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rocks and minerals

  1. 1. About the pagination of this eBookDue to the unique page numbering scheme of this book, theelectronic pagination of the eBook does not match the paginationof the printed version. To navigate the text, please use theelectronic Table of Contents that appears alongside the eBook orthe Search function.For citation purposes, use the page numbers that appear in the text.
  2. 2. ROCKS AND MINERALSBritannica Illustrated Science Library Encyclopædia Britannica, Inc. Chicago ■ London ■ New Delhi ■ Paris ■ Seoul ■ Sydney ■ Taipei ■ Tokyo
  3. 3. Britannica IllustratedScience Library© 2008 Editorial Sol 90All rights reserved.Idea and Concept of This Work: Editorial Sol 90Project Management: Fabián CassanPhoto Credits: Corbis, ESA, Getty Images, Graphic News,NASA, National Geographic, Science Photo LibraryIllustrators: Guido Arroyo, Pablo Aschei, Gustavo J. Caironi,Hernán Cañellas, Leonardo César, José Luis Corsetti, VaninaFarías, Joana Garrido, Celina Hilbert, Isidro López, DiegoMartín, Jorge Martínez, Marco Menco, Ala de Mosca, DiegoMourelos, Eduardo Pérez, Javier Pérez, Ariel Piroyansky, ArielRoldán, Marcel Socías, Néstor Taylor, Trebol Animation, JuanVenegas, Coralia Vignau, 3DN, 3DOM studio, Jorge Ivanovich,Fernando Ramallo, Constanza Vicco, Diego MourelosComposition and Pre-press Services: Editorial Sol 90Translation Services and Index: Publication Services, Inc.Portions © 2008 Encyclopædia Britannica, Inc.Encyclopædia Britannica, Britannica, and the thistle logo areregistered trademarks of Encyclopædia Britannica, Inc.Britannica Illustrated Science Library Staff Encyclopædia Britannica, Inc.Editorial Jacob E. Safra, Chairman of the BoardMichael Levy, Executive Editor, Core EditorialJohn Rafferty, Associate Editor, Earth Sciences Jorge Aguilar-Cauz, PresidentWilliam L. Hosch, Associate Editor, Mathematics and Computers Michael Ross, Senior Vice President, Corporate DevelopmentKara Rogers, Associate Editor, Life SciencesRob Curley, Senior Editor, Science and Technology Dale H. Hoiberg, Senior Vice President and EditorDavid Hayes, Special Projects Editor Marsha Mackenzie, Director of ProductionArt and CompositionSteven N. Kapusta, DirectorCarol A. Gaines, Composition SupervisorChristine McCabe, Senior Illustrator International Standard Book Number (set): 978-1-59339-797-5Media Acquisition International Standard Book Number (volume):Kathy Nakamura, Manager 978-1-59339-799-9 Britannica Illustrated Science Library:Copy Department Rocks and Minerals 2008Sylvia Wallace, DirectorJulian Ronning, Supervisor Printed in ChinaInformation Management and RetrievalSheila Vasich, Information ArchitectProduction ControlMarilyn L. BartonManufacturingKim Gerber, Director
  4. 4. Rocks and Minerals
  5. 5. Contents PHOTOGRAPH ON PAGE 1 A stone with a blue opal in its center is a product of time, since it forms over millions of years.Dynamics ofthe Earths CrustPage 6MineralsPage 18Formation andTransformationof RocksPage 40Classes of RocksPage 60Use of Rocksand MineralsPage 76
  6. 6. THE MONKS HOUSE This orthodox monk lives in a volcanic cave, very close to the 11 Christian churches located in the Ethiopian town of Lalibela. thing is that there will be rocks. Only stones will remain, and their chemical composition, shape, and texture will provide clues about previous geological events and about what the Earths surface was like in the past. In the pages of this book, illustrated with stunning images, you will find invaluable information about the language of rocks and natural forces inMemory of silent witnesses to the cataclysms our planet has experienced. They know the cold of the glacial era, the intense heat of general. You will also learn to identify the most important minerals, know their physical and chemical properties, andthe Planet the Earths interior, and the fury of the oceans. They store much information about how external agents, such as wind, discover the environments in which they form. rain, ice, and temperature changes, have id you know that the Earths crustR ocks, like airplane flight recorders, store in their interior very useful information about what has been altering the planets surface for millions of years. D and its oceans are sources of useful and essential minerals for human beings? Coal, petroleum, and natural gashappened in the past. Whether forming or ancient civilizations, stones found in the crust allow us to travel and tocaves in the middle of mountains, mixedamong folds, or lying at the bottom oflakes and oceans, stones are everywhere, F symbolized eternity. This idea has persisted throughout time because stones endure, but they are recycled time heat our homes. Furthermore, practically all the products that surround us have elements provided by rocks and minerals.and they hold clues to the past. By and again. Fifty million years from now, For example, aluminum is used to producestudying rocks, we can reconstruct the nothing will be as we now know it—not beverage cans; copper is used in electrichistory of the Earth. Even the most the Andes, nor the Himalayas, nor the ice cables; and titanium, mixed with otherinsignificant rocks can tell stories about of Antarctica, nor the Sahara Desert. durable metals, is used in the constructionother times, because rocks have been Weathering and erosion, though slow, will of spacecraft. We invite you to enjoy thisaround since the beginning of the universe. never stop. This should free us from any book. It is full of interesting and worthwhileThey were part of the cloud of dust and illusion of the immortality of the Earths information. Dont miss out on it!gases that revolved around the Sun over features. What will everything be like infour billion years ago. Rocks have been the future? We dont know. The only sure
  7. 7. Dynamics of the Earths Crust MOUNTAINS OF SAND Corkscrew Canyon in Arizona contains an array of shapes, colors, and textures. The sand varies from pink to yellow to red depending on TRAVERSING TIME 8-11 UNDER CONSTRUCTION 12-13 A CHANGING SURFACE 14-15 the sunlight it receives. BEFORE ROCK, MINERAL 16-17T he Earth is like a blender in weathering and erosion by wind and rain sediment that eventually become which rocks are moved around, wear down and transform the rock. This sedimentary rock. This rock cycle never broken, and crumbled. The produces mountains, cliffs, and sand stops. In 50 million years, no single fragments are deposited, dunes, among other features. The mountain we know will exist in the same forming different layers. Then deposited material settles into layers of condition as it does today.
  8. 8. 8 DYNAMICS OF THE EARTH’S CRUST ROCKS AND MINERALS 9Traversing Time 2 COLLISION AND FUSION Heavy elements migrate. 3 METALLIC CORE The light elements form the mantle. eologists and paleontologists use many sources to reconstructG the Earths history. The analysis of rocks, minerals, and fossils found on the Earths surface provides data about thedeepest layers of the planets crust and reveals both climatic andatmospheric changes that are often associated withcatastrophes. Craters caused by the impact of meteorites and THE CORE The Earths core isother bodies on the surface of the Earth also reveal valuableinformation about the history of the planet. extremely hot and is made mostly of iron and nickel. Mountains are external folds of the crust produced by extremely powerful forces occurring inside the Earth.Complex The region that will The fragments ofStructure THE FORMATION OF THE INTERIOR Cosmic materials began to 1 become North America moves toward the Equator, thus initiating continents combine to form a single continent called Pangea.accumulate, forming a growing celestial Small bodies and dust accumulate 542 OROGENIES the development of thebody, the precursor of the Earth. High to become the size The supercontinent Geological history recognizes long periods (lasting most important The Appalachiantemperatures combined with gravity of an asteroid. Panotia forms, containing millions of years) of intense mountain formation carboniferous formations. Mountains form.caused the heaviest elements to portions of present-day called orogenies. Each orogeny is characterized by Gondwana moves slowly; The formation of slatemigrate to the center of the planet continents. North America its own particular materials and location. the ocean floor spreads through sedimentation isand the lighter ones to move toward separates from Panotia. at a similar speed. at its peak.the surface. Under a rain of meteors, The oldest 1,100 The first major Laurentia andthe external layers began to minerals, such as Rodinia, an early orogeny Baltica converge, Baltica and Siberiaconsolidate and form the Earths crust. zircon, form. supercontinent, (Caledonian creating the clash, forming the UralIn the center, metals such as iron forms. folding) begins. Caledonian range. Mountains.concentrated into a red-hot nucleus. The oldest rocks Gondwana moves Gneiss forms on metamorphose, A meteorite falls in toward the South the coast of Eruptions of basalt forming gneiss. Sudbury, Ontario, Pole. Scotland. occur in Siberia. Canada.Age in millionsof years 4,600 2,500 542 488.3 443.7 416 359.2 299ERA Hadean Proterozoic Paleozoic THE ERA OF PRIMITIVE LIFEPERIOD Pregeologic Precambrian Cambrian Ordovician Silurian Devonian Carboniferous PermianEPOCHClimate Consolidation The Earth cools 2,500 Temperatures fall. It is thought that the By this period, Temperatures were Hot, humid climates The largest carbon begins under a and the first The level of carbon Earths atmosphere vertebrates with typically warmer than produce exuberant deposits we observe Glaciations: White Earth rain of meteors. ocean is formed. dioxide (CO2) in the contained far less carbon mandibles, such today, and oxygen forests in today form where The Earth undergoes the first of its atmosphere is 16 dioxide during the as the placoderms, (O2) levels attained swamplands. forests previously massive global cooling events times higher than it Ordovician than today. osteichthyans their maximum. existed. ELEMENTS PRESENT ACCORDING TO THE TABLE (glaciations). is today. Temperatures fluctuate (bony fish), and Existing in different combinations, the crust of the Earth within a range similar to acanthodians, O contains the same elements today as those that were present when the planet was formed. The most abundant 46.6% 800 Second glaciation what we experience today. have already emerged. element in the crust is oxygen, which bonds with metals Si Metals Transition metals 600 Last massive glaciation and nonmetals to form different compounds. 27.7% NonmetalsLife Noble gases THE FIRST ANIMALS THE CAMBRIAN EXPLOSION SILURIAN Amphibians diversify Palm trees and Mg Lanthanide series Ca Among the most mysterious fossils of the Fossils from this time attest to One of the first and reptiles originate conifers replace the 2.1% Actinide series 3.6% the great diversity of marine pisciform vertebrates, from one amphibian vegetation from the Na Precambrian Period are the remains of the Al an armored fishK 2.8% Fe 8.1% Ediacaran fauna, the Earths first-known animals and the emergence group to become the Carboniferous Period. without mandibles2.6% 5.0% animals. They lived at the bottom of the of different types of first amniotes. Winged ocean. Many were round and reminiscent of skeletal structures, such insects such as MASS EXTINCTION as those found in sponges The rocks of this period dragonflies emerge. Near the end of the jellyfish, while others were flat and sheetlike. and trilobites. contain an abundance Permian Period, an of fish fossils. estimated 95 percent of TRILOBITES marine organisms and over Marine arthropods Areas of solid ground two thirds of terrestrial with mineralized are populated by ones perish in the greatest exoskeletons gigantic ferns. known mass extinction.
  9. 9. 10 DYNAMICS OF THE EARTH’S CRUST ROCKS AND MINERALS 11IMPACT FROM THE OUTSIDE The heat caused by theIt is believed that a large meteor fell onChicxulub, on the Yucatán Peninsula(Mexico), about 65 million years ago. The expansion of fragments from the impact together with the greenhouse effect Elements in CRUST The Earths crust can reachimpact caused an explosion that created acloud of ash mixed with carbon rocks. Whenthe debris fell back to Earth, some experts brought about by the spreading of ashes in the stratosphere provoked a Equilibrium a thickness of up to 6 miles (10 km) at the bottom of the ocean and up to 30 miles (50 km) on the continents.believe it caused a great global fire. series of climatic changes. Minerals, such as iron and silicates, are It is believed that this widely spread among the major constituents process resulted in the of the crust. Only the movements of the62 miles extinction of the dinosaurs. crust on the molten mantle disrupt their equilibrium.(100 km)The diameter of the crater produced by North America and LITHOSPHEREthe impact of the meteor on the Yucatán The solid rock coating Europe drift apart.Peninsula. It is now buried under almost of the Earth, which North and South2 miles (3 km) of limestone. includes the exterior of America are joined at the mantle the end of this time period. The formation of MANTLE Patagonia concludes, The mantle is 1,800 miles and an important (2,900 km) thick and is overthrust raises the composed mainly of solid rock. Its temperature Andes mountain range. increases with depth. A notable component of the upper mantle is the asthenosphere, which is FORMATION OF semisolid. In the asthenosphere, MOUNTAIN CHAINS superficial rock layers that will Gondwana eventually form the Earths reappears. 60 Central Rocky Mountains crust are melted. 30 Alps Africa separates 20 Himalayas The African Rift Zone and from South America, the Red Sea open up. The and the South Atlantic Indian protocontinent Ocean appears. collides with Eurasia. 251 Mesozoic 199.6 THE ERA OF REPTILES 145.5 65.5 Cenozoic THE AGE OF MAMMALS 23.03 Triassic Jurassic Cretaceous Paleogene Neogene Paleocene Eocene Oligocene Miocene Pliocene Pleistocene Holocene THE AGE OF FLOWERING PLANTS CORE The level of oxygen At the end of the Cretaceous Period, THE LAST GLACIATION Outer Core Carbon dioxide The global Temperatures drop The outer core is 1,400 (O2) in the the first angiosperms—plants with The most recent period of levels increase. average to levels similar to miles (2,270 km) thick atmosphere is much protected seeds, flowers, and glaciation begins three million years Average temperature is those of today. The and contains melted iron, lower than today. fruits—appear. ago and intensifies at the beginning nickel, and other minor temperatures at least 62° F lower temperatures of the Quaternary period. North chemical compounds. are higher than (17° C). The ice cause forests to Pole glaciers advance, and much of today. layer covering shrink and grasslands Inner Core the Northern Hemisphere becomes The inner core has a diameter of Antarctica later to expand. covered in ice. 756 miles (1,216 km). It is made of thickens. iron and nickel, which are solidified due to their exposure to high pressure and temperature conditions. Vast development ANOTHER MASS EXTINCTION of feathered bird HUMAN BEINGS APPEAR ON EARTH. Proliferation of Birds emerge. Toward the end of the Cretaceous species and Although the oldest hominid fossils insects Period, about 50 percent of existing mammals covered (Sahelanthropus) date back to seven million The dinosaurs species disappear. The dinosaurs, the with long fur years ago, it is believed that modern humans Appearance of undergo adaptive large marine reptiles (such as the emerged in Africa at the end of the dinosaurs radiation. Plesiosaurs), the flying creatures of that Pleistocene. Humans migrated to Europe period (such as the Pterosaurs), and the 100,000 years ago, although settling there ammonites (cephalopod mollusks) was difficult because of the glacial climate. The first mammals ALLOSAURUS disappear from the Earth. At the MAMMOTHS According to one hypothesis, our ancestors evolve from a group This carnivore beginning of the Cenozoic Era, most of Mammoths lived in Siberia. reached the American continent about of reptiles called measured 39 feet the habitats of these extinct species The cause of their extinction 10,000 years ago by traveling across the Therapsida. (12 m) long. begin to be occupied by mammals. is still under debate. area now known as the Bering Strait.
  10. 10. 12 DYNAMICS OF THE EARTH’S CRUST ROCKS AND MINERALS 13Under Construction KILAUEA CRATER Hawaii ur planet is not a dead body, complete and unchanging. It is an ever-changing system whoseO activity we experience all the time: volcanoes erupt, earthquakes occur, and new rocks emerge on the Earths surface. All these phenomena, which originate in the interior of theplanet, are studied in a branch of geology called internal geodynamics. This science analyzes Latitude 19° N Longitude 155° Wprocesses, such as continental drift and isostatic movement, which originate with themovement of the crust and result in the raising and sinking of large areas. Themovement of the Earths crust also generates the conditions that form new rocks.This movement affects magmatism (the melting of materials that solidifyto become igneous rocks) and metamorphism (the series oftransformations occurring in solid materials that give rise tometamorphic rocks).Magmatism Metamorphism Folding Fracture Magma is produced when the temperature in the mantle or crust reaches a level at An increase in pressure and/or temperature causes Although solid, the materials forming the Earths When the forces acting upon rocks become too intense, which minerals with the lowest fusion point begin to melt. Because magma is less rocks to become plastic and their minerals to crust are elastic. The powerful forces of the Earth the rocks lose their plasticity and break, creating twodense than the solid material surrounding it, it rises, and in so doing it cools and begins to become unstable. These rocks then chemically react with place stress upon the materials and create folds in the types of fractures: joints and faults. When this process happenscrystallize. When this process occurs in the interior of the crust, plutonic or intrusive the substances surrounding them, creating different rock. When this happens, the ground rises and sinks. When too abruptly, earthquakes occur. Joints are fissures and cracks,rocks, such as granite, are produced. If this process takes place on the outside, volcanic chemical combinations and thus causing new rocks to this activity occurs on a large scale, it can create mountain whereas faults are fractures in which blocks are displacedor effusive rocks, such as basalt, are formed. form. These rocks are called metamorphic rocks. Examples ranges or chains. This activity typically occurs in the parallel to a fracture plane. of this type of rock are marble, quartzite, and gneiss. subduction zones.OUTER PRESSURE TEMPERATURE FOLDS RUPTURECRUST This force gives rise to new High temperatures make For folds to form, rocks When rocksVolcanic Crust metamorphic rocks, as older the rocks plastic and must be relatively rupture quickly, anrocks rocks fuse with the minerals their minerals unstable. plastic and be acted earthquake occurs. Sea that surround them. upon by a force. Oceanic Level Plate Zone of 62 miles SubductionINNERCRUST Magmatic (100 km)Plutonic ChamberRocks 124 miles Convective (200 km) Currents Asthenosphere
  11. 11. 14 DYNAMICS OF THE EARTH’S CRUST ROCKS AND MINERALS 15A Changing Surface CORKSCREW he molding of the Earths crust is the product of two great destructive forces: weathering and CANYONT erosion. Through the combination of these processes, rocks merge, disintegrate, and join again. Living organisms, especially plant roots and digging animals, cooperate withthese geologic processes. Once the structure of the minerals Arizona Latitude 36° 30´ N Longitude 111° 24´ Wthat make up a rock is disrupted, the mineralsdisintegrate and fall to the mercy of therain and wind, which erode them. Water currentErosion Weathering CHEMICAL External agents, such as water, wind, air, and living Mechanical agents can disintegrate rocks, and PROCESSES beings, either acting separately or together, wear The mineral components chemical agents can decompose them. Disintegration of rocks are altered.down, and their loose fragments may be transported. and decomposition can result from the actions of plant They either become newThis process is known as erosion. In dry regions, the roots, heat, cold, wind, and acid rain. The breaking down of minerals or are releasedwind transports grains of sand that strike and rock is a slow but inexorable process. in solution.polish exposed rocks. On the coast, wave River Caveaction slowly eats away at the rocks. MECHANICAL PROCESSES Limestone TEMPERATURE A variety of forces can cause rock When the temperature of the fragments to break into smaller air changes significantly over a pieces, either by acting on the rocks few hours, it causes rocks to Wind directly or by transporting rock expand and contract abruptly. Transportation and fragments that chip away at the rock surface. The daily repetition of this phenomenon can cause rocks Sedimentation to rupture. In this process, materialsEOLIAN HYDROLOGIC PROCESSES eroded by the wind or water All types of moving water slowly wear are carried away andPROCESSES down rock surfaces and carry loose deposited at lower elevations,The wind drags small particles particles away. The size of the particles that WATER and these new deposits canagainst the rocks. This wears them are carried away from the rock surface In a liquid or frozen state, later turn into other rocks.down and produces new deposits depends on the volume and speed of the water penetrates into theof either loess or sand depending flowing water. High-volume and high- rock fissures, causing themon the size of the particle. velocity water can move larger particles. to expand and shatter.
  12. 12. 16 DYNAMICS OF THE EARTH’S CRUST ROCKS AND MINERALS 17Before Rock, Mineral he planet on which we live can be seen as a large rock or, more precisely, as a From Minerals to RocksT large sphere composed of many types of rocks. These rocks are composed of tiny fragments of one or more materials. These materials are minerals, whichresult from the interaction of different chemical elements, each of which is stable From a chemical perspective, a mineral is a homogeneous substance. A rock, on the other hand, is composed of different chemical substances, which, in turn, are components of minerals. The QUARTZ Composed of silica, quartz gives rock a white color.only under specific conditions of pressure and temperature. Both rocks and mineral components of rocks are also those of mountains. Thus, according to this perspective, it isminerals are studied in the branches of geology possible to distinguish between rocks and minerals.called petrology and mineralogy. MICA Composed of thin, shiny sheets of silicon, aluminum, potassium,12 millionago and other minerals, mica can be black or colorless. years GRANITE Rock composed of FELDSPAR A light-colored feldspar, quartz, and silicate, feldspar rock batholiths formed during a mica makes up a large period of great volcanic activity part of the crust. and created the Torres del Paine and its high mountains. TORRES DEL PAINE Chilean Patagonia Latitude 52° 20´ S Longitude 71° 55´ WComposition GraniteHighest summit Paine Grande (10,000 feet [3,050 m]) CHANGE OF STATESurface 598 acres (242 ha) Temperature and pressure play a prominent part in rock transformation. Inside the Earth, liquid magma is produced.Torres del Paine National Park is located in Chile When it reaches the surface, it solidifies. A similar processbetween the massif of the Andes and the Patagonian happens to water when it freezes upon reaching 32° F (0° C).steppes.
  13. 13. Minerals DALLOL VOLCANO Located in Ethiopia, Dallol is the only non- oceanic volcano on Earth below sea level, making it one of the hottest places on the planet. Sulfur and other minerals that spring YOU ARE WHAT YOU HAVE 20-21 A QUESTION OF STYLE 22-23 HOW TO RECOGNIZE MINERALS 24-25 CRYSTALLINE SYMMETRY 30-31 PRECIOUS CRYSTALS 32-33 DIAMONDS IN HISTORY 34-35 from this volcano create very vivid colors. THE MOST COMMON MINERALS 36-37 A DESERT OF MINERALS 26-27 THE ESSENCE OF CRYSTALS 28-29 THE NONSILICATES 38-39 Graphite, for instance, is used toD allol is basically a desert of shades of orange. Some minerals Did you know it took human beings minerals whose ivory- belong to a very special class. thousands of years to separate metal make pencils; gypsum is used in colored crust is scattered Known as gems, they are sought and from rock? Did you also know that construction; and halite, also known with green ponds and hoarded for their great beauty. The certain nonmetallic minerals are as salt, is used in cooking. towers of sulfur salts in most valuable gems are diamonds. valued for their usefulness?
  14. 14. 20 MINERALS ROCKS AND MINERALS 21You Are What You Have Polymorphism inerals are the “bricks” of materials that make up theM Earth and all other solid bodies in the universe. They are usually defined both by their chemical composition and bytheir orderly internal structure. Most are solid crystalline A phenomenon in which the same chemical composition can create multiple structures and, consequently, result in the creation of several different minerals. The transition of onesubstances. However, some minerals have a disordered internal polymorphous variant into another, facilitated by temperature or pressurestructure and are simply amorphous solids similar to glass. conditions, can be fast or slow and eitherStudying minerals helps us to understand the origin of the Earth. reversible or irreversible.Minerals are classified according to their composition andinternal structure, as well as by the properties of hardness,weight, color, luster, and transparency. Although more than Chemical Crystallization Mineral4,000 minerals have been discovered, only about 30 are Composition Systemcommon on the Earths surface. CaCO3 Trigonal Calcite CaCO3 Rhombic Aragonite FeS2 Cubic PyriteComponents MINERALS COME FROM FeS2 Rhombic Marcasite The basic components of minerals are the chemical elements listed on the periodictable. Minerals are classified as native if they arefound in isolation, contain only one element, andoccur in their purest state. On the other hand, they 112 elements C C Cubic Hexagonal Diamond Graphiteare classified as compound if they are composed of listed in thetwo or more elements. Most minerals fall into the periodic table.compound category. DIAMOND AND GRAPHITE A minerals internal structure influences its hardness. Both 1 NATIVE MINERALS graphite and diamond are composed only of carbon; however, These minerals are classified into: they have different degrees of hardness.A- METALS AND INTERMETALSNative minerals have high thermal and electrical Diamond Graphiteconductivity, a typically metallic luster, lowhardness, ductility, and malleability. They are easyto identify and include gold, copper, and lead.GOLD SILVERAn excellent thermal and electrical conductor. The close-upAcids have little or no effect on it. image shows the MORE THAN dendrites formed by the stacking of octahedrons, sometimes in an elongated form. Microphotograph of 4,000 types of minerals have been recognized by the Carbon silver crystal dendrites International Association of Mineralogy. Atom 2 COMPOUND MINERALS Isotypic Minerals Compound minerals Isomorphism happens when minerals with the same structure, such as halite and galena, are created when exchange cations. The structure remains the same, but the resulting substance is different, chemical bonds form because one ion has been exchanged for another. An example of this process is siderite (rhombic B- SEMIMETALS C- NONMETALS HALITE between atoms of FeCO3), which gradually changes to magnesite (MgCO3) when it trades its iron (Fe) for similarly- Native minerals that are more An important group of is composed of fragile than metals and have minerals, which includes more than one element. sized magnesium (Mg). Because the ions are the same size, the structure remains unchanged. Model demonstrating chlorine and sodium. a lower conductivity. sulfur The properties of a how one atom bonds Examples are arsenic, compound mineral differ to the other four antimony, and bismuth. from those of its HALITE AND GALENA constituent elements. Halite NaCl Galena PbS Each atom is joined to four other Atoms form hexagons that Cl Na S Pb atoms of the same type. The are strongly interconnected BISMUTH SULFUR carbon network extends in three in parallel sheets. This dimensions by means of strong structure allows the sheets covalent bonds. This provides the to slide over one another. mineral with an almost unbreakable hardness. Cubic Internal Hardness of 10 Hardness of 1 Structure on the Mohs scale on the Mohs scale