesis silica accumulates toform chert or flint nodules within the carbonate rock.CoquinaCoquina outcropping on the beach at Washington Oaks State Gardens, FloridaCoquina (Spanish, "cockle"; pronounced /koʊˈkiːnə/) is an incompletely consolidatedsedimentary rock. Coquina was formed in association with marine reefs and is a variety of "coralrag", technically a subset of limestone.Composition and distribution
Coquina is mainly composed of mineral calcite, often including some phosphate, in the form ofseashells or coral. It is found in surface exposures along the east coast of Florida from St. JohnsCounty to Palm Beach County. It may occur up to 20 miles inland from the coast in the sub-surface. It is found as far north as Fort Fisher, North Carolina. It has also been formed in theSouth Island of New Zealand, where it outcrops in a disused quarry near Oamaru. The Oligocenedeposits here are composed primarily of very well preserved brachiopod shells, in a matrix ofbrachiopod, echinoid, and bryozoan detritus and foraminifera.History and useCoquina from Florida.Close-up of coquina from Florida. The scale bar is 10 mm.Occasionally quarried or mined and used as a building stone in Florida for over 400 years,coquina forms the walls of the Castillo de San Marcos, Saint Augustine. The stone makes a verygood material for forts, particularly those built during the period of heavy cannon use. Becauseof coquinas softness, cannon balls would sink into, rather than shatter or puncture, the walls ofthe Castillo de San Marcos.When first quarried, coquina is extremely soft. This softness makes it very easy to remove fromthe quarry and cut into shape. However, the stone is also at first much too soft to be used for
building. In order to be used as a building material, the stone is left out to dry for approximatelyone to three years, which causes the stone to harden into a usable, but still comparatively soft,form.Coquina has also been used as a source of paving material. It is usually poorly cemented andeasily breaks into component shell or coral fragments, which can be substituted for gravel orcrushed harder rocks. Large pieces of coquina of unusual shape are sometimes used as landscapedecoration.Because coquina often includes a component of phosphate, it is sometimes mined for use asfertilizer.Notable exposures of coquina Washington Oaks State Gardens, Flagler County, Florida House of Refuge, Hutchinson Island, Martin County, Florida Blowing Rocks Preserve (and along Country Club Road), Palm Beach County, Florida North Carolina Aquarium at Fort Fisher, features a "Coquina Outcrop Touch Pool"Lithographic LimestoneA lithographic limestone printing plate after use to print a map. Note the uniform fine texture of thestone.Lithographic limestone is hard limestone that is sufficiently fine-grained, homogeneous anddefect free to be used for lithography. Geologists use the term lithographic texture to refer to agrain size under 1/250 mm. The term sublithographic is sometimes used for homogeneousfine-grained limestone with a somewhat coarser texture. OriginThe generally accepted theory for the origin of lithographic and sublithographic limestones isthat they were formed in shallow stagnant hypersaline and anoxic lagoons. The combination of
mild hypersalinity and low oxygen content is believed to have inhibited the formation ofmicrobial mats and prevented the invasion of bottom dwelling organisms. Microbial mats andbottom dwelling organisms would have left fossils, and bottom dwelling organisms would havechurned the accumulating sediment, producing a less homogeneous rock. Stagnancy wasrequired to avoid churning or sculpting of the sediment by currents or wave action. DistributionEuropeThe original source for lithographic limestone was the Solnhofen Limestone named after thequarries of Solnhofen where it was first found. This is a late Jurassic deposit, part of a deposit ofplattenkalk (a very fine-grained limestone that splits into thin plates, usually Micrite) thatextends through the Swabian Alb and Franconian Alb in Southern Germany. Only a smallfraction of plattenkalk is suitable for lithography. For many years, the Solnhofen deposits were the only source of lithographic limestone. Frenchlithographic limestone from quarries near Montdardier, about 6 km south of le Vigan, Gard wasexhibited at the Great Exhibition of 1851, where it earned an honorable mention. This stone isfrom the upper Lias Group, from the early Jurassic. The largest lithographic printing stoneever quarried came from Le Vigan, 230x150cm (90x59 in).  Théophile Steinlen used acomparable stone for some of his posters.  Several quarries are visible today on the chalkyplateau above Montdardier, between 2 km north ( 43°56′54.46″N 3°35′3.44″E / 43.9484611°N3.5842889°E), and 2 km west ( 43°55′52.61″N 3°33′38.78″E / 43.9312806°N 3.5607722°E) ofthe town.Shortly before 1867, a second lithographic limestone quarry was opened in France near Cerinand Crey, Isère ( 45°46′45.77″N 5°33′14.06″E / 45.7793806°N 5.5539056°E). Thelithographic limestones of Cerin are from the Kimmeridgian stage of the Upper Jurassic, and aswith the Solnhofen deposits, they preserve numerous interesting fossils.Lithographic limestone from the Lower Cretaceous has been quarried near Santa Maria de Meiàon the south flank of the Serra del Montsec in Spain. In 1902, L. M. Vidal, a mining engineer,recognized the importance of the fossils found there. The AmericasThe American Lithographic Stone Company was organized in Louisville, Kentucky in late 1868.It initially focused its operation on quarries in Overton County, Tennessee, but shortly before1900, it opened a quarry at Brandenburg, Kentucky. This quarry was the only commercial sourceof lithographic stone in the United States at the turn of the 20 th century. Unlike the Solnhofenstone, Kentucky lithographic limestone was slightly dolomitic, and it was judged to becompetitive with Solnhofen stone for some purposes, but not for the highest quality work. This stone source was sub-Carboniferous (Mississipian). In 1917, the Brandenburg quarry wasjudged the most important source of Lithographic stone in the United States.  Prior to 1916, the
output of the Brandenburg quarry was small, but in 1916, as World War I cut off access toSolnhofen stone, the quarry produced 20 tons of finished lithographic stone.  The Remains ofthe Brandenburg Lithograph Quarry are located along the Buttermilk Falls Historic WalkingTrail ( 38°0′3.54″N 86°9′34.74″W / 38.0009833°N 86.15965°W).In 1903, Clement L. Webster discovered a bed of lithographic limestone about 2 miles southwestof Orchard, Iowa. His company, the Interstate Investment & Development Company platted atown named Lithograph City nearby and opened a quarry ( 43°11′38.2″N 92°48′59.52″W /43.193944°N 92.8165333°W). The Lithograph City Formation of the Cedar Valley Groupstraddles the border between the Middle and Late Devonian and was named for its exposure inthis quarry. Outcrops of this formation extend from near Cedar Falls, Iowa north intoMinnesota. The suitability of Lithograph City limestone for lithography was tested by A. B.Hoen who reported that stone from two layers in the Lithograph City quarry was excellent forlithography and finer grained than the finest Solnhofen stone.  Lithograph City was animportant source of lithographic stone in the United States during World War I, but the quarriesclosed as metal printing plates replaced stone. In 1918, the Devonian Products Company tookover the operation, focusing on the production of crushed rock and renaming the townDevonia. By 1938, the town had disappeared.OoliteModern ooids from a beach on Joulters Cay, The Bahamas
Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern UtahThin-section of calcitic ooids from an oolite within the Carmel Formation (Middle Jurassic) of southernUtahOolite (egg stone) is a sedimentary rock formed from ooids, spherical grains composed ofconcentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolitesconsist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are calledpisolites. The term oolith can refer to oolite or individual ooids.CompositionOoids are most commonly composed of calcium carbonate (calcite or aragonite), but can becomposed of phosphate, chert, dolomite or iron minerals, including hematite. Dolomitic andchert ooids are most likely the result of the replacement of the original texture in limestone.Oolitic hematite occurs at Red Mountain near Birmingham, Alabama, along with ooliticlimestone.Oolites are often used in the home aquarium industry because their small grain size (0.2 to 1.22mm) is ideal for shallow static beds and bottom covering of up to 1" in depth. Also known as"oolitic" sand, the sugar-sized round grains of this sand pass easily through the gills of gobiesand other sand-sifting organisms. Importantly, this incredibly smooth sand promotes the growthof bacteria, which are important biofilters in home aquaria. Because of its extremely small grainsize, oolitic sand has a lot of surface area, which promotes high bacterial growth.OccurrenceSome exemplar oolitic limestone, a common term for an oolite, was formed in England duringthe Jurassic period, and forms the Cotswold Hills on the Isle of Portland with its famous PortlandStone, and part of the North Yorkshire Moors. A particular type, Bath Stone, gives thebuildings of the World Heritage City of Bath their distinctive appearance.The islands of the Lower Keys in the Florida Keys, as well as some barrier islands east of Miamibordering Biscayne Bay, are mainly oolitic limestone, which was formed by deposition when
shallow seas covered the area between periods of glaciation. The material consolidated anderoded during later exposure above the ocean surface.This type of limestone is also found in Indiana in the United States. The town of Oolitic, Indiana,was founded for the trade of limestone and bears its name. Quarries in Oolitic, Bedford, Indiana,and Bloomington, Indiana contributed the materials for such iconic US landmarks as the EmpireState Building and The Pentagon. The Soldiers and Sailors Monument in downtownIndianapolis is built mainly of grey oolitic limestone.The 1979 movie Breaking Away, centers around the sons of quarry workers in Bloomington, thehome of Indiana University. Many of the buildings on the Indiana University campus are builtwith native oolitic limestone material.Roggenstein is a term describing a specific type of oolite in which the cementing matter isargillaceousTravertineTravertine terraces at Mammoth Hot Springs, Yellowstone National Park
Calcium-carbonate-encrusted, yet growing moss, early stage of porous travertine formation.Travertine is a form of limestone deposited by mineral springs, especially hot springs.Travertine often has a fibrous or concentric appearance and exists in white, tan and cream-colored varieties. It is formed by a process of rapid precipitation of calcium carbonate, often atthe mouth of a hot spring or in a limestone cave. In the latter it can form stalactites, stalagmitesand other speleothems. It is frequently used in Italy and elsewhere as a building material.Travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate mineralsfrom solution in ground and surface waters, and/or geothermally heated hot-springs. Similar(but softer and extremely porous) deposits formed from ambient-temperature water are known astufa.FeaturesTravertine forms the stalactites and stalagmites of limestone caves, and the filling of some veinsand hot spring conduits. Travertine forms from geothermal springs and is often linked tosiliceous systems which form siliceous sinter. [Macrophyte]]s, bryophytes, algae, cyanobacteriaand other organisms often colonise the surface of travertine and are preserved, giving travertineits distinctive porosity.Some springs have temperatures high enough to exclude macrophytes and bryophytes from thedeposits, consequently, deposits are generally less porous than tufa. Thermophilic microbes areimportant in these environments and stromatolitic fabrics are common. When deposits areapparently devoid of any biological component, they are often referred to as calcareous sinter.Geochemistry
Modern travertine is formed from geothermally heated supersaturated alkaline waters, withraised pCO2 (see partial pressure). On emergence, waters degas CO2 due to the loweratmospheric pCO2, resulting in an increase in pH. Since carbonate solubility decreases withincreased pH, precipitation is induced. Supersaturation may be enhanced by factors leading toa reduction in pCO2, for example increased air-water interactions at waterfalls may beimportant, as may photosynthesis. Precipitation may also be enhanced by evaporation insome springs.Both calcite and aragonite are found in hot spring travertines; aragonite is preferentiallyprecipitated when temperatures are hot, while calcite dominates when temperatures arecooler. When pure and fine, travertine is white, but often it is brown to yellow due toimpurities.Travertine forming at Jupiter Terrace, Fountain Geyser Pool, Yellowstone National Park. Photo by AnselAdams, 1941.Travertine may precipitate out directly onto rock and other inert materials as in Pamukkale orYellowstone for example. Travertine may also precipitate out onto growing moss as in PlitviceLakes.OccurrenceTravertine waterfalls exist not only in the U.S. in Oklahoma and Texas but most famously inItaly, in Tivoli and Guidonia Montecelio where we can find most important quarries sinceAncient Roman like the old quarry Bernini in Guidonia. The latter has a major historic value,because it was one of the quarries that Gian Lorenzo Bernini selected material from to build thefamous (colonnato di Piazza S.Pietro ) The Colonnade of St. Peters Square in Rome in 1500.Travertine derives its name from the former town, known as Tibur in ancient Roman times. Theancient name for the stone was lapis tiburtinus, meaning tibur stone, which was graduallycorrupted to travertine. Detailed studies of the Tivoli and Guidonia travertine deposits revealeddiurnal and annual rhythmic banding and laminae, which have potential use in geochronology.In Central Europes last post-glacial palaeoclimatic optimum (Atlantic Period, 8000-5000 B.C.),huge deposits of tufa formed from karst springs. Important geotopes are found at the Swabian
Alb, mainly in valleys at the foremost northwest ridge of the cuesta; in many valleys of theeroded periphery of the karstic Franconian Jura; at the northern Alpine foothills; and the northernKarst Alps. On a smaller scale, these karst processes are still working. Travertine has been animportant building material since the Middle Ages.Travertine has formed sixteen huge, natural dams in a valley in Croatia known as Plitvice LakesNational Park. Clinging to moss and rocks in the water, the travertine has built up over severalmillennia to form waterfalls up to 70 metres (230 ft) in height.Cascades of natural lakes formed behind travertine dams can be seen in Mahallat, Abbass Abad,and Atash Kooh in Iran; Pamukkale, Turkey; Band-i-Amir, Afghanistan; HuangLong Valley,Sichuan, China; and Semuc Champey, Guatemala.In the U.S., the most well-known place for travertine formation is Yellowstone National Park,where the geothermal areas are rich in travertine deposits. Oklahoma has two parks are dedicatedto this natural wonder. Turner Falls, the tallest waterfall in Oklahoma, is a 77 feet (23 m) cascadeof spring water flowing over a travertine cave. Honey Creek feeds this waterfall and createsmiles of travertine shelves both up and downstream. Many small waterfalls upstream in thedense woods repeat the travertine-formation effect. The city of Davis now owns thousands ofacres of this land and has made it a tourist attraction. Another travertine resource is in Sulphur,Oklahoma, 10 miles (16 km) east of Turner Falls. Travertine Creek flows through a spring-waternature preserve within the boundaries of the Chickasaw National Recreation Area.In Texas, the city of Austin and its surrounding "Hillcountry" to the south is built on limestone.The area has many travertine formations, such as those found at Gorman Falls within ColoradoBend State Park, the nature preserve known as Hamilton Pool, the West Cave Preserve, andKrause Springs in Spicewood.TufaTufa columns at Mono Lake, California.
Tufa is a variety of limestone, formed by the precipitation of carbonate minerals from ambienttemperature water bodies. Geothermally heated hot-springs sometimes produce similar (but lessporous) carbonate deposits known as travertine. Tufa is sometimes referred to as (meteogene)travertine; care must be taken when searching through literature to prevent confusion with hotspring (thermogene) travertine. Calcareous tufa should not be confused with tuff, a porousvolcanic rock with parallel etymological origins.Classification and featuresModern and fossil tufa deposits abound with wetland plants; as such many tufa deposits arecharacterised by their large macrobiological component and are highly porous. Tufa forms eitherin fluvial channels or in lacustrine settings. Ford and Pedley (1996)  provide a review of tufasystems worldwide.Barrage Tufa at Cwm Nash, South WalesFluvial depositsDeposits can be classified by their depositional environment (or otherwise by vegetation orpetrographically). Pedley (1990) provides an extensive classification system, which includesthe following classes of fluvial tufa:- Spring - Deposits form on emergence from a spring/seep. Morphology can vary from mineratrophic wetlands to spring aprons (see calcareous sinter) Braided channel - Deposits form within a fluvial channel, dominated by oncoids (see oncolite) Cascade - Deposits form at waterfalls, deposition is focussed here due to accelerated flow (see Geochemistry) Barrage - Deposits form as a series of phytoherm barrages across a channel, which may grow up to several metres in height. Barrages often contain a significant detrital component, composed of organic material (leaf litter, branches etc.).Lacustrine deposits
Tufa deposits and columns and their flora on the lakeside of Mono Lake.Lacustrine tufas are generally formed at the periphery of lakes and build up phytoherms(freshwater reefs) and stromatolites. Oncoids are also common in these environments.Other depositsWhile fluvial and lacustrine systems make up the bulk of tufa systems worldwide, there areseveral other important tufa environments.Calcareous SinterAlthough sometimes regarded as a distinct carbonate deposit, calcareous sinter formed fromambient temperature water can be considered a sub-type of tufa.SpeleothemsCalcareous speleothems may be regarded as a form of calcareous sinter. They lack anysignificant macrophyte component due to the absence of light, for this reason they are oftenmorphologically closer to travertine or calcareous sinter.Tufa columnsTufa columns are an unusual form of tufa typically associated with saline lakes. They are distinctfrom most tufa deposits in that they lack any significant macrophyte component; this is due tothe salinity excluding mesophilic organisms. Some tufa columns may actually form from hot-springs and therefore actually be a form of travertine. It is generally thought that such featuresform from CaCO3 precipitated when carbonate rich source waters emerge into alkaline sodalakes. They have also been found in marine settings. Biology
Tufa deposits form an important habitat for a diverse flora. Bryophytes (non-vascular landplants) and diatoms are well represented. The porosity of the deposits creates a wet habitat idealfor these plants.GeochemistryModern tufa is formed from supersaturated alkaline waters, with raised pCO 2. On emergence,waters degas CO2 due to the lower atmospheric pCO2 (see partial pressure), resulting in anincrease in pH. Since carbonate solubility decreases with increased pH,  precipitation isinduced. Supersaturation may be enhanced by factors leading to a reduction in pCO 2, forexample increased air-water interactions at waterfalls may be important,  as mayphotosynthesis.Recently it has been demonstrated that microbially induced precipitation may be more importantthan physico-chemical precipitation. Pedley et al. (2009)  showed with flume experiments thatprecipitation does not occur unless a biofilm is present, despite supersaturation.Calcite is the dominant mineral precipitate found; however, the polymorph aragonite is alsofound.OccurrenceTufa is common in many parts of the world. Some notable deposits include:- Pyramid Lake, Nevada, USA - tufa formations Mono Lake, California, USA- tufa columns Trona Pinnacles, California, USA - tufa columns North Dock Tufa, United KingdomSome sources suggest that "tufa" was used as the primary building material for most of thechâteaux of the Loire Valley, France. This results from a mis-translation of the terms "tuffeaujaune" and "tuffeau blanc", which are porous varieties of the Late Cretaceous marine limestoneknown as chalk.Dolomite Dolomite
Dolomite and magnesite - Spain GeneralCategory Carbonate mineralChemical formula CaMg(CO3)2 IdentificationColor white, gray to pink tabular crystals, often with curvedCrystal habit faces, also columnar, stalactitic, granular, massive.Crystal system trigonal - rhombohedral, bar3Twinning common as simple contact twinsCleavage rhombohedral cleavage (3 planes)Fracture brittle - conchoidalMohs scale 3.5 to 4hardnessLuster vitreous to pearly
Streak whiteSpecific gravity 2.84–2.86Optical properties Uniaxial (-)Refractive index nω = 1.679–1.681 nε = 1.500Birefringence δ = 0.179–0.181 Poorly soluble in dilute HCl unlessSolubility powdered.Other May fluoresce white to pink under UV;characteristics triboluminescent. ReferencesDolomite druse with calcite crystals from Lawrence County, Arkansas, USA (size: 17.0 x 6.3 x 2.8 cm)Dolomite.Dolomite (pronounced /ˈdɒləmaɪt/) is the name of a sedimentary carbonate rock and a mineral,both composed of calcium magnesium carbonate CaMg(CO3)2 found in crystals.
Dolomite rock (also dolostone) is composed predominantly of the mineral dolomite. Limestonethat is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologicliterature as magnesian limestone. Dolomite was first described in 1791 as the rock by the Frenchnaturalist and geologist, Déodat Gratet de Dolomieu (1750–1801) for exposures in what are nowknown as the Dolomite Alps of northern Italy.PropertiesThe mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, gray topink, commonly curved crystals, although it is usually massive. It has physical properties similarto those of the mineral calcite, but does not rapidly dissolve or effervesce (fizz) in dilutehydrochloric acid unless it is scratched or in powdered form. The Mohs hardness is 3.5 to 4 andthe specific gravity is 2.85. Refractive index values are nω = 1.679 - 1.681 and nε = 1.500.Crystal twinning is common. A solid solution series exists between dolomite and iron richankerite. Small amounts of iron in the structure give the crystals a yellow to brown tint.Manganese substitutes in the structure also up to about three percent MnO. A high manganesecontent gives the crystals a rosy pink color noted in the image above. A series with themanganese rich kutnohorite may exist. Lead and zinc also substitute in the structure formagnesium.FormationDolomite bedrock underneath a Bristlecone Pine, White Mountains, California.Vast deposits are present in the geological record, but the mineral is relatively rare in modernenvironments. Laboratory synthesis of stoichiometric dolomite has been carried out only attemperatures of greater than 100 degrees Celsius (conditions typical of burial in sedimentarybasins), even though much dolomite in the rock record appears to have formed in low-temperature conditions. The high temperature is likely to speed up the movement of calcium andmagnesium ions so that they can find their places in the ordered structure within a reasonableamount of time. This suggests that the lack of dolomite that is being formed today is likely due tokinetic factors. I.e. due to the lack of kinetic energy or temperature.
Modern dolomite does occur as a precipitating mineral in specialized environments on thesurface of the earth today. In the 1950s and 60s, dolomite was found to be forming in highlysaline lakes in the Coorong region of South Australia. Dolomite crystals also occur in deep-seasediments, where organic matter content is high. This dolomite is termed "organogenic"dolomite.Recent research has found modern dolomite formation under anaerobic conditions insupersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelhaand Brejo do Espinho. One interesting reported case was the formation of dolomite in thekidneys of a Dalmatian dog. This was believed to be due to chemical processestriggered by bacteria. Dolomite has been speculated to develop under these conditions with thehelp of sulfate-reducing bacteria.The actual role of bacteria in the low-temperature formation of dolomite remains to bedemonstrated. The specific mechanism of dolomitization, involving sulfate-reducing bacteria,has not yet been demonstrated.Dolomite appears to form in many different types of environment and can have varyingstructural, textural and chemical characteristics. Some researchers have stated "there aredolomites and dolomites", meaning that there may not be one single mechanism by whichdolomite can form. Much modern dolomite differs significantly from the bulk of the dolomitefound in the rock record, leading researchers to speculate that environments where dolomiteformed in the geologic past differ significantly from those where it forms today.Reproducible laboratory syntheses of dolomite (and magnesite) leads first to the initialprecipitation of a metastable "precursor" (such as magnesium calcite), to be changed graduallyinto more and more of the stable phase (such as dolomite or magnesite) during periodicalintervals of dissolution and reprecipitation. The general principle governing the course of thisirreversible geochemical reaction has been coined Ostwalds step rule.For a very long time scientists had difficulties synthesizing dolomite. However, in a 1999 study,through a process of dissolution alternating with intervals of precipitation, measurable levels ofdolomite were synthesized at low temperatures and pressures. Coral atollsDolomitization of calcite also occurs at certain depths of coral atolls where water isundersaturated in calcium carbonate but saturated in dolomite. Convection created by tides andsea currents enhance this change. Hydrothermal currents created by volcanoes under the atollmay also play an important role.Magnesite Magnesite
GeneralCategory Carbonate mineralChemical formula MgCO3 Identification Colorless, white, pale yellow, paleColor brown, faintly pink, lilac-rose Usually massive, rarely asCrystal habit rhombohedrons or hexagonal prisms Trigonal - Hexagonal Scalenohedral H-MCrystal system Symbol 32/m Space Group: R3cCleavage  perfectFracture ConchoidalTenacity BrittleMohs scale 3.5 - 4.5hardnessLuster VitreousStreak White
Diaphaneity Transparent to translucentSpecific gravity 3.0 - 3.2Optical Uniaxial (-)propertiesRefractive index nω=1.508 - 1.510 nε=1.700Fusibility InfusibleSolubility Effervesces in hot HCl May exhibit pale green to pale blueOther fluorescence and phosphorescencecharacteristics under UV; triboluminescent ReferencesMagnesite is magnesium carbonate, MgCO3. Iron (as Fe2+) substitutes for magnesium (Mg) witha complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel mayalso occur in small amounts. Dolomite, (Mg,Ca)CO3, is almost indistinguishable from magnesite.OccurrenceMagnesite occurs as veins in and an alteration product of ultramafic rocks, serpentinite and othermagnesium rich rock types in both contact and regional metamorphic terranes. These magnesitesoften are cryptocrystalline and contain silica as opal or chert.Magnesite is also present within the regolith above ultramafic rocks as a secondary carbonatewithin soil and subsoil, where it is deposited as a consequence of dissolution of magnesium-bearing minerals by carbon dioxide within groundwaters.FormationMagnesite can be formed via talc carbonate metasomatism of peridotite and other ultrabasicrocks. Magnesite is formed via carbonation of olivine in the presence of water and carbondioxide, and is favored at moderate temperatures and pressures typical of greenschist facies;
Magnesite can also be formed via the carbonation of magnesian serpentine (lizardite) via thefollowing reaction:Serpentine + carbon dioxide → Talc + magnesite + Water 2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + H2OForsterite magnesia-rich olivine compositions favor production of magnesite from peridotite.Fayalitic (iron-rich) olivine favors production of magnetite-magnesite-silica compositions.Magnesite can also be formed from metasomatism in skarn deposits, in dolomitic limestones,associated with wollastonite, periclase, and talc.Magnesite is also found in a number of Precambrian carbonate hosted sediments, and is thoughtto have formed as an evaporite.GeyseriteGeyserite from IcelandGeyserite is a form of opaline silica that is often found around hot springs and geysers.Botryoidal geyserite is known as fiorite. It is sometimes referred to as sinter.Diatomaceous earth
A sample of diatomaceous earthDiatomaceous earth (pronounced /ˌdaɪ.ətəˌmeɪʃəs ˈɜrθ/) also known as diatomite or kieselgur,is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine whiteto off-white powder. It has a particle size ranging from less than 1 micron to more than 1millimeter, but typically 10 to 200 microns.  This powder has an abrasive feel, similar topumice powder, and is very light, due to its high porosity. The typical chemical composition ofoven dried diatomaceous earth is 80 to 90% silica, with 2 to 4% alumina (attributed mostly toclay minerals) and 0.5 to 2% iron oxide.Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled algae. It isused as a filtration aid, as a mild abrasive, as a mechanical insecticide, as an absorbent forliquids, as cat litter, as an activator in blood clotting studies, and as a component of dynamite. Asit is also heat-resistant, it can be used as a thermal insulator.Geology and occurrenceDiatomaceous earth as viewed under bright field illumination on a light microscope. Diatomaceous earthis made up of the cell walls/shells of single cell diatoms and readily crumbles to a fine powder. Diatomcell walls are made up of biogenic silica; silica synthesised in the diatom cell by the polymerisation ofsilicic acid. This image of diatomaceous earth particles in water is at a scale of 6.236 pixels/μm, theentire image covers a region of approximately 1.13 by 0.69 mm.FormationDiatomite forms by the accumulation of the amorphous silica (opal, SiO2·nH2O) remains of deaddiatoms (microscopic single-celled algae) in lacustrine or marine sediments. The fossil remainsconsist of a pair of symmetrical shells or frustules.Discovery
In 1836 or 1837, the peasant and goods waggoner, Peter Kasten,  discovered kieselgur whensinking a well on the northern slopes of the Haußelberg hill, in the Lüneburg Heath in northGermany. Initially, it was thought that limestone had been found, which could be used asfertiliser. Alfred Nobel used the properties of kieselgur in the manufacture of dynamite. TheCelle engineer, Wilhelm Berkefeld, recognised its ability to filter, and developed filter candlesfired from kieselgur. During the cholera epidemic in Hamburg in 1892, these Berkefeld filterswere used successfully.Extraction and storage sites in the Lüneburg Heath Neuohe - Abbau from 1863 to 1994 Wiechel from 1871 to 1978 Hützel from 1876 to 1969 Hösseringen from ca. 1880 to 1894 Hammerstorf from ca. 1880 to 1920 Oberohe from 1884 to 1970 Schmarbeck from 1896 to ca. 1925 Steinbeck from 1897 to 1928 Breloh from 1907 to 1975 Schwindebeck from 1913 to 1975 Hetendorf from 1970 to 1994The deposits are up to 28 metres thick and are all of freshwater kieselgur. ca.1900–1910 a drying area: one 1913 Staff at the Neuohe factory,ca. 1900–1910 with workers and a female cook inKieselgur pit at Neuohe firing pile is being prepared; another is under way front of a drying shedUntil the First World War almost the entire worldwide production of kieselgur was from thisregion.Other depositsIn Germany kieselgur was also extracted at Altenschlirf  on the Vogelsberg (Upper Hesse) andat Klieken  (Saxony-Anhalt).There is a layer of kieselgur up to 4 metres thick in the nature reserve of Soos in the CzechRepublic.In Colorado and in Clark, Nevada (USA), there are deposits that are up to several hundred metresthick in places.
Sometimes kieselgur is found on the surface in deserts. Research has shown that the erosion ofkieselgur in such areas (such as the Bodélé Depression in the Sahara) is one of the mostimportant sources of climate-affecting dust in the atmosphere.The commercial deposits of diatomite are restricted to Tertiary or Quaternary periods. Olderdeposits from as early as the Cretaceous Period are known, but are of low quality.  Marinedeposits have been worked in the Sisquoc Formation in Santa Barbara County, California nearLompoc and along the southern California coast. Additional marine deposits have been workedin Maryland, Virginia, Algeria and the MoClay of Denmark. Fresh water lake deposits occur inNevada, Oregon, Washington and California. Lake deposits also occur in interglacial lakes in theeastern US and Canada and in Europe in Germany, France, Denmark and the Czech Republic.The worldwide association of diatomite deposits and volcanic deposits suggests that theavailability of silica from volcanic ash may be needed for thick diatomite deposits.EvaporiteCobble encrusted with halite evaporated from the Dead Sea, Israel.Evaporites (pronounced /ɨˈvæpəraɪt/) are water-soluble mineral sediments that result from theevaporation of bodies of surficial water. Evaporites are considered sedimentary rocks.Formation of evaporite rocksAlthough all water bodies on the surface and in aquifers contain dissolved salts, the water mustevaporate into the atmosphere for the minerals to precipitate. For this to happen the water bodymust enter a restricted environment where water input into this environment remains below thenet rate of evaporation. This is usually an arid environment with a small basin fed by a limitedinput of water. When evaporation occurs, the remaining water is enriched in salts, and theyprecipitate when the water becomes oversaturated.Evaporite depositional environments
Evaporite depositional environments which meet the above conditions include; Graben areas and half-grabens within continental rift environments fed by limited riverine drainage, usually in subtropical or tropical environments o Example environments at the present which match this is the Denakil Depression, Ethiopia; Death Valley, California Graben environments in oceanic rift environments fed by limited oceanic input, leading to eventual isolation and evaporation o Examples include the Red Sea, and the Dead Sea in Jordan and Israel Internal drainage basins in arid to semi-arid temperate to tropical environments fed by ephemeral drainage o Example environments at the present include the Simpson Desert, Western Australia, the Great Salt Lake in Utah Non-basin areas fed exclusively by groundwater seepage from artesian waters o Example environments include the seep-mounds of the Victoria Desert, fed by the Great Artesian Basin, Australia Restricted coastal plains in regressive sea environments o Examples include the sabkha deposits of Iran, Saudi Arabia and the Red Sea; the Garabogazköl of the Caspian Sea Drainage basins feeding into extremely arid environments o Examples include the Chilean deserts, certain parts of the Sahara and the NamibThe most significant known evaporite depositions happened during the Messinian salinity crisisin the basin of the Mediterranean.Evaporitic formationsHopper crystal cast of halite in a Jurassic rock, Carmel Formation, southwestern Utah.Evaporite formations need not be composed entirely of halite salt. In fact, most evaporiteformations do not contain more than a few percent of evaporite minerals, the remainder beingcomposed of the more typical detrital clastic rocks and carbonates.
For a formation to be recognised as evaporitic it may simply require recognition of halitepseudomorphs, sequences composed of some proportion of evaporite minerals, and recognitionof mud crack textures or other textures.Economic importance of evaporitesEvaporites are important economically because of their mineralogy, their physical properties in-situ and their behaviour within the subsurface.Evaporite minerals, especially nitrate minerals, are economically important in Peru and Chile.Nitrate minerals are often mined for use in the production on fertilizer and explosives.Thick halite deposits are expected to become an important location for the disposal of nuclearwaste because of their geologic stability, predictable engineering and physical behaviour andimperviousness to groundwater.Halite formations are famous for their ability to form diapirs which produce ideal locations fortrapping petroleum deposits.Major groups of evaporite minerals Halides: halite, sylvite (KCl), and fluorite Sulfates: such as gypsum, barite, and anhydrite Nitrates: nitratine (soda niter) and niter Borates: typically found in arid-salt-lake deposits plentiful in the southwestern US. A common borate is borax, which has been used in soaps as a surfactant. Carbonates: such as trona, formed in inland brine lakes.Evaporite minerals start to precipitate when their concentration in water reaches such a level thatthey can no longer exist as solutes.The minerals precipitate out of solution in the reverse order of their solubilities, such that theorder of precipitation from sea water is 1. Calcite (CaCO3) and dolomite (CaMg(CO3)2) 2. Gypsum (CaSO4-2H2O) and anhydrite (CaSO4). 3. Halite (i.e. common salt, NaCl) 4. Potassium and magnesium saltsThe abundance of rocks formed by seawater precipitation is in the same order as the precipitationgiven above. Thus, limestone (calcite) and dolomite are more common than gypsum, which ismore common than halite, which is more common than potassium and magnesium salts.Metamorphic rock
Quartzite, a form of metamorphic rock, from the Museum of Geology at University of Tartu collection.Metamorphic rock is the transformation of an existing rock type, the protolith, in a processcalled metamorphism, which means "change in form". The protolith is subjected to heat andpressure (temperatures greater than 150 to 200 °C and pressures of 1500 bars) causingprofound physical and/or chemical change. The protolith may be sedimentary rock, igneous rockor another older metamorphic rock. Metamorphic rocks make up a large part of the Earths crustand are classified by texture and by chemical and mineral assemblage (metamorphic facies).They may be formed simply by being deep beneath the Earths surface, subjected to hightemperatures and the great pressure of the rock layers above it. They can form from tectonicprocesses such as continental collisions, which cause horizontal pressure, friction and distortion.They are also formed when rock is heated up by the intrusion of hot molten rock called magmafrom the Earths interior. The study of metamorphic rocks (now exposed at the Earths surfacefollowing erosion and uplift) provides us with information about the temperatures and pressuresthat occur at great depths within the Earths crust.Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.Metamorphic mineralsMetamorphic minerals are those that form only at the high temperatures and pressures associatedwith the process of metamorphism. These minerals, known as index minerals, includesillimanite, kyanite, staurolite, andalusite, and some garnet.Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz, may befound in metamorphic rocks, but are not necessarily the result of the process of metamorphism.These minerals formed during the crystallization of igneous rocks. They are stable at hightemperatures and pressures and may remain chemically unchanged during the metamorphicprocess. However, all minerals are stable only within certain limits, and the presence of someminerals in metamorphic rocks indicates the approximate temperatures and pressures at whichthey formed.The change in the particle size of the rock during the process of metamorphism is calledrecrystallization. For instance, the small calcite crystals in the sedimentary rock limestone
change into larger crystals in the metamorphic rock marble, or in metamorphosed sandstone,recrystallization of the original quartz sand grains results in very compact quartzite, in which theoften larger quartz crystals are interlocked. Both high temperatures and pressures contribute torecrystallization. High temperatures allow the atoms and ions in solid crystals to migrate, thusreorganizing the crystals, while high pressures cause solution of the crystals within the rock attheir point of contact.FoliationFolded foliation in a metamorphic rock from near Geirangerfjord, NorwayThe layering within metamorphic rocks is called foliation (derived from the Latin word folia,meaning "leaves"), and it occurs when a rock is being shortened along one axis duringrecrystallization. This causes the platy or elongated crystals of minerals, such as mica andchlorite, to become rotated such that their long axes are perpendicular to the orientation ofshortening. This results in a banded, or foliated, rock, with the bands showing the colors of theminerals that formed them.Textures are separated into foliated and non-foliated categories. Foliated rock is a product ofdifferential stress that deforms the rock in one plane, sometimes creating a plane of cleavage. Forexample, slate is a foliated metamorphic rock, originating from shale. Non-foliated rock does nothave planar patterns of strain.Rocks that were subjected to uniform pressure from all sides, or those that lack minerals withdistinctive growth habits, will not be foliated. Slate is an example of a very fine-grained, foliatedmetamorphic rock, while phyllite is medium, schist coarse, and gneiss very coarse-grained.Marble is generally not foliated, which allows its use as a material for sculpture and architecture.Another important mechanism of metamorphism is that of chemical reactions that occur betweenminerals without them melting. In the process atoms are exchanged between the minerals, andthus new minerals are formed. Many complex high-temperature reactions may take place, andeach mineral assemblage produced provides us with a clue as to the temperatures and pressuresat the time of metamorphism.
Metasomatism is the drastic change in the bulk chemical composition of a rock that often occursduring the processes of metamorphism. It is due to the introduction of chemicals from othersurrounding rocks. Water may transport these chemicals rapidly over great distances. Because ofthe role played by water, metamorphic rocks generally contain many elements absent from theoriginal rock, and lack some that originally were present. Still, the introduction of new chemicalsis not necessary for recrystallization to occur.Types of metamorphismContact metamorphismA contact metamorphic rock made of interlayered calcite and serpentine from the Precambrian ofCanada. Once thought to be a fossil called Eozoön canadense. Scale in mm.Contact metamorphism is the name given to the changes that take place when magma isinjected into the surrounding solid rock (country rock). The changes that occur are greatestwherever the magma comes into contact with the rock because the temperatures are highest atthis boundary and decrease with distance from it. Around the igneous rock that forms from thecooling magma is a metamorphosed zone called a contact metamorphism aureole. Aureoles mayshow all degrees of metamorphism from the contact area to unmetamorphosed (unchanged)country rock some distance away. The formation of important ore minerals may occur by theprocess of metasomatism at or near the contact zone.When a rock is contact altered by an igneous intrusion it very frequently becomes moreindurated, and more coarsely crystalline. Many altered rocks of this type were formerly calledhornstones, and the term hornfels is often used by geologists to signify those fine grained,compact, non-foliated products of contact metamorphism. A shale may become a darkargillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone maychange to a grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough andsplintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is animportant component. A diabase or andesite may become a diabase hornfels or andesite hornfelswith development of new hornblende and biotite and a partial recrystallization of the originalfeldspar. Chert or flint may become a finely crystalline quartz rock; sandstones lose their clastic
structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphicrock called quartzite.If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliatedcalc-schist) this character may not be obliterated, and a banded hornfels is the product; fossilseven may have their shapes preserved, though entirely recrystallized, and in many contact-alteredlavas the vesicles are still visible, though their contents have usually entered into newcombinations to form minerals that were not originally present. The minute structures, however,disappear, often completely, if the thermal alteration is very profound; thus small grains of quartzin a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass oflavas is entirely reconstructed.By recrystallization in this manner peculiar rocks of very distinct types are often produced. Thusshales may pass into cordierite rocks, or may show large crystals of andalusite (and chiastolite),staurolite, garnet, kyanite and sillimanite, all derived from the aluminous content of the originalshale. A considerable amount of mica (both muscovite and biotite) is often simultaneouslyformed, and the resulting product has a close resemblance to many kinds of schist. Limestones, ifpure, are often turned into coarsely crystalline marbles; but if there was an admixture of clay orsand in the original rock such minerals as garnet, epidote, idocrase, wollastonite, will be present.Sandstones when greatly heated may change into coarse quartzites composed of large cleargrains of quartz. These more intense stages of alteration are not so commonly seen in igneousrocks, because their minerals, being formed at high temperatures, are not so easily transformedor recrystallized.In a few cases rocks are fused and in the dark glassy product minute crystals of spinel, sillimaniteand cordierite may separate out. Shales are occasionally thus altered by basalt dikes, andfeldspathic sandstones may be completely vitrified. Similar changes may be induced in shales bythe burning of coal seams or even by an ordinary furnace.There is also a tendency for metasomatism between the igneous magma and sedimentary countryrock, whereby the chemicals in each are exchanged or introduced into the other. Granites mayabsorb fragments of shale or pieces of basalt. In that case, hybrid rocks called skarn arise, whichdont have the characteristics of normal igneous or sedimentary rocks. Sometimes an invadinggranite magma permeates the rocks around, filling their joints and planes of bedding, etc., withthreads of quartz and feldspar. This is very exceptional but instances of it are known and it maytake place on a large scale.Regional metamorphism
Mississippian marble in Big Cottonwood Canyon, Wasatch Mountains, Utah.Regional metamorphism is the name given to changes in great masses of rock over a wide area.Rocks can be metamorphosed simply by being at great depths below the Earths surface,subjected to high temperatures and the great pressure caused by the immense weight of the rocklayers above. Much of the lower continental crust is metamorphic, except for recent igneousintrusions. Horizontal tectonic movements such as the collision of continents create orogenicbelts, and cause high temperatures, pressures and deformation in the rocks along these belts. Ifthe metamorphosed rocks are later uplifted and exposed by erosion, they may occur in long beltsor other large areas at the surface. The process of metamorphism may have destroyed the originalfeatures that could have revealed the rocks previous history. Recrystallization of the rock willdestroy the textures and fossils present in sedimentary rocks. Metasomatism will change theoriginal composition.Regional metamorphism tends to make the rock more indurated and at the same time to give it afoliated, shistose or gneissic texture, consisting of a planar arrangement of the minerals, so thatplaty or prismatic minerals like mica and hornblende have their longest axes arranged parallel toone another. For that reason many of these rocks split readily in one direction along mica-bearingzones (schists). In gneisses, minerals also tend to be segregated into bands; thus there are seamsof quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral. Alongthe mineral layers composed of soft or fissile minerals the rocks will split most readily, and thefreshly split specimens will appear to be faced or coated with this mineral; for example, a pieceof mica schist looked at facewise might be supposed to consist entirely of shining scales of mica.On the edge of the specimens, however, the white folia of granular quartz will be visible. Ingneisses these alternating folia are sometimes thicker and less regular than in schists, but mostimportantly less micaceous; they may be lenticular, dying out rapidly. Gneisses also, as a rule,contain more feldspar than schists do, and they are tougher and less fissile. Contortion orcrumbling of the foliation is by no means uncommon, and then the splitting faces are undulose orpuckered. Schistosity and gneissic banding (the two main types of foliation) are formed bydirected pressure at elevated temperature, and to interstitial movement, or internal flow arrangingthe mineral particles while they are crystallizing in that directed pressure field.Rocks that were originally sedimentary and rocks that were undoubtedly igneous convert intoschists and gneisses. If originally of similar composition they may be very difficult to distinguishfrom one another if the metamorphism has been great. A quartz-porphyry, for example, and a
fine feldspathic sandstone, may both the converted into a grey or pink mica-schist. They aremade by heat and pressure.Metamorphic rock texturesThe five basic metamorphic textures with typical rock types are slaty (includes slate and phyllite)(the foliation is called "slaty cleavage"), "schistose" (includes schist) (the foliation is called("schistosity"), gneissose (gneiss) (the foliation is called "gneissosity"), granoblastic (includesgranulite, some marbles and quartzite), and hornfelsic (includes hornfels and skarn).GneissGneiss rockAugen gneiss from Rio de Janeiro, BrazilGranitic gneiss from Enfield, New York
Study of Gneiss Rock, Glenfinlas, the Trossachs, Scotland. A pen and ink study by John Ruskin, 1853, isnow in the Ashmolean Museum, Oxford.Gneiss (pronounced /ˈnaɪs/ "nice") is a common and widely distributed type of rock formed byhigh-grade regional metamorphic processes from pre-existing formations that were originallyeither igneous or sedimentary rocks.EtymologyThe etymology of the word "gneiss" is disputed. Some sources say it comes from the MiddleHigh German verb gneist (to spark; so called because the rock glitters) and has occurred inEnglish at least since 1757. Other sources claim the root to be an old Saxon mining term thatseems to have meant decayed, rotten, or possibly worthless material. CompositionGneissic rocks are usually medium- to coarse-foliated and largely recrystallized but do not carrylarge quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosedigneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Depending ontheir composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc.Gneiss displays compositional banding where the minerals are arranged into bands of moremafic minerals and more felsic minerals. This is developed under high temperature and pressureconditions.
Types of gneissOrthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from asedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.LewisianMost of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss. These areamongst the oldest rocks in Europe and some of the oldest in the world, having been formed inthe Precambrian "super-eon", up to 3 billion years ago. In addition to the Outer Hebrides, theyform basement deposits on the Scottish mainland west of the Moine Thrust and on the islands ofColl and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble,quartzite and mica schist and intruded by later basaltic dykes and granite magma. The gneisssdelicate pink colours are exposed throughout the islands and it is sometimes referred to bygeologists as "The Old Boy".Augen gneissAugen gneiss, from the German Augen [ˈaʊɡən], meaning "eye", is a coarse-grained gneiss,interpreted as resulting from metamorphism of granite, which contains characteristic elliptic orlenticular shear bound feldspar porphyroclasts, normally microcline, within the layering of thequartz, biotite and magnetite bands.SchistSchistThe schists constitute a group of medium-grade metamorphic rocks, chiefly notable for thepreponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, andothers. Quartz often occurs in drawn-out grains to such an extent that a particular form calledquartz schist is produced. By definition, schist contains more than 50% platy and elongatedminerals, often finely interleaved with quartz and feldspar. Schist is often garnetiferous.
The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can beseen by the naked eye. Schist is characteristically foliated, meaning the individual mineral grainssplit off easily into flakes or slabs. The word schist is derived from the Greek word σχίζεινmeaning "to split", which is a reference to the ease with which schists can be split along theplane in which the platy minerals lie.Most schists have been derived from clays and muds which have passed through a series ofmetamorphic processes involving the production of shales, slates and phyllites as intermediatesteps. Certain schists have been derived from fine-grained igneous rocks such as basalts andtuffs. Most schists are mica schists, but graphite and chlorite schists are also common.Thin section of Garnet-Mica-SchistView of cut Garnet-Mica-SchistSchists are named for their prominent or perhaps unusual mineral constituents, such as garnetschist, tourmaline schist, glaucophane schist, etc.Schists are frequently used as dimension stone.Historical mining terminology
Before the mid 19th century, the terms slate, shale and schist were not sharply distinguished. Inthe context of underground coal mining, shale was frequently referred to as slate well into the20th century.FormationDuring metamorphism, rocks which were originally sedimentary or igneous are converted intoschists and gneisses. If the composition of the rocks was originally similar, they may be verydifficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry,for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pinkmica-schist. Usually, however, it is possible to distinguish between sedimentary and igneousschists and gneisses. If the whole district, for example, occupied by these rocks have traces ofbedding, clastic structure, or unconformability then it may be a sign that the original rock wassedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyriticstructure may prove that in its original condition a metamorphic gneiss was an igneous rock. Thelast appeal is often to the chemistry, for there are certain rock types which occur only assediments, while others are found only among igneous masses, and however advanced themetamorphism may be, it rarely modifies the chemical composition of the mass very greatly.Such rocks, for example, as limestones, dolomites, quartzites and aluminous shales have verydefinite chemical characters which distinguish them even when completely recrystallized.Manhattan schist, from Southeastern New York
Chlorite schist forms from shale or mudstone.The schists are classified principally according to the minerals they consist of and on theirchemical composition. For example, many metamorphic limestones, marbles, and calc-schists,with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite,quartz and feldspar. They are derived from calcareous sediments of different degrees of purity.Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variableamounts of white and black mica, garnet, feldspar, zoisite and hornblende. These were oncesandstones and arenaceous rocks. The graphitic schists may readily be believed to representsediments once containing coal or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon. Among schists ofigneous origin there are the silky calc-schists, the foliated serpentines (once ultramafic massesrich in olivine), and the white mica-schists, porphyroids and banded halleflintas, which havebeen derived from rhyolites, quartz-porphyries and felsic tuffs. The majority of mica-schists,however, are altered claystones and shales, and pass into the normal sedimentary rocks throughvarious types of phyllite and mica-slates. They are among the most common metamorphic rocks;some of them are graphitic and others calcareous. The diversity in appearance and composition isvery great, but they form a well-defined group not difficult to recognize, from the abundance ofblack and white micas and their thin, foliated, schistose character. A subgroup is the andalusite,staurolite, kyanite and sillimanite-schists which usually make their appearance in the vicinity ofgneissose granites, and have presumably been affected by contact metamorphism. Quartzite
QuartziteSwan Peak Quartzite (Ordovician) exposed just north of Tony Grove Lake, Cache County, Utah.The quartzite of the Prospect Mountain Formation on top of Wheeler Peak, the highest peak withinNevada, United States.Quartzite (from German Quarzit) is a hard metamorphic rock which was originallysandstone. Sandstone is converted into quartzite through heating and pressure usually related totectonic compression within orogenic belts. Pure quartzite is usually white to grey, thoughquartzites often occur in various shades of pink and red due to varying amounts of iron oxide(Fe2O3). Other colors, such as yellow and orange, are due to other mineral impurities.
When sandstone is metamorphosed to quartzite, the individual quartz grains recrystallize alongwith the former cementing material to form an interlocking mosaic of quartz crystals. Most or allof the original texture and sedimentary structures of the sandstone are erased by themetamorphism. Minor amounts of former cementing materials, iron oxide, carbonate and clay,often migrate during recrystallization and metamorphosis. This causes streaks and lenses to formwithin the quartzite.Orthoquartzite is a very pure quartz sandstone composed of usually well rounded quartz grainscemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxideand trace resistant minerals such as zircon, rutile and magnetite. Although few fossils arenormally present, the original texture and sedimentary structures are preserved. The term is oftenmisused, and should be used for only tightly-cemented metamorphic quartzites, not quartz-cemented quartz arenites. The typical distinction between the two (since each is a gradationinto the other) is a proper quartzite is so highly cemented, diagentically altered, andmetamorphosed that it will fracture and break across grain boundaries, not around them.Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops.The nearly pure silica content of the rock provides little to form soil from and therefore thequartzite ridges are often bare or covered only with a very thin layer of soil and little vegetation.Abandoned quartzite mine in Kakwa Provincial Park, British Columbia, Canada
Biface in quartzite - Stellenbosch, South AfricaOccurrencesIn the United States, formations of quartzite can be found in some parts of Pennsylvania, easternSouth Dakota, Central Texas, southwest Minnesota, Devils Lake State Park in the BarabooHills in Wisconsin, the Wasatch Range in Utah, near Salt Lake City, Utah and as resistantridges in the Appalachians and other mountain regions. Quartzite is also found in the MorenciCopper Mine in Arizona. The town of Quartzsite in western Arizona derives its name from thequartzites in the nearby mountains in both Arizona and Southeastern California. A glassyvitreous quartzite has been described from the Belt Supergroup in the Coeur d’Alene district ofnorthern Idaho.In the United Kingdom, a craggy ridge of quartzite called the Stiperstones (early Ordovician -Arenig Epoch, 500 Ma) runs parallel with the Pontesford-Linley fault, 6 km north-west of theLong Mynd in south Shropshire. Also to be found in England are the Cambrian "Wrekinquartizite" (in Shropshire), and the Cambrian "Hartshill quartzite" (Nuneaton area). In Wales,Holyhead mountain and most of Holy island off Anglesey sport excellent Precambrian quartzitecrags and cliffs. In the Scottish Highlands, several mountains (e.g. Foinaven, Arkle) composed ofCambrian quartzite can be found in the far north-west Moine Thrust Belt running in a narrowband from Loch Eriboll in a south-westerly direction to Skye.In Canada, the La Cloche Mountains in Ontario are composed primarily of white quartzite.Slate Slate — Metamorphic Rock —
Slate CompositionPrimary quartz, muscovite/illiteSecondary biotite, chlorite, hematite, pyriteSlate Macro (~ 6 cm long and ~ 4 cm high)Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regionalmetamorphism. The result is a foliated rock in which the foliation may not correspond to theoriginal sedimentary layering. When expertly "cut" by striking with a specialized tool in thequarry, many slates will form smooth flat sheets of stone which have long been used for roofingand floor tiles and other purposes. Slate is frequently grey in color, especially when seen inmasse covering roofs. However, slate occurs in a variety of colors even from a single locality; forexample, slate from North Wales can be found in many shades of grey, from pale to dark, andmay also be purple, green or cyan. Slate is not to be confused with shale, from which it may beformed, or schist. Ninety percent of Europes natural slate used for roofing originates fromSpain.
The word "slate" is also used for some objects made from slate. It may mean a single roofingslate, or a writing slate, traditionally a small piece of slate, often framed in wood, used withchalk as a notepad or noticeboard etc., and especially for recording charges in pubs and inns. Thephrase "clean slate" or "blank slate" comes from this use.Historical mining terminologyBefore the mid-19th century, the terms slate, shale and schist were not sharply distinguished.In the context of underground coal mining, the term slate was commonly used to refer to shalewell into the 20th century. For example, roof slate refers to shale above a coal seam, and drawslate refers to roof slate that falls from the mine roof as the coal is removed. Mineral compositionSlate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite,hematite, and pyrite and, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, orzircon as well as feldspar. Occasionally, as in the purple slates of North Wales, ferrous reductionspheres form around iron nuclei, leaving a light green spotted texture. These spheres aresometimes deformed by a subsequent applied stress field to ovoids, which appear as ellipseswhen viewed on a cleavage plane of the specimen.Slate extractionExhibition Slate mine Fell.
Historical Pit Vogelsberg 1 at FellMain article: Slate industryIn EurasiaSlate-producing regions in Europe include Wales (see slate industry in Wales), Cornwall(famously the village of Delabole), Cumbria (see Burlington Slate Quarries, Honister Slate Mineand Skiddaw Slate) in the United Kingdom; parts of France (Anjou, Ardennes, Brittany, Savoie);Belgium (Ardenne); Liguria in northern Italy, especially between the town of Lavagna (whichmeans chalkboard in Italian) and Fontanabuona valley; Portugal especially around Valongo inthe north of the country; Germanys (Moselle River-region, Hunsrück, Eifel, Westerwald,Thuringia and north Bavaria); Alta, Norway (actually schist not a true slate) and Galicia. Someof the slate from Wales and Cumbria is colored slate (non-blue): purple and formerly green inWales and green in Cumbria. China has vast slate deposits; in recent years its export of finishedand unfinished slate has increased, it has slate in various colors.In the AmericasSlate is abundant in Brazil (the second-biggest producer of slate) around Papagaios in MinasGerais (responsible for 95% of the extraction of slate in Brazil). An independent report byConsultant Geologist J. A. Walsh describes how certain products originating from Brazil on salein the UK, are not entitled to bear the CE mark.Other areas known for slate production are the east coast of Newfoundland, the Slate Belt ofEastern Pennsylvania, Buckingham County Virginia (Buckingham Slate), and the Slate Valley ofVermont and New York, where colored slate is mined in the Granville, New York area.A major slating operation existed in Monson, Maine, during the late 19th- and early-20thcenturies. The slate found in Monson is usually a dark purple to blackish color, and many localstructures are still roofed with slate tiles. The roof of St. Patricks Cathedral was made of roofingslate from Monson, as was the headstone of John F. Kennedy.Slate is also found in the Arctic and was used by the Inuit to make the blades for ulus.Fossils
Shale can metamorphose into slate; sometimes the fossils may remain intact.Because slate was formed in low heat and pressure, compared to a number of other metamorphicrocks, some fossils can be found in slate; sometimes even microscopic remains of delicateorganisms.MarbleMarble.
Folded and weathered marble at General Carrera Lake, Chile.The Taj Mahal is made of marble.
Ancient greek statue of Venus de Milo, sculpted from marble.Natural patterns on the polished surface of Breccia or "landscape marble" can resemble a city skyline oreven trees, and were used as inlays for furniture etc.Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonlycalcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasonsuse the term more broadly to encompass unmetamorphosed limestone. Marble is commonly used for sculpture and as a building material.EtymologyThe word "marble" derives from the Greek "μάρμαρον" (mármaron), from "μάρμαρος"(mármaros), "crystalline rock", "shining stone",  perhaps from the verb "μαρμαίρω"(marmaírō), "to flash, sparkle, gleam". This stem is also the basis for the English wordmarmoreal, meaning "marble-like."Whilst the English term resembles the French marbre, most other european languages (egSpanish mármol, Italian marmo, Portuguese mármore, German and Swedish marmor, Dutchmarmer, Polish marmur, Czech mramor and Russian мрáмор ) follow the original Greek.Physical OriginsMarble is a rock resulting from metamorphism of sedimentary carbonate rocks, (most commonlylimestone or dolomite rock). Metamorphism causes variable recrystallization of the originalcarbonate mineral grains.The resulting marble rock is typically composed of an interlocking mosaic of carbonate crystals.Primary sedimentary textures and structures of the original carbonate rock (protolith) havetypically been modified or destroyed.Pure white marble is the result of metamorphism of a very pure (silicate-poor) limestone ordolomite protolith. The characteristic swirls and veins of many colored marble varieties are
usually due to various mineral impurities such as clay, silt, sand, iron oxides, or chert which wereoriginally present as grains or layers in the limestone.Green coloration is often due to serpentine resulting from originally high magnesium limestoneor dolostone with silica impurities. These various impurities have been mobilized andrecrystallized by the intense pressure and heat of the metamorphism.TypesExamples of historically notable marble varieties and locations:Marble name Color Location CountryBucova Băuţar, Caraş-Severin County (applied in white, gray RomaniaMarble Ulpia Traiana Sarmizegetusa)Carrara white or blue-gray Carrara ItalymarbleMacael white Macael, Almeria SpainmarbleMakrana white Makrana IndiaMarbleMurphy white Pickens and Gilmer Counties, Georgia United StatesMarbleParian marble pure-white, fine-grained Island of Paros GreecePentelic pure-white, fine-grained Penteliko Mountain, Athens Greecemarble semitranslucentPhrygian purple Phrygia TurkeyMarbleRuskeala white near Ruskeala, Karelia RussiaMarbleSienese yellow, yellowish-white near Sovicille, Tuscany ItaliaMarbleBianco Sivec white near Prilep Republic of
MacedoniaSylacauga white Talladega County, Alabama United StatesmarbleTennessee Knox, Blount and Hawkins Counties, pale pink to cedar-red United Statesmarble TennesseeVermont white Proctor, Vermont United StatesMarbleYule Marble uniform pure white near Marble, Colorado United StatesWunsiedel white Wunsiedel, Bavaria GermanyMarbleSerpentiniteA sample of serpentinite rock, partially made up of chrysotileBoulder of Serpentinite at Soldiers Delight Natural Environmental Area, Maryland
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in thisgroup are formed by serpentinization, a hydration and metamorphic transformation ofultramafic rock from the Earths mantle. The alteration is particularly important at the sea floor attectonic plate boundaries.FormationSerpentinization is a geological low-temperature metamorphic process involving heat and waterin which low-silica mafic and ultramafic rocks are oxidized (anaerobic oxidation of Fe2+ by theprotons of water leading to the formation of H2) and hydrolyzed with water into serpentinite.Peridotite, including dunite, at and near the seafloor and in mountain belts is converted toserpentine, brucite, magnetite, and other minerals — some rare, such as awaruite (Ni3Fe), andeven native iron. In the process large amounts of water are absorbed into the rock increasing thevolume and destroying the structure. The density changes from 3.3 to 2.7 g/cm3 with a concurrent volume increase of about 40%. Thereaction is exothermic and large amounts of heat energy are produced in the process. Rock temperatures can be raised by about 260 °C,  providing an energy source for formation ofnon-volcanic hydrothermal vents. The magnetite-forming chemical reactions produce hydrogengas under anaerobic conditions prevailing deep in the mantle, far from the Earth atmosphere.Carbonates and sulfates are subsequently reduced by hydrogen and form methane and hydrogensulfide. The hydrogen, methane, and hydrogen sulfide provide energy sources for deep seachemotroph microorganisms.Serpentinite reactionsSerpentinite is formed from olivine via several reactions, some of which are complementary.Olivine is a solid solution between the magnesium-endmember forsterite and the iron-endmember fayalite. Serpentinite reactions 1a and 1b, below, exchange silica between forsteriteand fayalite to form serpentine group minerals and magnetite. These are highly exothermicreactions.Reaction 1a:Fayalite + water → magnetite + aqueous silica + hydrogen 3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2Reaction 1b:Forsterite + aqueous silica → serpentine 3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4
Reaction 1c:Forsterite + water → serpentine + brucite 2Mg2SiO4 + 3H2O → Mg3Si2O5(OH)4 + Mg(OH)2Reaction 1c describes the hydration of olivine with water only to yield serpentine and Mg(OH) 2(brucite). Serpentine is stable at high pH in the presence of brucite like calcium silicate hydrate,(C-S-H) phases formed along with portlandite (Ca(OH)2) in hardened Portland cement paste afterthe hydration of belite (Ca2SiO4), the artificial calcium equivalent of forsterite.Analogy of reaction 1c with belite hydration in ordinary Portland cement:Belite + water → C-S-H phase + portlandite 2 Ca2SiO4 + 4 H2O → 3 CaO · 2 SiO2 · 3 H2O + Ca(OH)2After reaction, the poorly soluble reaction products (aqueous silica or dissolved magnesium ions)can be transported in solution out of the serpentinized zone by diffusion or advection.A similar suite of reactions involves pyroxene-group minerals, though less readily and withcomplication of the additional end-products due to the wider compositions of pyroxene andpyroxene-olivine mixes. Talc and magnesian chlorite are possible products, together with theserpentine minerals antigorite, lizardite, and chrysotile. The final mineralogy depends both onrock and fluid compositions, temperature, and pressure. Antigorite forms in reactions attemperatures that can exceed 600°C during metamorphism, and it is the serpentine group mineralstable at the highest temperatures. Lizardite and chrysotile can form at low temperatures verynear the Earths surface. Fluids involved in serpentinite formation commonly are highly reactiveand may transport calcium and other elements into surrounding rocks; fluid reaction with theserocks may create metasomatic reaction zones enriched in calcium and called rodingites.In the presence of carbon dioxide, however, serpentinitization may form either magnesite(MgCO3) or generate methane (CH4). It is thought that some hydrocarbon gases may beproduced by serpentinite reactions within the oceanic crust.Reaction 2a: Olivine + water + carbonic acid → serpentine + magnetite + methane (Fe,Mg)2SiO4 + nH2O + CO2 → Mg3Si2O5(OH)4 + Fe3O4 + CH4or, in balanced form: 18Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4
Reaction 2b: Olivine + water + carbonic acid → serpentine + magnetite + magnesite + silica (Fe,Mg)2SiO4 + nH2O + CO2 → Mg3Si2O5(OH)4 + Fe3O4 + MgCO3 + SiO2Reaction 2a is favored if the serpentinite is Mg-poor or if there isnt enough carbon dioxide topromote talc formation. Reaction 2b is favored in highly magnesian compositions and low partialpressure of carbon dioxide.The degree to which a mass of ultramafic rock undergoes serpentinisation depends on thestarting rock composition and on whether or not fluids transport calcium, magnesium and otherelements away during the process. If an olivine composition contains sufficient fayalite, thenolivine plus water can completely metamorphose to serpentine and magnetite in a closed system.In most ultramafic rocks formed in the Earths mantle, however, the olivine is about 90%forsterite endmember, and for that olivine to react completely to serpentine, magnesium must betransported out of the reacting volume.Serpentinitization of a mass of peridotite usually destroys all previous textural evidence becausethe serpentine minerals are weak and behave in a very ductile fashion. However, some masses ofserpentinite are less severely deformed, as evidenced by the apparent preservation of texturesinherited from the peridotite, and the serpentinites may have behaved in a rigid fashion.Hydrogen production by anaerobic oxidation of fayalite ferrous ionsIn the absence of atmospheric oxygen (O2), in deep geological conditions prevailing far awayfrom Earth atmosphere, hydrogen (H2) is produced by the anaerobic oxidation of ferrous ions(Fe2+) present in the crystal lattice of the iron-endmember fayalite by the protons (H+) of water.Considering three formula units of fayalite (Fe2(SiO4)) for the purpose of stoechiometry andreaction mass balance, four ferrous ions will undergo oxidation by water protons while the tworemaining will stay unoxidised. Neglecting the orthosilicate anions not involved in the redoxprocess, it is then possible to schematically write the two half-redox reactions as follows: 4 (Fe2+ → Fe3+ + e–) (oxidation of ferrous ions) 2 (H2O + 2 e– → O2– + H2) (reduction of protons into hydrogen)This leads to the global redox reaction involving ferrous ions oxidation by water: 4 Fe2+ + 2 H2O → 4 Fe3+ + 2 O2– + 2 H2
The two unoxidised ferrous (Fe2+) ions still available in the three formula units of fayalite finallycombine with the four ferric (Fe3+) cations and oxide anions (O2–) to form two formula units ofmagnetite (Fe3O4).Finally, considering the required rearrangement of the orthosilicate anions into free silica (SiO 2)and free oxide anions (O2–), it is possible to write the complete reaction of anaerobic oxidationand hydrolysis of fayalite according to the following mass balance: 3 Fe2SiO4 + 2 H2O → 2 Fe3O4 + 3 SiO2 + 3 H2 fayalite + water → magnetite + quartz + hydrogenThis reaction closely resembles the Schikorr reaction observed in the anaerobic oxidation of theferrous hydroxide in contact with water: 3 Fe(OH)2 → Fe3O4 + 2 H2O + H2 ferrous hydroxide → magnetite + water + hydrogenCarbon sequestrationSerpentinite, along with olivine, its precursor, has been proposed as an efficient reagent forcarbon sequestration using the magnesite reaction, mentioned hereabove, or a variation whereserpentine is reacted with carbon dioxide and hydrogen to form magnesite, magnetite, and silica.The ideal composition of olivine or serpentinite for this process is thus highly magnesian, tofavor the production of magnesite and the fixation of carbon.Swiss ovenstoneA variety of chlorite talc schist associated with Alpine serpentinite is found in Val d’Anniviers,Switzerland and was used as ovenstone in stove construction.Serpentinization on MarsThe presence of traces of methane in the atmosphere of Mars has been hypothesized to bepossible evidence for life on Mars. Serpentinization has been proposed as an alternative non-biological source for the observed methane traces. Hornfels
hornfelsHornfels (German, meaning "hornstone," after its frequent association with glacial "horn peaks"in the Alps, being a very hard rock and thus more likely to resist glacial action and form horn-shaped peaks such as Matterhorn) is the group designation for a series of contact metamorphicrocks that have been baked and indurated by the heat of intrusive igneous masses and have beenrendered massive, hard, splintery, and in some cases exceedingly tough and durable.Most hornfels are fine-grained, and while the original rocks (such as sandstone, shale, slate,limestone and diabase) may have been more or less fissile owing to the presence of bedding orcleavage planes, this structure is effaced or rendered inoperative in the hornfels. Though theymay show banding, due to bedding, etc., they break across this as readily as along it; in fact, theytend to separate into cubical fragments rather than into thin plates.The most common hornfels (the biotite hornfelses ) are dark-brown to black with a somewhatvelvety luster owing to the abundance of small crystals of shining black mica. The lime hornfelsare often white, yellow, pale-green, brown and other colors. Green and darkgreen are theprevalent tints of the hornfels produced by the alteration of igneous rocks. Although for the mostpart the constituent grains are too small to be determined by the unaided eye, there are oftenlarger crystals of cordierite, garnet or andalusite scattered through the fine matrix, and these maybecome very prominent on the weathered faces of the rock.StructureThe structure of the hornfels is very characteristic. Very rarely do any of the minerals showcrystalline form, but the small grains fit closely together like the fragments of a mosaic; they areusually of nearly equal dimensions. This has been called pfiaster or pavement structure from theresemblance to rough pavement work. Each mineral may also enclose particles of the others; inthe quartz, for example, small crystals of graphite, biotite, iron oxides, sillimanite or feldsparmay appear in great numbers. Often the whole of the grains are rendered semi-opaque in this
way. The minutest crystals may show traces of crystalline outlines; undoubtedly they are of newformation and have originated in situ. This leads us to believe that the whole rock has beenrecrystallized at a high temperature and in the solid state so that there was little freedom for themineral molecules to build up well-individualized crystals. The regeneration of the rock has beensufficient to efface most of the original structures and to replace the former minerals more-or-less completely by new ones. But crystallization has been hampered by the solid condition of themass and the new minerals are formless and have been unable to reject impurities, but havegrown around them.Compositions of HornfelsSlates, shales and clays yield biotite hornfels in which the most conspicuous mineral is blackmica, the small scales of which are transparent under the microscope and have a dark reddishbrown color and strong dichroism. There is also quartz, and often a considerable amount offeldspar, while graphite, tourmaline and iron oxides frequently occur in lesser quantity. In thesebiotite hornfels the minerals, which consist of aluminiun silicates, are commonly found; they areusually andalusite and sillimanite, but kyanite appears also in hornfels, especially in those thathave a schistose character. The andalusite may be pink and is then often pleochroic in thinsections, or it may be white with the cross-shaped dark enclosures of the matrix that arecharacteristic of chiastolite. Sillimanite usually forms exceedingly minute needles embedded inquartz.In the rocks of this group cordierite also occurs, not rarely, and may have the outlines ofimperfect hexagonal prisms that are divided up into six sectors when seen in polarized light. Inbiotite hornfels, a faint striping may indicate the original bedding of the unaltered rock andcorresponds to small changes in the nature of the sediment deposited. More commonly there is adistinct spotting, visible on the surfaces of the hand specimens. The spots are round or elliptical,and may be paler or darker than the rest of the rock. In some cases they are rich in graphite orcarbonaceous matter; in others they are full of brown mica; some spots consist of rather coarsergrains of quartz than occur in the matrix. The frequency with which this feature reappears in theless altered slates and hornfels is rather remarkable, especially as it seems certain that the spotsare not always of the same nature or origin. Tourmaline hornfels are found sometimes near themargins of tourmaline granites; they are black with small needles of schorl that under themicroscope are dark brown and richly pleochroic. As the tourmaline contains boron, there musthave been some permeation of vapors from the granite into the sediments. Rocks of this groupare often seen in the Cornish tin-mining districts, especially near the ludes.A second great group of hornfels are the calcite-silicate hornfels that arise from the thermalalteration of impure limestone. The purer beds recrystallize as marbles, but where there has beenoriginally an admixture of sand or clay lime-bearing silicates are formed, such as diopside,epidote, garnet, sphene, vesuvianite and scapolite; with these phlogopite, various feldspars,pyrites, quartz and actinolite often occur. These rocks are fine-grained, and though often banded,are tough and much harder than the original limestones. They are excessively variable in theirmineralogical composition, and very often alternate in thin seams with biotite hornfels andindurated quartzites. When perfused with boric and fluoric vapors from the granite they may
contain much axinite, fluorite and datolite, but the altiminous silicates (andalusite, &c.) areabsent from these rocks.From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced.They consist essentially of feldspar with hornblende (generally of brown color) and palepyroxene. Sphene, biotite and iron oxides are the other common constituents, but these rocksshow much variety of composition and structure. Where the original mass was decomposed andcontained calcite, zeolites, chlorite and other secondary minerals either in veins or in cavities,there are usually rounded a reas or irregular streaks containing a suite of new minerals, whichmay resemble those of the calcium-silicate hornfelses above described. The original porphyritic,fluidal, vesicular or fragmental structures of the igneous rock are clearly visible in the lessadvanced stages of hornfelsing, but become less evident as the alteration progresses.In some districts hornfelsed rocks occur that have acquired a schistose structure throughshearing, and these form transitions to schists and gneisses that contain the same minerals as thehornfels, but have a schistose instead of a hornfels structure. Among these may be mentionedcordierite and sillimanite gneisses, andalusite and kyanite mica-schists, and those schistosecalcite-silicate rocks that are known as cipolins. That these are sediments that have undergonethermal alteration is generally admitted, but the exact conditions under which they were formedare not always clear. The essential features of hornfelsing are ascribed to the action of heat,pressure and permeating vapors, regenerating a rock mass without the production of fusion (atleast on a large scale). It has been argued, however, that often there is extensive chemical changeowing to the introduction of matter from the granite into the rocks surrounding it. The formationof new feldspar in the hornfelses is pointed out as evidence of this. While this felspathizationmay have occurred in a few localities, it seems conspicuously absent from others. Mostauthorities at the present time regard the changes as being purely of a physical and not of achemical nature.