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Igneous rock textures
 

Igneous rock textures

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Rock textures

Rock textures

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    Igneous rock textures Igneous rock textures Presentation Transcript

    • IGNEOUS ROCKIGNEOUS ROCK TEXTURESTEXTURES Prepared by Dr. F. Clark,Prepared by Dr. F. Clark, Department of Earth and AtmosphericDepartment of Earth and Atmospheric Sciences, University of AlbertaSciences, University of Alberta Oct. 05Oct. 05
    • ROCK TEXTURES DEFINEDROCK TEXTURES DEFINED The use of the term “texture” as applied to rocks is not at allThe use of the term “texture” as applied to rocks is not at all intuitive. One might expect rock textures to includeintuitive. One might expect rock textures to include possibilities such as smooth, rough, silky, hard, soft,possibilities such as smooth, rough, silky, hard, soft, waxy, or other such properties, but this is not the case.waxy, or other such properties, but this is not the case. Whether applied to igneous or other rocks, the termWhether applied to igneous or other rocks, the term texture embraces the size, shape, and arrangement oftexture embraces the size, shape, and arrangement of mineral grains [the general term we use rather thanmineral grains [the general term we use rather than “crystals”, although mineral grains are crystals, by“crystals”, although mineral grains are crystals, by definition]. It is generally by the texture that a geologistdefinition]. It is generally by the texture that a geologist will first recognize whether a rock is igneous,will first recognize whether a rock is igneous, sedimentary, or metamorphic, before they get down tosedimentary, or metamorphic, before they get down to the business of identifying the rock or naming it.the business of identifying the rock or naming it.
    • IGNEOUS ROCK TEXTURES -IGNEOUS ROCK TEXTURES - PRINCIPLEPRINCIPLE The fundamental principle behind igneous rock textures isThe fundamental principle behind igneous rock textures is that grain size is controlled by cooling rate. Thus, rapidthat grain size is controlled by cooling rate. Thus, rapid cooling at the Earth’s surface of extrusive moltencooling at the Earth’s surface of extrusive molten material, or lava, results in the growth of smaller crystals,material, or lava, results in the growth of smaller crystals, or prevents crystal growth altogether. Conversely, slowor prevents crystal growth altogether. Conversely, slow cooling within the Earth’s crust of intrusive moltencooling within the Earth’s crust of intrusive molten material, called magma, results in the growth of fewermaterial, called magma, results in the growth of fewer but larger crystals, because atoms are able to migratebut larger crystals, because atoms are able to migrate through the liquid to attach themselves to crystals thatthrough the liquid to attach themselves to crystals that have already begun to form. The many igneous rockhave already begun to form. The many igneous rock textures are simply variations on or modifications of thistextures are simply variations on or modifications of this principle.principle.
    • Igneous Extrusive, or Volcanic, RocksIgneous Extrusive, or Volcanic, Rocks These rocks, formed by cooling and solidification (notThese rocks, formed by cooling and solidification (not always crystallization) of lava, are typically fine-grained,always crystallization) of lava, are typically fine-grained, to the extent that detailed analysis of the mineralogy ofto the extent that detailed analysis of the mineralogy of these rocks is only possible with the high magnificationthese rocks is only possible with the high magnification of a petrographic microscope, or even higher resolutionof a petrographic microscope, or even higher resolution techniques. As we shall see in the file on igneous rocktechniques. As we shall see in the file on igneous rock classification, the colour of the rock is an important clueclassification, the colour of the rock is an important clue to its bulk or average composition, which controls itsto its bulk or average composition, which controls its mineralogy, and other clues may tell us what mineralsmineralogy, and other clues may tell us what minerals are present.are present.
    • AphaniticAphanitic Crystals areCrystals are uniformly fine-uniformly fine- grained andgrained and interlocking,interlocking, with individualwith individual crystalscrystals invisible to theinvisible to the unaided eye.unaided eye. This is the generic, base texture for most igneous extrusive rocks, theThis is the generic, base texture for most igneous extrusive rocks, the fine-grained nature being a result of rapid cooling that prevents growthfine-grained nature being a result of rapid cooling that prevents growth of large crystals. Even with a hand lens, virtually nothing can beof large crystals. Even with a hand lens, virtually nothing can be identified, and such rocks can be very frustrating to deal with.identified, and such rocks can be very frustrating to deal with.
    • PorphyriticPorphyritic Large, evidentLarge, evident crystals calledcrystals called phenocrystsphenocrysts [red arrows][red arrows] are surroundedare surrounded by an aphaniticby an aphanitic matrix ormatrix or groundmassgroundmass [blue arrows].[blue arrows]. This texture represents two stage cooling. Slower cooling of magmaThis texture represents two stage cooling. Slower cooling of magma within the crust leads to growth of the phenocrysts, whose early growthwithin the crust leads to growth of the phenocrysts, whose early growth leads to the development of well-formed faces. Once erupted as lava,leads to the development of well-formed faces. Once erupted as lava, the remaining liquid crystallizes as the aphanitic groundmass.the remaining liquid crystallizes as the aphanitic groundmass.
    • VesicularVesicular Vesicles areVesicles are simply bubblessimply bubbles produced byproduced by gasesgases escaping fromescaping from lava as itlava as it solidifies.solidifies. The vesicles [red arrows] are in a full range of sizes, enclosed in anThe vesicles [red arrows] are in a full range of sizes, enclosed in an aphanitic groundmass [blue arrows]. The few slightly larger crystalsaphanitic groundmass [blue arrows]. The few slightly larger crystals [green arrows] are of the mineral olivine, a common constituent of[green arrows] are of the mineral olivine, a common constituent of Hawaiian basalt volcanic rocks such as this one.Hawaiian basalt volcanic rocks such as this one.
    • VesiclesVesicles andand XenolithsXenoliths This sample fromThis sample from the previous slidethe previous slide shows a moreshows a more complex face. Thecomplex face. The red, blue, andred, blue, and green arrows aregreen arrows are the same as in thethe same as in the previous slide.previous slide. Volatiles dissolved in magma under high pressure within the EarthVolatiles dissolved in magma under high pressure within the Earth escape when lava is erupted, like COescape when lava is erupted, like CO22 from soda pop. A xenolithfrom soda pop. A xenolith [literally, “foreign rock”, purple arrow] is a fragment of a previously[literally, “foreign rock”, purple arrow] is a fragment of a previously crystallized rock incorporated in the magma as it nears the surface.crystallized rock incorporated in the magma as it nears the surface.
    • Scoria – a Highly Vesicular BasaltScoria – a Highly Vesicular Basalt If basalt, which is a mafic [high Fe+Mg content, low silica content] volcanic rock, is highly vesicular, then we have scoria, which for lack of a better way to describe it, resembles the guts or interior of an Aero chocolate bar. Note how the iron content in this chemically unstable mafic rock weathers to produce a rust-coloured surface [brown arrows], versus the greenish grey fresh surface [cut surface; green arrows].
    • PumicePumice This is anThis is an extremelyextremely vesicular felsicvesicular felsic [low Fe+Mg, high[low Fe+Mg, high silica content]silica content] volcanic rock. Thisvolcanic rock. This material willmaterial will actually float onactually float on water because itwater because it has so manyhas so many bubbles orbubbles or vesicles.vesicles. Gases do not escape easily from high viscosity [stiff] felsic lavas, andGases do not escape easily from high viscosity [stiff] felsic lavas, and so a light-coloured volcanic froth is produced. Pumice is used in theso a light-coloured volcanic froth is produced. Pumice is used in the cosmetic industry as an exfoliant, effective at removing calluses due tocosmetic industry as an exfoliant, effective at removing calluses due to the abrasive nature of the glass-hard frothy surface.the abrasive nature of the glass-hard frothy surface.
    • Amygdaloidal – With Filled VesiclesAmygdaloidal – With Filled Vesicles It may be that a vesicular rock, such as this basalt [red arrows point to vesicles set in the aphanitic groundmass highlighted by blue arrows], has fluids circulating through the vesicles, from which minerals may precipitate or crystallize. These deposits, with rounded outlines reflecting their origin as bubble-filling, are called amygdules [purple arrows]. Phenocrysts would be angular in outline.
    • GlassyGlassy Very rapid coolingVery rapid cooling may preventmay prevent crystal growthcrystal growth altogether, and wealtogether, and we get naturalget natural volcanic glass,volcanic glass, called obsidian.called obsidian. Note theNote the conchoidalconchoidal fracture [greenfracture [green arrows] we wouldarrows] we would expect for glass.expect for glass. Note the contrast between the black fresh surface [yellow arrows] andNote the contrast between the black fresh surface [yellow arrows] and the rusty brown weathered surface [red arrows], which reveals thethe rusty brown weathered surface [red arrows], which reveals the minor iron content that stains the glass black. This is a black igneousminor iron content that stains the glass black. This is a black igneous rock whose colour index [% mafic minerals] is nevertheless zero.rock whose colour index [% mafic minerals] is nevertheless zero.
    • GlassyGlassy This texture isThis texture is more likely inmore likely in felsic [high silicafelsic [high silica content, notcontent, not necessarily asnecessarily as quartz] lavas,quartz] lavas, which are viscouswhich are viscous or stiff. This sharpor stiff. This sharp natural glass cannatural glass can be fashioned intobe fashioned into effective weaponeffective weapon tips.tips. Obsidian is an exception to the definition of rocks, which among otherObsidian is an exception to the definition of rocks, which among other things are aggregates of one or more minerals, which by definition arethings are aggregates of one or more minerals, which by definition are crystalline solids. Glass is an amorphous solid, which means it lackscrystalline solids. Glass is an amorphous solid, which means it lacks the ordered arrangement of atoms that characterizes crystals.the ordered arrangement of atoms that characterizes crystals.
    • Igneous Intrusive, or Plutonic, RocksIgneous Intrusive, or Plutonic, Rocks The coarse crystal size associated with slow cooling meansThe coarse crystal size associated with slow cooling means that the hand specimen properties of minerals can bethat the hand specimen properties of minerals can be easily applied and exploited to identify the mineralseasily applied and exploited to identify the minerals present in the rock. This may extend as far as being ablepresent in the rock. This may extend as far as being able to recognize cleavage intersection angles in the fewto recognize cleavage intersection angles in the few crystals that may be favourably oriented in the samplecrystals that may be favourably oriented in the sample under consideration.under consideration.
    • Phaneritic – With Evident CrystalsPhaneritic – With Evident Crystals Igneous intrusive rocks have evident crystals [the Greek word phaneros means visible or evident] that one can easily distinguish with the unaided eye, even if one doesn’t have the skill to identify what minerals they are. In these samples, one sees grey glassy quartz, black biotite and amphibole, and cream-coloured potassium feldspar.
    • Phaneritic – the Igneous Intrusive StaplePhaneritic – the Igneous Intrusive Staple Although these samples have smaller crystals than the previous two, the individual crystals or mineral grains are still readily distinguished without magnification. The coarse crystal size makes many igneous intrusive rocks quite attractive, and they are also durable and reasonably stable chemically. This makes them good choices for grave markers and facing stone for buildings.
    • Phaneritic – Seeing Cleavage on GrainsPhaneritic – Seeing Cleavage on Grains The coarse grain size of phaneritic intrusive rocks allows us to see cleavage faces developed on many grains. Essentially, when the rock is stressed and breaks when samples are taken, many of the randomly oriented crystals have their cleavage directions more or less parallel to the face of the sample, and so they break along cleavage. This gives excellent reflections [blue arrows] from potassium feldspar in this case.
    • PhaneriticPhaneritic andand PorphyriticPorphyritic PorphyriticPorphyritic textures aretextures are classicallyclassically developed indeveloped in extrusiveextrusive rocks, but it isrocks, but it is not restricted tonot restricted to them.them. This mafic intrusive rock is called a gabbro, a mixture of pyroxene andThis mafic intrusive rock is called a gabbro, a mixture of pyroxene and plagioclase feldspar. The bulk of the rock is phaneritic, with grainsplagioclase feldspar. The bulk of the rock is phaneritic, with grains approximately 1-2 mm across. However, there is a megacryst, a largeapproximately 1-2 mm across. However, there is a megacryst, a large pyroxene crystal, whose extent is indicated by the yellow arrows.pyroxene crystal, whose extent is indicated by the yellow arrows.
    • Phaneritic and PorphyriticPhaneritic and Porphyritic The same specimen as in the previous slide, we take advantage of the development of cleavage to highlight the megacryst [width indicated by yellow arrow], which is somewhat obscure in the previous image. The cleavage direction meets the surface of the specimen at something like 30 degrees, but the crystal still breaks along cleavage in a series of several steps, which reflect at the same time.
    • PegmatitePegmatite This term appliesThis term applies to extremelyto extremely coarse-grainedcoarse-grained igneous intrusiveigneous intrusive rocks, usually of arocks, usually of a felsic composition.felsic composition. Some restrict theSome restrict the term to rocks withterm to rocks with a grain sizea grain size exceeding 2 cm,exceeding 2 cm, but others allowbut others allow more latitude.more latitude. In the late stages of cooling, volatiles tend to be concentrated in theIn the late stages of cooling, volatiles tend to be concentrated in the magma. This lowers magma viscosity, accounting for the abnormallymagma. This lowers magma viscosity, accounting for the abnormally large crystals. In this specimen, there are crystals of an unusuallarge crystals. In this specimen, there are crystals of an unusual turquoise variety of potassium feldspar called amazonite [red arrows].turquoise variety of potassium feldspar called amazonite [red arrows].
    • PegmatitePegmatite Finer crystalsFiner crystals on the upperon the upper surface of thesurface of the specimen mayspecimen may represent morerepresent more rapid cooling atrapid cooling at the exterior ofthe exterior of the intrusion;the intrusion; field data couldfield data could confirm this.confirm this. The large crystals [see cleavage face, yellow arrow] may be gemThe large crystals [see cleavage face, yellow arrow] may be gem quality crystals, often of minerals that are significant sources of unusualquality crystals, often of minerals that are significant sources of unusual elements such as lithium, fluorine, and boron. Pegmatites are thuselements such as lithium, fluorine, and boron. Pegmatites are thus attractive as well as economically significant.attractive as well as economically significant.