44 Chapter 2

2.2 Minerals

Key ConceptsWhat are five characteristicsof a mineral?

What processes result inthe formation of minerals?

How can minerals beclassified?

What are some of themajor groups of minerals?

Vocabulary◆ mineral◆ silicate◆ silicon-oxygen tetrahedron

Reading StrategyPreviewing Copy the organizer below. Skim the material on mineralgroups on pages 47 to 49. Place each group name into one of the ovalsin the organizer. As you read this section, complete the organizer withcharacteristics and examples of each major mineral group.

Figure 9 A Table salt is the mineral halite. B The glasscontainer is made from the mineral quartz. Bauxite isone of the minerals that provides aluminum for the cap.

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Mineral Groups

Look at the salt shaker in Figure 9B. This system is made up of themetal cap, glass container, and salt grains. Each component is madeof elements or compounds that either are minerals or that areobtained from minerals. In fact, practically every manufactured

product that you might use in a typical day con-tains materials obtained from minerals. Whatother minerals do you probably use regularly?The lead in your pencils actually contains a soft

black mineral called graphite. Most bodypowders and many kinds of make-up containfinely ground bits of the mineral talc. Yourdentist’s drill bits contain tiny pieces of the

mineral diamond. It is hard enough to drillthrough your tooth enamel. The mineral quartzis the main ingredient in the windows in yourschool and the drinking glasses in your family’skitchen. What do all of these minerals have incommon? How do they differ?

A

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FOCUS

Section Objectives2.6 List five characteristics of

minerals.2.7 Describe the processes that

result in mineral formation.2.8 Explain how minerals can be

classified.2.9 List some of the major groups

of minerals.

Build VocabularyWord Parts Help students understandthe meaning of the word tetrahedron bybreaking the word down into parts. Thepart tetra comes from the Greek word for“four.” The part hedron comes from theGreek word for “face.” So a tetrahedronis a shape that has four faces.

Reading StrategySilicates: made of tetrahedra; quartz;feldsparCarbonates: contain carbon, oxygen,and one or more other metallicelements; calcite; dolomiteOxides: contain oxygen, and one ormore other elements; rutile; corundumSulfate and sulfides: contain sulfur;gypsum; galenaHalides: contain a halogen ion plus oneor more other elements; halite; fluoriteNative elements: substances that existas free elements; gold; silver

INSTRUCT

Students may think that the mineralsdiscussed in this chapter are the same asthe minerals found in vitamin pills. Theyare related, but not the same. Remindstudents of the Q&A on p. 38. In earthscience, a mineral is a naturallyoccurring inorganic crystalline solid. Incontrast, minerals found in vitamin pillsare inorganic compounds made in thelaboratory that contain elements neededby the body. Many elements needed bythe body are metals such as calcium,potassium, phosphorus, magnesium,and iron.Logical

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MineralsA mineral in Earth science is different from the minerals in foods.

A mineral is a naturally occurring, inorganic solid with anorderly crystalline structure and a definite chemical composition.For an Earth material to be considered a mineral, it must have the fol-lowing characteristics:1. Naturally occurring A mineral forms by natural geologic

processes. Therefore, synthetic gems, such as synthetic diamondsand rubies, are not considered minerals.

2. Solid substance Minerals are solids within the temperature rangesthat are normal for Earth’s surface.

3. Orderly crystalline structure Minerals are crystalline substanceswhich means that their atoms or ions are arranged in an orderlyand repetitive manner. You saw this orderly type of packing inFigure 5 for halite (NaCl). The gemstone opal is not a mineral eventhough it contains the same elements as quartz. Opal does not havean orderly internal structure.

4. Definite chemical composition Most minerals are chemical com-pounds made of two or more elements. A few, such as gold andsilver, consist of only a single element (native form). The commonmineral quartz consists of two oxygen atoms for every silicon atom.Thus the chemical formula for quartz would be SiO2.

5. Generally considered inorganic Most minerals are inorganic crys-talline solids found in nature. Table salt (halite) is one suchmineral. However, sugar, another crystalline solid is not consid-ered a mineral because it is classified as an organic compound.Sugar comes from sugar beets or sugar cane. We say “generallyinorganic” because many marine animals secrete inorganic com-pounds, such as calcium carbonate (calcite). This compound isfound in their shells and in coral reefs. Most geologists considerthis form of calcium carbonate a mineral.

How Minerals FormMinerals form nearly everywhere on Earth under different conditions.For example, minerals called silicates often form deep in the crust ormantle where temperatures and pressures are very high. Most of theminerals known as carbonates form in warm, shallow ocean waters.Most clay minerals form at or near Earth’s surface when existing min-erals are exposed to weathering. Still other minerals form when rocksare subjected to changes in pressure or temperature. There arefour major processes by which minerals form: crystallization frommagma, precipitation, changes in pressure and temperature, and for-mation from hydrothermal solutions.

Figure 10 These minerals often form as the result ofcrystallization from magma.

Feldspar

Quartz

Muscovite

Hornblende

MineralsBuild Reading LiteracyRefer to p. 334D in Chapter 12, whichprovides guidelines for this strategy.

Outline Have students read the text onpp. 45–49 about how minerals form andmineral groups. Encourage students touse the headings as major divisions in anoutline. Have students refer to theiroutlines when answering the questionsin the Section 2.2 Assessment.Verbal, Logical

How Minerals Form

Crystallizationof SulfurPurpose Students observe how therate of cooling of a mineral affectscrystal size.

Materials two crucibles, 50-mLbeaker, water, teaspoon, sulfur powder,tongs, Bunsen burner, magnifying glass

Procedure Put a teaspoon of powderedsulfur into one of the crucibles. Usingtongs, hold the crucible near the burnerflame until the sulfur melts. Set thecrucible aside to cool. Put a teaspoonof powdered sulfur into the secondcrucible. Put about 200 mL of water inthe beaker. Melt the sulfur in the secondcrucible and carefully pour the moltensulfur into the beaker. Pour off thewater. As soon as the first crucible hascooled, allow students to look at bothsets of crystals using the magnifyingglass.

Safety Wear goggles, apron, and heat-resistant mitts while melting the sulfur.Make sure students are at a safe distancefrom the crucible and flame and that theroom is well ventilated.

Expected Outcome The sulfur in thefirst crucible, which cooled slowly, willhave bigger crystals than the sulfur thatcooled quickly.Visual, Logical

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Customize for English Language Learners

To help students learn and understand thenames of the different mineral groups, havethem focus on the roots of the names andignore the endings. The beginning of silicates,for example, is silic-. This is similar to siliconand indicates that silicates contain silicon.

Have students go though all the other names,isolate the beginnings of the names, and figureout what the name indicates about whatelements the minerals contain (carbonates:carbon-, carbon; oxides: ox-, oxygen; sulfatesand sulfides: sulf-, sulfur; halides: hal-, halogens).

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Crystallization from Magma Magma is molten rock. It formsdeep within Earth. As magma cools, elements combine to form miner-als such as those shown in Figure 10 on page 45. The first minerals tocrystallize from magma are usually those rich in iron, calcium, and mag-nesium. As minerals continue to form, the composition of the magmachanges. Minerals rich in sodium, potassium, and aluminum then form.

Precipitation The water in Earth’s lakes, rivers, ponds, oceans, andbeneath its surface contains many dissolved substances. If this waterevaporates, some of the dissolved substances can react to form miner-als. Changes in water temperature may also cause dissolved material toprecipitate out of a body of water. The minerals are left behind, or pre-cipitated, out of the water. Two common minerals that form in this wayare shown in Figure 11.

Pressure and Temperature Some minerals, including talcand muscovite, form when existing minerals are subjected to changesin pressure and temperature. An increase in pressure can cause a min-eral to recrystallize while still solid. The atoms are simply rearrangedto form more compact minerals. Changes in temperature can alsocause certain minerals to become unstable. Under these conditions,new minerals form, which are stable at the new temperature.

Hydrothermal Solutions A hydrothermal solution is a veryhot mixture of water and dissolved substances. Hydrothermal solu-tions have temperatures between about 100°C and 300°C. When thesesolutions come into contact with existing minerals, chemical reactionstake place to form new minerals. Also, when such solutions cool, someof the elements in them combine to form minerals such as quartz andpyrite. The sulfur minerals in the sample shown in Figure 12 formedfrom thermal solutions.

Describe what happens when a mineral is subjected to changes in pressure or temperature.

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Figure 11 A This limestone caveformation is an obvious exampleof precipitation. B Halite andcalcite are also formed byprecipitation.

Figure 12 Bornite (blue andpurple) and chalcopyrite (gold)are sulfur minerals that formfrom thermal solutions.

A B

46 Chapter 2

Precipitationof a MineralPurpose Students observe how amineral can form by precipitation.

Materials rock salt, spoon warm water,400-mL beaker, shallow pan or tray

Procedure Out of view of students,add rock salt spoon by spoon to abeaker of warm water, stirring as yougo. Stop when no more salt will dissolve.Show the beaker to students and askthem if there is a mineral in it. Most willsay no. Then pour the liquid into thepan and leave it in a warm and/or sunnyspot. (If you don’t have a suitable warmand/or sunny spot or want to save time,pour some of the solution into anevaporation dish and heat it on a hotplate.) When the water has evaporated,show the pan to students and ask themto identify the substance in it (halitecrystals).

Expected Outcome As the waterevaporates, the salt will precipitate outof the solution and form crystals.Visual, Logical

Build Science SkillsInferring Sometimes rocks are burieddeep under Earth’s surface. When theyreach the surface again they are oftenchanged into a different type of rock.Ask students why this happens. (Therocks are subjected to great heat andpressure deep below Earth’s surface. Theheat and pressure cause reactions thatchange the minerals and thus producesnew types of rocks.)Logical

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Mineral GroupsOver 3800 minerals have been named, and several new ones are iden-tified each year. You will be studying only the most abundant minerals.

Common minerals, together with the thousands of others thatform on Earth, can be classified into groups based on their compo-sition. Some of the more common mineral groups include the silicates,the carbonates, the oxides, the sulfates and sulfides, the halides, andthe native elements. First, you will learn about the most commongroups of minerals on Earth—the silicates.

Silicates If you look again at Table 1, you can see that the two most abundant elements in Earth’s crust are silicon and oxygen.

Silicon and oxygen combine to form a structure called thesilicon-oxygen tetrahedron. This structure is shown in Figure 13. Thetetrahedron, which consists of one silicon atom and four oxygenatoms, provides the framework of every silicate mineral. Except for afew silicate minerals, such as pure quartz (SiO2), most silicates alsocontain one or more other elements.

Silicon-oxygen tetrahedra can join in a variety of ways, as you cansee in Figure 14 on the next page. The silicon-oxygen bonds are verystrong. Some minerals, such as olivine, are made of millions of singletetrahedra. In minerals such as augite, the tetrahedra join to formsingle chains. Double chains are formed in minerals such as horn-blende. Micas are silicates in which the tetrahedra join to form sheets.Three-dimensional network structures are found in silicates such asquartz and feldspar. As you will see, the internal structure of a mineralaffects its properties.

What is the silicon-oxygen tetrahedron, and inhow many ways can it combine?

Figure 13 A The silicon-oxygentetrahedron is made of one siliconatom and four oxygen atoms. The rods represent chemicalbonds between silicon and theoxygen atoms. B Quartz is themost common silicate mineral. A typical piece of quartz like thiscontains millions of silicon-oxygentetrahedra.

A B

Mineral Groups

Students may think that the silicatesdescribed in this section are the samematerials as those in silicon computerchips or silicone sealant. They aredifferent materials, although all threecontain silicon. Silicon chips are madefrom pure silicon, which does notcontain the oxygen found in silicates.Silicone is an artificially-made silicon-oxygen polymer gel that feels rubberyand is water repellent, chemically inert,and stable at extreme temperatures.Verbal

Use VisualsFigure 13 Use this diagram to explainthe basic structure of silicates. Ask:What is the atom at the center of thetetrahedron? (a silicon atom) What arethe four atoms at the corners of thetetrahedron? (oxygen atoms) What dothe rods represent? (chemical bondsbetween the atoms) Why is this structurecalled a tetrahedron? (it has four sides,or faces)Visual

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Answer to . . .

The mineral oftenbecomes unstable, and

its atoms react to form a new mineral.

The silicon-oxygentetrahedron consists of

one silicon atom and four oxygenatoms and provides the framework ofevery silicate mineral. These tetrahedracan join to form single chains, doublechains, sheets, and three-dimensionalnetworks. In these arrangements thecorner oxygen atoms are sharedbetween silicon atoms so the ratiois not necessarily 1 to 4.

Judging from the enormous number ofknown minerals, one might think that alarge number of elements are needed to makethem. Surprisingly, the bulk of these mineralsare made up of only eight elements. Theseelements, in order of abundance, are oxygen(O), silicon (Si), aluminum (Al), iron (Fe),calcium (Ca), sodium (Na), potassium (K),and magnesium (Mg). These eight elementsrepresent over 98 percent by weight of the

continental crust. The two most abundantelements, oxygen and silicon, comprise nearlythree-fourths of Earth’s continental crust.These elements are the main building blocksof silicates. The most common group ofsilicates, the feldspars, compose over50 percent of Earth’s crust. Quartz is thesecond most abundant mineral in the conti-nental crust. It is a silicate made mostly ofoxygen and silicon.

Facts and Figures

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Recall that most silicate minerals crystallizefrom magma as it cools. This cooling can occur ator near Earth’s surface, where temperatures andpressures are relatively low. The formation of sili-cates can also occur at great depths, wheretemperatures and pressures are high. The place offormation and the chemical composition of themagma determine which silicate minerals willform. For example, the silicate olivine crystallizesat temperatures of about 1200°C. Quartz crystal-lizes at about 700°C.

Some silicate minerals form at Earth’s surfacewhen existing minerals are exposed to weathering.Clay minerals, which are silicates, form this way.Other silicate minerals form under the extremepressures that occur with mountain building.Therefore, silicate minerals can often provide sci-entists with clues about the conditions in whichthe minerals formed.

Carbonates Carbonates are the second mostcommon mineral group. Carbonates areminerals that contain the elements carbon,oxygen, and one or more other metallic ele-ments. Calcite (CaCO3) is the most commoncarbonate mineral. Dolomite is another carbon-ate mineral that contains magnesium andcalcium. Both limestone and marble are rockscomposed of carbonate minerals. Both types ofrock are used in building and construction.

Oxides Oxides are minerals that containoxygen and one or more other elements, whichare usually metals. Some oxides, including themineral called rutile (TiO2), form as magma coolsdeep beneath Earth’s surface. Rutile is titaniumoxide. Other oxides, such as corundum (Al2O3),form when existing minerals are subjected tochanges in temperature and pressure. Corundumis aluminum oxide. Still other oxides, such ashematite (Fe2O3), form when existing minerals areexposed to liquid water or to moisture in the air.Hematite is one form of iron oxide.

Silicate Structures

Single tetrahedron

Single chains

Double chains

Sheets

Three-dimensional

networks

Figure 14 Silicon-oxygen tetrahedra can form chains,sheets, and three-dimensional networks. Formulating Hypotheses What type of chemicalbond is formed by silicon atoms in an SiO4tetrahedron?

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Build Science SkillsUsing Models Havestudents use Figures 13and 14 to build modelsof the various silicatestructures. They can use balls ofmodeling clay or gumdrops to representsilicon and oxygen atoms and toothpicksto represent chemical bonds. First haveeach student build several silicon-oxygentetrahedra as shown in Figure 13. Thenhave students work in groups to combinetheir tetrahedra into chains and otherstructures as shown in Figure 14.Kinesthetic, Visual, Logical

Use VisualsFigure 14 Use this diagram to explainthe different structures that silicatetetrahedra can form. Ask: How does asingle chain form? (A series of tetrahedraare joined together end-to-end.) How aredouble chains formed? (Two singlechains are joined together side-by-side.)How are sheets formed? (Many singlechains are joined together side-by-side.)Visual

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Section 2.2 Assessment

Reviewing Concepts1. What are five characteristics of a mineral?

2. Describe four processes that result in theformation of minerals.

3. How can minerals be classified?

4. Name the major groups of minerals, andgive at least two examples of minerals in eachgroup.

Critical Thinking5. Comparing and Contrasting Compare

and contrast sulfates and sulfides.

6. Formulating Conclusions When hit with ahammer, quartz shows an uneven breakage

pattern. Using Figure 14, what can yousuggest about its structure?

7. Applying Concepts To which mineral groupdo each of the following minerals belong:bornite (Cu5FeS4), cuprite (Cu2O), magnesite(MgCO3), and barite (BaSO4)?

Sulfates and Sulfides Sulfates and sul-fides are minerals that contain the element sulfur.Sulfates, including anhydrite (CaSO4) and gypsum(CaSO4 • 2H2O), form when mineral-rich watersevaporate. Sulfides, which include the minerals galena(PbS), sphalerite (ZnS), and pyrite (FeS2), often formfrom thermal, or hot-water, solutions. Figure 15 showstwo of these sulfides.

Halides Halides are minerals that contain ahalogen ion plus one or more other elements.Halogens are elements from Group 7A of the periodictable. This group includes the elements fluorine (F)and chlorine (Cl). The mineral halite (NaCl), tablesalt, is a common halide. Fluorite (CaF2) is also acommon halide and is used in making steel. It formswhen salt water evaporates.

Native Elements Native elements are agroup of minerals that exist in relatively pure form.You are probably familiar with many native elements,such as gold (Au), silver (Ag), copper (Cu), sulfur (S),and carbon (C). Native forms of carbon are diamondand graphite. Some native elements form fromhydrothermal solutions.

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Explanatory Paragraph Coal formsfrom ancient plant matter that has beencompressed over time. Do you think coal isa mineral? Write a paragraph that explainsyour reasoning.

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Figure 15 Sulfides A Galena is a sulfide mineralthat can be mined for its lead. B Pyrite is anothersulfide that is often called fool’s gold. Inferring What element do you think pyrite isgenerally mined for?

Use CommunityResourcesInvite a geologist from a local collegeor company to visit the classroom anddiscuss different groups of minerals withstudents. Ask the geologist to bring insamples from each group of mineralsto show to students.Visual, Interpersonal

ASSESSEvaluateUnderstandingHave students describe the four majorprocesses by which minerals can form.

ReteachReview the five characteristics that amaterial must have to be considered amineral. Have students list and defineall of the science terms used in thedescription of the five characteristics.Then review each of the five pointsagain to be sure students understandthem.

Students should explain that becausecoal is formed from once-living things, itis not considered a mineral. Also, coal isa carbon-based material that falls intothe class of organic compounds.

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Answer to . . .

Figure 14 covalent

Figure 15 Iron (Fe)

hydrothermal solutions is another way inwhich minerals form.3. Minerals can be classified according totheir compositions.4. silicates (quartz, feldspar, olivine, andmica); carbonates (calcite, dolomite);oxides (rutile, corundum, hematite); sulfates(anhydrite, gypsum); sulfides (galena, pyrite,sphalerite); halides (fluorite, halite); nativeelements (gold, silver, copper, iron, sulfur,diamond)

Section 2.2 Assessment

1. A mineral is a natural, inorganic solid withan orderly internal structure and a definitechemical composition.2. Crystallization occurs when minerals formas magma cools. Precipitation is a processwhereby minerals form as waters rich in dis-solved substances evaporate. Changes inpressure and temperature can cause theatoms in a mineral to change places toform a new mineral. Precipitation from

5. Both contain the element sulfur. Sulfatesalso contain oxygen and a metallic element.Sulfides contain only sulfur and one or moreother metallic elements.6. The tetrahedra that combine to formquartz share very strong bonds, resultingin an uneven breakage.7. Bornite is a sulfide mineral, cuprite is anoxide, magnesite is a carbonate mineral,and barite is a sulfate.

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