20705866 3 minerals

ick up any rock and look at it carefully.You will probably see small, differently colored specks like those

in granite (Fig. 3–1). Each speck is a mineral. A rock is anaggregate of minerals. Some rocks are made of only onemineral, but most contain two to five abundant mineralsplus minor amounts of several others.

C H A P T E R

3Minerals

P

Figure 3–1 Each of the differently colored grains in thisgranite is a different mineral. The pink grains are feldspar, theblack ones are biotite, and the glassy-white ones are quartz.

37

A sample of basalt, one of the most abundant rocks in the Earth’scrust, viewed through a microscope.The intense colors are pro-duced by polarized light. (� 1997 Kent Wood)

37

� 3.1 WHAT IS A MINERAL?

A mineral is a naturally occurring inorganic solidwith a characteristic chemical composition and a crys-talline structure. Chemical composition and crystallinestructure are the two most important properties of a min-eral: They distinguish any mineral from all others. Beforediscussing them, however, let us briefly consider theother properties of minerals described by this definition.

NATURAL OCCURRENCE

A synthetic diamond can be identical to a natural one,but it is not a true mineral because a mineral must formby natural processes. Like diamond, most gems that oc-cur naturally can also be manufactured by industrialprocesses. Natural gems are valued more highly thanmanufactured ones. For this reason, jewelers should always tell their customers whether a gem is natural orartificial, and they usually preface the name of a manu-factured gem with the term synthetic.

INORGANIC SOLID

Organic substances are made up mostly of carbon that ischemically bonded to hydrogen or other elements.Although organic compounds can be produced in labo-ratories and by industrial processes, plants and animalscreate most of the Earth’s organic material. In contrast,inorganic compounds do not contain carbon-hydrogenbonds and generally are not produced by living organisms.All minerals are inorganic and most form independentlyof life. An exception is the calcite that forms limestone.Limestone is commonly composed of the shells of deadcorals, clams, and similar marine organisms. Shells, inturn, are made of the mineral calcite or a similar mineralcalled aragonite. Although produced by organisms andcontaining carbon, the calcite and aragonite are trueminerals.

� 3.2 ELEMENTS, ATOMS, AND THE CHEMICAL COMPOSITION OF MINERALS

To consider the chemical composition and crystallinestructure of minerals, we must understand the nature ofchemical elements—the fundamental components ofmatter. An element cannot be broken into simpler parti-cles by ordinary chemical processes. Most common min-erals consist of a small number—usually two to five—ofdifferent chemical elements.

A total of 88 elements occur naturally in the Earth’scrust. However, eight elements—oxygen, silicon, alu-

minum, iron, calcium, magnesium, potassium, andsodium—make up more than 98 percent of the crust(Table 3–1).

A complete list of all elements is given in Table 3–2.Each element is represented by a one- or two-letter sym-bol, such as O for oxygen and Si for silicon. The tableshows a total of 108 elements, not 88, because 20 ele-ments are produced in nuclear reactors but do not occurnaturally.

An atom is the basic unit of an element. An atom istiny; the diameter of the average atom is about 10�10

meters (1/10,000,000,000). A single copper penny con-tains about 1.56 � 1022 (1.56 followed by 22 zeros) cop-per atoms. An atom consists of a small, dense, positivelycharged center called a nucleus surrounded by nega-tively charged electrons (Fig. 3–2).

An electron is a fundamental particle; it is not madeup of smaller components. An electron orbits the nu-cleus, but not in a clearly defined path like that of theEarth around the Sun. Rather, an electron travels in arapidly undulating path and is usually portrayed as acloud of negative charge surrounding the nucleus.Electrons concentrate in spherical layers, or shells,around the nucleus. Each shell can hold a certain num-ber of electrons.

The nucleus is made up of several kinds of particles;the two largest are positively charged protons anduncharged neutrons. A neutral atom contains equalnumbers of protons and electrons. Thus, the positive andnegative charges balance each other so that a neutralatom has no overall electrical charge.

An atom is most stable when its outermost shell iscompletely filled with electrons. But in their neutral

38 CHAPTER 3 MINERALS

Table 3–1 • THE EIGHT MOSTABUNDANT CHEMICAL ELEMENTS INTHE EARTH’S CRUST

WEIGHT ATOM VOLUMEPERCENT PERCENT PERCENT*

O 46.60 62.55 93.8Si 27.72 21.22 0.9Al 8.13 6.47 0.5Fe 5.00 1.92 0.4Ca 3.63 1.94 1.0Na 2.83 2.64 1.3K 2.59 1.42 1.8Mg 2.09 1.84 0.3Total 98.59 100.00 100.00

From Principles of Geochemistry by Brian Mason and Carleton B.Moore. Copyright © 1982 by John Wiley & Sons, Inc.

*These numbers will vary somewhat as a function of the ionic radiichosen for the calculations.

states, most atoms do not have a filled outer shell. Suchan atom may fill its outer shell by acquiring extra elec-trons until the shell becomes full. Alternatively, an atommay give up electrons until the outermost shell becomesempty. In this case, the next shell in, which is full, thenbecomes the outermost shell. When an atom loses one ormore electrons, its protons outnumber its electrons and itdevelops a positive charge. If an atom gains one or moreextra electrons, it becomes negatively charged. A chargedatom is called an ion.

A positively charged ion is a cation. All of the abun-dant crustal elements except oxygen release electrons tobecome cations, as shown in Table 3–3. For example,each potassium atom (K) loses one electron to form acation with a charge of 1�. Each silicon atom loses fourelectrons, forming a cation with a 4� charge. In contrast,

Elements, Atoms, and the Chemical Composition of Minerals 39

Table 3–2 • THE PERIODIC TABLE

H11

2

3

4

5

6

Transition Elements

Per

iods

*Lanthanoids

✝Actinoids

Groups of Main-Group Elements

7

1 2 8 76543

Li3

Be4

Na11

Mg12

Ca20

Sc21

Ti22

V23

Cr24

Mn25

Fe26

Co27

Ni28

Cu29

Zn30

Ga31

Ge32

As33

Se34

Br35

Kr36

Al13

Si14

P15

S16

Cl17

Ar18

B5

C6

N7

O8

F9

Ne10

He2

Sr38

Y39

Zr40

Nb41

Mo42

Tc43

Ru44

Rh45

Pd46

Ag47

Cd48

In49

Sn50

Sb51

Te52

I53

Xe54

Ba56

La57

Hf72

Ta73

W74

Re75

Os76

Ir77

Pt78

Au79

Hg80

Tl81

Pb82

Bi83

Po84

At85

Ce58

Pr59

Nd60

Pm61

Sm62

Eu63

Gd64

Tb65

Dy66

Ho67

Er68

Tm69

Yb70

Lu71

Th90

Pa91

U92

Np93

Pu94

Am95

Cm96

Bk97

Cf98

Es99

Fm100

Md101

No102

Lr103

Rn86

Ra88

Ac89

Unq104

Unp105

Unh106

Uns107 108

Une109

K19

Rb37

Cs55

Fr87

*

✝

Of the 108 elements that appear here, only 88 occur naturally in the Earth’s crust. The other 20 are synthetic. The eight most abundant elementsare shaded in orange. Elements with filled outer electron shells are shaded in violet.

Atomicnucleus

Cloud ofelectrons

–

+

Figure 3–2 An atom consists of a small, dense, positive nu-cleus surrounded by a much larger cloud of negative electrons.

oxygen gains two extra electrons to acquire a 2� charge.Atoms with negative charges are called anions.

Atoms and ions rarely exist independently. Instead,they unite to form compounds. The forces that holdatoms and ions together to form compounds are calledchemical bonds (see the “Focus On” box entitled“Chemical Bonds”).

Most minerals are compounds. When ions bond to-gether to form a mineral, they do so in proportions sothat the total number of negative charges exactly bal-ances the total number of positive charges. Thus, mineralsare always electrically neutral. For example, the mineralquartz proportionally consists of one (4�) silicon cationand two (2�) oxygen anions.

Recall that a mineral has a definite chemical compo-sition. A substance with a definite chemical compositionis made up of chemical elements that are bonded togetherin definite proportions. Therefore, the composition canbe expressed as a chemical formula, which is written bycombining the symbols of the individual elements.

A few minerals, such as gold and silver, consist ofonly a single element. Their chemical formulas, respec-tively, are Au (the symbol for gold) and Ag (the symbolfor silver). Most minerals, however, are made up of twoto five essential elements. For example, the formula ofquartz is SiO2: It consists of one atom of silicon (Si) forevery two of oxygen (O). Quartz from anywhere in theUniverse has that exact composition. If it had a differentcomposition, it would be some other mineral. The com-positions of some minerals, such as quartz, do not varyby even a fraction of a percent. The compositions ofother minerals vary slightly, but the variations are lim-ited, as explained in Section 3.7.

The 88 elements that occur naturally in the Earth’scrust can combine in many ways to form many different

minerals. In fact, about 3500 minerals are known.However, the eight abundant elements commonly com-bine in only a few ways. As a result, only nine rock-forming minerals (or mineral “groups”) make up mostrocks of the Earth’s crust. They are olivine, pyroxene,amphibole, mica, the clay minerals, quartz, feldspar, cal-cite, and dolomite.

� 3.3 CRYSTALS: THE CRYSTALLINE NATURE OF MINERALS

A crystal is any substance whose atoms are arranged ina regular, periodically repeated pattern. All minerals arecrystalline. The mineral halite (common table salt) hasthe composition NaCl: one sodium ion (Na�) for everychlorine ion (Cl�). Figure 3–3a is an “exploded” view of the ions in halite. Figure 3–3b is more realistic, show-ing the ions in contact. In both sketches the sodium andchlorine ions alternate in orderly rows and columns in-tersecting at right angles. This arrangement is the crys-talline structure of halite.

Think of a familiar object with an orderly, repetitivepattern, such as a brick wall. The rectangular bricks re-peat themselves over and over throughout the wall. As aresult, the whole wall also has the shape of a rectangleor some modification of a rectangle. In every crystal, asmall group of atoms, like a single brick in a wall, re-peats itself over and over. This small group of atoms iscalled a unit cell. The unit cell for halite is shown inFigure 3–3a. If you compare Figures 3–3a and 3–3b,you will notice that the simple halite unit cell repeatsthroughout the halite crystal.

Most minerals initially form as tiny crystals thatgrow as layer after layer of atoms is added to their sur-faces. A halite crystal might grow, for example, as saltyseawater evaporates from a tidal pool. At first, a tinygrain might form, similar to the sketch of halite in Figure3–3b. This model shows a halite crystal containing 125atoms; it would be only about one millionth of a mil-limeter long on each side. As evaporation continued,more and more sodium and chlorine ions would precip-itate onto the faces of the growing crystal. Minerals crys-tallize from cooling magma in a similar manner.

The shape of a large, well-formed crystal like that ofhalite in Figure 3–3c is determined by the shape of theunit cell and the manner in which the crystal grows. Forexample, it is obvious from Figure 3–4a that the stack-ing of small cubic unit cells can produce a large cubiccrystal. Figure 3–4b shows that a different kind of stack-ing of the same cubes can also produce an eight-sidedcrystal, called an octahedron. Halite can crystallize as acube or as an octahedron. All minerals consist of unit


Table 3–3 • THE MOST COMMON IONSOF THE EIGHT MOST ABUNDANTCHEMICAL ELEMENTS IN THE EARTH’SCRUST

CHEMICAL COMMONELEMENT SYMBOL ION(S)

Oxygen O O2�

Silicon Si Si4�

Aluminum Al Al3�

Iron Fe Fe2� and Fe3�

Calcium Ca Ca2�

Magnesium Mg Mg2�

Potassium K K1�

Sodium Na Na1�

cells stacked face to face as in halite, but not all unit cellsare cubic.

A crystal face is a planar surface that develops if acrystal grows freely in an uncrowded environment. Thesample of halite in Figure 3–3c has well-developed crystal faces. In nature, the growth of crystals is often impeded by adjacent minerals that are growing simulta-neously or that have formed previously. For this reason,minerals rarely show perfect development of crystalfaces.

� 3.4 PHYSICAL PROPERTIES OF MINERALS

How does a geologist identify a mineral in the field?Chemical composition and crystal structure distinguisheach mineral from all others. For example, halite alwaysconsists of sodium and chlorine in a one-to-one ratio,with the atoms arranged in a cubic fashion. But if youpick up a crystal of halite, you cannot see the ions. You

Physical Properties of Minerals 41

(a)

Cl�Na�

(b) (c)

Figure 3–3 (a and b) The orderly arrangement of sodium and chlorine ions in halite.(c) Halite crystals. The crystal model in (a) is exploded so that you can see into it; the ionsare actually closely packed as in (b). Note that ions in (a) and (b) form a cube, and thecrystals in (c) are also cubes. (c, American Museum of Natural History)

Figure 3–4 Both a cubic crystal (a) and an “octahedron”(b) can form by different kinds of stacking of identical cubes.

a)

(b)(a)

(continued on p. 44)


F O C U S O N

C H E M I C A L B O N D S

E lectrons concentrate in shells around the nu-cleus of an atom (Fig. 1). The laws of quantum

physics allow only certain shells, or energy levels, toexist around a nucleus. Electrons can only occupythose shells and cannot orbit in areas between shells.

Look at the periodic table (Table 3–2). The ele-ments in the right-hand column, colored blue, have afilled outer shell in their normal, neutral state. Thoseelements are stable and chemically nonreactive be-cause their outer electron shells are full. Other ele-ments gain or lose electrons so that they acquire afilled outer shell like those shaded in blue.

For example, sodium has one electron more thana filled outer shell. Therefore, it tends to give up oneelectron. When it loses that electron, its outer shell isperfectly filled, but it becomes a positively chargedcation because it then has one more proton than it haselectrons. Oxygen acquires a filled shell by gainingtwo electrons and forming an anion. The common an-ions and cations of the eight abundant elements areshown in Table 3–3.

Four types of chemical bonds are found in min-erals: ionic, covalent, metallic, and van der Waalsforces.

Ionic BondsCations and anions are attracted by their oppositeelectronic charges and thus bond together. This unionis called an ionic bond. An ionic compound (made upof two or more ions) is neutral because the positiveand negative charges balance each other. For example,when sodium and chlorine form an ionic bond, thesodium atom loses one electron to become a cationand chlorine gains one to become an anion. Whenthey combine, the �1 charge balances the �1 charge(Fig. 2).

Covalent BondsA covalent bond develops when two or more atomsshare their electrons to produce the effect of filledouter electron shells. For example, carbon needs fourelectrons to fill its outermost shell. It can achieve thisby forming four covalent bonds with four adjacentcarbon atoms. It “gains” four electrons by sharing onewith another carbon atom at each of the four bonds.Diamond consists of a three-dimensional network ofcarbon atoms bonded into a network of tetrahedra,similar to the framework structure of quartz (Fig. 3).The strength and homogeneity of the bonds through-out the crystal make diamond the hardest of all min-erals.

In most minerals, the bonds between atoms arepartly covalent and partly ionic. The combined char-acteristics of the different bond types determine thephysical properties of those minerals.

Metallic BondsIn a metallic bond, the outer electrons are loose; thatis, they are not associated with particular atoms. Themetal atoms sit in a “sea” of outer-level electrons thatare free to move from one atom to another. Thatarrangement allows the nuclei to pack together asclosely as possible, resulting in the characteristic highdensity of metals and metallic minerals, such as pyrite.Because the electrons are free to move through the

Figure 1 Electrons concentrate in spherical layers, or shells,around the nucleus of an atom.

Concentric shellsof electrons

Nucleus


entire crystal, metallic minerals are excellent conduc-tors of electricity and heat.

Van der Waals ForcesWeak electrical forces called van der Waals forcesalso bond molecules together. These weak bonds re-sult from an uneven distribution of electrons aroundindividual molecules, so that one portion of a mole-cule may have a greater density of negative chargewhile another portion has a partial positive charge.Because van der Waals forces are weak, minerals inwhich these bonds are important, such as talc andgraphite, tend to be soft and cleave easily along planesof van der Waals bonds.

DISCUSSION QUESTION

Why do some minerals, such as native gold, silver,and graphite, conduct electricity, whereas others,such as quartz and feldspar, do not? Discuss rela-tionships among other physical properties of miner-als and the types of chemical bonds found in thoseminerals.

Figure 3 Carbon atoms in diamond form a tetrahedral net-work similar to that of quartz.

Sodium (Na�) Chlorine (Cl�)

Electron

Figure 2 When sodium and chlorine atoms combine, sodium loses one electron, becominga cation, Na�. Chlorine acquires the electron to become an anion, Cl�.

Carbon

Covalentbond

could identify a sample of halite by measuring its chem-ical composition and crystal structure using laboratoryprocedures, but such analyses are expensive and time-consuming. Instead, geologists commonly identifyminerals by visual recognition, and they confirm theidentification with simple tests.

Most minerals have distinctive appearances. Onceyou become familiar with common minerals, you willrecognize them just as you recognize any familiar object.For example, an apple just looks like an apple, eventhough apples come in many colors and shapes. In thesame way, quartz looks like quartz to a geologist. The

color and shape of quartz may vary from sample to sam-ple, but it still looks like quartz. Some minerals, how-ever, look enough alike that their physical propertiesmust be examined further to make a correct identifi-cation. Geologists commonly use physical propertiessuch as crystal habit, cleavage, and hardness to identifyminerals.

CRYSTAL HABIT

Crystal habit is the characteristic shape of a mineral andthe manner in which aggregates of crystals grow. If a


(a)

(b)

(c)

Figure 3–5 (a) Equant garnet crystals have about the samedimensions in all directions. (b) Asbestos is fibrous. (c) Kyaniteforms bladed crystals. (Geoffrey Sutton)

(a)

(b)

Figure 3–6 (a) Prismatic quartz grows as elongated crystals.(b) Massive quartz shows no characteristic shape. (ArkansasGeological Commission, J. M. Howard, Photographer)


crystal grows freely, it develops a characteristic shapecontrolled by the arrangement of its atoms, as in thecubes of halite shown in Figure 3–3c. Figure 3–5 showsthree common minerals with different crystal habits.Some minerals occur in more than one habit. For exam-ple, Figure 3–6a shows quartz with a prismatic (pencil-shaped) habit, and Figure 3–6b shows massive quartz.

When crystal growth is obstructed by other crystals,a mineral cannot develop its characteristic habit. Figure3–7 is a photomicrograph (a photo taken through a mi-croscope) of a thin slice of granite in which the crystalsfit like pieces of a jigsaw puzzle. This interlocking tex-ture developed because some crystals grew around oth-ers as the magma solidified.

CLEAVAGE

Cleavage is the tendency of some minerals to breakalong flat surfaces. The surfaces are planes of weakbonds in the crystal. Some minerals, such as mica andgraphite, have one set of parallel cleavage planes (Fig.3–8). Others have two, three, or even four different sets,as shown in Figure 3–9. Some minerals, like the micas,have excellent cleavage. You can peel sheet after sheetfrom a mica crystal as if you were peeling layers froman onion. Others have poor cleavage. Many mineralshave no cleavage at all because they have no planes of

Figure 3–7 A photomicrograph of a thin slice of granite.When crystals grow simultaneously, they commonly interlockand show no characteristic habit. To make this photo, a thinslice of granite was cut with a diamond saw, glued to a micro-scope slide, and ground to a thickness of 0.02 millimeters.Most minerals are transparent when such thin slices areviewed through a microscope.

Figure 3–8 Cleavage in mica.This large crystal is the varietyof mica called muscovite. (Geoffrey Sutton)

weak bonds. The number of cleavage planes, the qualityof cleavage, and the angles between cleavage planes allhelp in mineral identification.

A flat surface created by cleavage and a crystal facecan appear identical because both are flat, smooth sur-faces. However, a cleavage surface is duplicated when acrystal is broken, whereas a crystal face is not. So if youare in doubt, break the sample with a hammer—unless,of course, you want to save it.

FRACTURE

Fracture is the pattern in which a mineral breaks otherthan along planes of cleavage. Many minerals fractureinto characteristic shapes. Conchoidal fracture createssmooth, curved surfaces (Fig. 3–10). It is characteristicof quartz and olivine. Glass, although not a mineral be-cause it has no crystalline structure, also typically frac-tures in a conchoidal pattern. Some minerals break intosplintery or fibrous fragments. Most fracture into irregu-lar shapes.

HARDNESS

Hardness is the resistance of a mineral to scratching. Itis easily measured and is a fundamental property of eachmineral because it is controlled by bond strength be-tween the atoms in the mineral. Geologists commonly

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gauge hardness by attempting to scratch a mineral witha knife or other object of known hardness. If the bladescratches the mineral, the mineral is softer than the knife.If the knife cannot scratch the mineral, the mineral isharder.

To measure hardness more accurately, geologistsuse a scale based on ten minerals, numbered 1 through10. Each mineral is harder than those with lower num-bers on the scale, so 10 (diamond) is the hardest and 1(talc) is the softest. The scale is known as the Mohshardness scale after F. Mohs, the Austrian mineralogistwho developed it in the early nineteenth century.

The Mohs hardness scale shows, for example, that amineral scratched by quartz but not by orthoclase has ahardness between 6 and 7 (Table 3–4). Because the min-

erals of the Mohs scale are not always handy, it is use-ful to know the hardness values of common materials. Afingernail has a hardness of slightly more than 2, a cop-per penny about 3, a pocketknife blade slightly morethan 5, window glass about 5.5, and a steel file about 6.5.If you practice with a knife and the minerals of the Mohsscale, you can develop a “feel” for minerals with hard-nesses of 5 and under by how easily the blade scratchesthem.

When testing hardness, it is important to determinewhether the mineral has actually been scratched by theobject, or whether the object has simply left a trail of itsown powder on the surface of the mineral. To check,simply rub away the powder trail and feel the surface ofthe mineral with your fingernail for the groove of the


(a) (b) (c)

Figure 3–9 Some minerals have more than one cleavage plane. (a) Feldspar has twocleavages intersecting at right angles. (b) Calcite has three cleavage planes. (c) Fluorite hasfour cleavage planes. (Arthur R. Hill, Visuals Unlimited)

Figure 3–10 Quartz typically fractures along smoothlycurved surfaces, called conchoidal fractures.This sample issmoky quartz. (Breck P. Kent)

Table 3–4 • THE MINERALS OF THEMOHS HARDNESS SCALE

MINERALS OF COMMONMOHS SCALE OBJECTS

1. Talc2. Gypsum Fingernail3. Calcite Copper penny4. Fluorite5. Apatite Knife blade

Window glass6. Orthoclase Steel file7. Quartz8. Topaz9. Corundum

10. Diamond

scratch. Fresh, unweathered mineral surfaces must beused in hardness measurements because weathering of-ten produces a soft rind on minerals.

SPECIFIC GRAVITY

Specific gravity is the weight of a substance relative tothat of an equal volume of water. If a mineral weighs 2.5times as much as an equal volume of water, its specificgravity is 2.5. You can estimate a mineral’s specific grav-ity simply by hefting a sample in your hand. If youpractice with known minerals, you can develop a feel forspecific gravity. Most common minerals have specificgravities of about 2.7. Metals have much greater specificgravities; for example, gold has the highest specific grav-ity of all minerals, 19. Lead is 11.3, silver is 10.5, andcopper is 8.9.

COLOR

Color is the most obvious property of a mineral, but it iscommonly unreliable for identification. Color would bea reliable identification tool if all minerals were pure andhad perfect crystal structures. However, both smallamounts of chemical impurities and imperfections incrystal structure can dramatically alter color. For exam-ple, corundum (Al2O3) is normally a cloudy, translucent,brown or blue mineral. Addition of a small amount ofchromium can convert corundum to the beautiful, clear,red gem known as ruby. A small quantity of iron or tita-nium turns corundum into the striking blue gem calledsapphire.

STREAK

Streak is the color of a fine powder of a mineral. It isobserved by rubbing the mineral across a piece ofunglazed porcelain known as a streak plate. Many min-erals leave a streak of powder with a diagnostic color onthe plate. Streak is commonly more reliable than thecolor of the mineral itself for identification.

LUSTER

Luster is the manner in which a mineral reflects light. Amineral with a metallic look, irrespective of color, has ametallic luster. The luster of nonmetallic minerals is usu-ally described by self-explanatory words such as glassy,pearly, earthy, and resinous.

OTHER PROPERTIES

Properties such as reaction to acid, magnetism, radio-activity, fluorescence, and phosphorescence can be

characteristic of specific minerals. Calcite and some othercarbonate minerals dissolve rapidly in acid, releasingvisible bubbles of carbon dioxide gas. Minerals contain-ing radioactive elements such as uranium emit radioac-tivity that can be detected with a scintillometer.Fluorescent materials emit visible light when they areexposed to ultraviolet light. Phosphorescent mineralscontinue to emit light after the external stimulus ceases.

� 3.5 ROCK-FORMING MINERALS,ACCESSORY MINERALS, GEMS, OREMINERALS, AND INDUSTRIALMINERALS

Although about 3500 minerals are known to exist in theEarth’s crust, only a small number—between 50 and100—are important because they are common or valu-able.

ROCK-FORMING MINERALS

The rock-forming minerals make up the bulk of mostrocks in the Earth’s crust. They are important to geolo-gists simply because they are the most common miner-als. They are olivine, pyroxene, amphibole, mica, theclay minerals, feldspar, quartz, calcite, and dolomite.The first six minerals in this list are actually mineral“groups,” in which each group contains several varieties

Rock-Forming Minerals, Accessory Minerals, Gems, Ore Minerals, and Industrial Minerals 47

Figure 3–11 Pyrite is a common accessory mineral.(American Museum of Natural History)

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with very similar chemical compositions, crystallinestructures, and appearances. The rock-forming mineralsare described in Section 3.6.

ACCESSORY MINERALS

Accessory minerals are minerals that are common butusually are found only in small amounts. Chlorite, gar-net, hematite, limonite, magnetite, and pyrite are com-mon accessory minerals (Fig. 3–11).

GEMS

A gem is a mineral that is prized primarily for its beauty,although some gems, like diamonds, are also used in-dustrially. Depending on its value, a gem can be eitherprecious or semiprecious. Precious gems include dia-mond, emerald, ruby, and sapphire (Fig. 3–12). Severalvarieties of quartz, including amethyst, agate, jasper,and tiger’s eye, are semiprecious gems. Garnet, olivine,topaz, turquoise, and many other minerals sometimes occur as aesthetically pleasing semiprecious gems (Fig.3–13).

ORE MINERALS

Ore minerals are minerals from which metals or otherelements can be profitably recovered. A few, such as nativegold and native silver, are composed of a single element.However, most metals are chemically bonded to anions.Copper, lead, and zinc are commonly bonded to sulfur toform the important ore minerals chalcopyrite, galena(Fig. 3–14), and sphalerite.

INDUSTRIAL MINERALS

Several minerals are industrially important, althoughthey are not considered ore because they are mined forpurposes other than the extraction of metals. Halite ismined for table salt, and gypsum is mined as the raw ma-terial for plaster and sheetrock. Apatite and other phos-phorus minerals are sources of the phosphate fertilizerscrucial to modern agriculture. Many limestones are madeup of nearly pure calcite and are mined as the raw mate-rial of cement.


Figure 3–12 Sapphire is one of the most costly preciousgems. (Smithsonian Institution)

Figure 3–13 Topaz is a popular semiprecious gem.(American Museum of Natural History)

Figure 3–14 Galena is the most important ore of lead andcommonly contains silver. (Ward’s Natural Science Establish-ment, Inc.)

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� 3.6 MINERAL CLASSIFICATION

Geologists classify minerals according to their anions(negatively charged ions). Anions can be either simple orcomplex. A simple anion is a single negatively chargedion, such as O2�. Alternatively, two or more atoms canbond firmly together and acquire a negative charge to form a complex anion. Two common examples arethe silicate, (SiO4)4�, and carbonate, (CO3)2�, complexanions.

Each mineral group (except the native elements) is named for its anion. For example, the oxides all con-tain O2�, the silicates contain (SiO4)4�, and the carbon-ates contain (CO3)2�.

NATIVE ELEMENTS

About 20 elements occur naturally in their native statesas minerals. Fewer than ten, however, are commonenough to be of economic importance. Gold, silver, plat-inum, and copper are all mined in their pure forms. Ironis rarely found in its native state in the Earth’s crust, butmetallic iron is common in certain types of meteorites.Native iron and nickel are thought to comprise most ofthe Earth’s core. Native sulfur, used to manufacture sul-furic acid, insecticides, fertilizer, and rubber, is minedfrom volcanic craters, where it is deposited from gasesemanating from the vents (Fig. 3–15).

Pure carbon occurs as both graphite and diamond.The minerals have identical compositions but differentcrystalline structures and are called polymorphs, afterthe ancient Greek for “several forms.” Graphite is one ofthe softest minerals and is opaque and an electrical conductor. Diamond, the hardest mineral known, is transparent and an electrical insulator. The contrasting

characteristics of graphite and diamond emphasize theimportance of crystalline structure in determining thephysical properties of minerals.

OXIDES

The oxides are a large group of minerals in which oxy-gen is combined with one or more metals. Oxide miner-als are the most important ores of iron, manganese, tin,chromium, uranium, titanium, and several other indus-trial metals. Hematite (iron oxide, Fe2O3) occurs widelyin many types of rocks and is the most abundant ore ofiron. Although typically red in color, it occasionally oc-curs as black crystals used as semiprecious gems.Magnetite (Fe3O4), a naturally magnetic iron oxide, isanother ore of iron. Spinel (MgAl2O4) often occurs as at-tractive red or blue crystals that are used as inexpensive,semiprecious gems. Synthetic spinels are also commonlyused in jewelry. Ice, the oxide of hydrogen (H2O), is acommon mineral at the Earth’s surface.

SULFIDES

Sulfide minerals consist of sulfur combined with one ormore metals. Many sulfides are extremely important oreminerals. They are the world’s major sources of copper,lead, zinc, molybdenum, silver, cobalt, mercury, nickel,and several other metals. The most common sulfides arepyrite (FeS2), chalcopyrite (CuFeS2), galena (PbS), andsphalerite (ZnS).

SULFATES

The sulfate minerals contain the sulfate complex anion(SO4)2�. Gypsum (CaSO4 � 2H2O) and anhydrite(CaSO4) are two important industrial sulfates used tomanufacture plaster and sheetrock. Both form by evapo-ration of seawater or salty lake water.

PHOSPHATES

Phosphate minerals contain the complex anion (PO4)3�.Apatite, Ca5(F,Cl,OH)(PO4)3, is the substance that makesup both teeth and bones. Phosphate is an essential fertil-izer in modern agriculture. It is mined from fossil bonebeds near Tampa, Florida, and from great sedimentaryapatite deposits in the northern Rocky Mountains.

CARBONATES

The complex carbonate anion (CO3)2� is the basis oftwo common rock-forming minerals, calcite (CaCO3)

Mineral Classification 49

Figure 3–15 Native sulfur is forming today in the vent ofOllagüe Volcano on the Chile–Bolivia border.

and dolomite [CaMg(CO3)2] (Figs. 3–16a and 3–16b).Most limestone is composed of calcite, and dolomitemakes up the similar rock that is also called dolomite orsometimes dolostone. Limestone is mined as a raw in-

gredient of cement. Aragonite is a polymorph of calcitethat makes up the shells of many marine animals.

SILICATES

The silicate minerals contain the (SiO4)4� complex an-ion. Silicates make up about 95 percent of the Earth’scrust. They are so abundant for two reasons. First, siliconand oxygen are the two most plentiful elements in the crust. Second, silicon and oxygen combine readily.To understand the silicate minerals, remember four prin-ciples:

1. Every silicon atom surrounds itself with four oxy-gens. The bonds between each silicon and its fouroxygens are very strong.

2. The silicon atom and its four oxygens form apyramid-shaped structure called the silicate tetrahe-dron with silicon in the center and oxygens at thefour corners (Fig. 3–17). The silicate tetrahedronhas a 4� charge and forms the (SiO4)4� complexanion. The silicate tetrahedron is the fundamentalbuilding block of all silicate minerals.


Si

O

O

O

O

(a)

4 ¯

Figure 3–17 The silicate tetrahedron consists of one siliconatom surrounded by four oxygens. It is the fundamental build-ing block of all silicate minerals. (a) A ball-and-stick representa-tion. (b) A proportionally accurate model.

Oxygen atoms on points of tetrahedron

Silicon atom incenter of tetrahedron

(b)

Figure 3–18 The five silicate structures are based on shar-ing of oxygens among silicate tetrahedra. (A) Independenttetrahedra share no oxygens. (B) In single chains, each tetrahe-dron shares two oxygens with adjacent tetrahedra, forming achain. (C) A double chain is a pair of single chains that arecrosslinked by additional oxygen sharing. (D) In the sheet sili-cates, each tetrahedron shares three oxygens with adjacenttetrahedra. (E) A three-dimensional silicate framework sharesall four oxygens of each tetrahedron.

Figure 3–16 Calcite (a) and dolomite (b) are two rock-forming carbonate minerals.(Ward’s Natural Science Establishment, Inc.)

(a) (b)

Mineral Classification 51

Frameworksilicates

Unitcomposition

(SiO4)4–

(SiO3)2–

(Si4O11)6–

(Si2O5)2–

SiO2

Arrangementof SiO4

tetrahedron

A

Class

B

C

D

E

Independenttetrahedra

Single chains

Double chains

Sheet silicates

Mineral examples

Olivine: The compositionvaries between Mg2SiO4and Fe2SiO4.

Pyroxene: The mostcommon pyroxene isaugite,Ca(Mg, Fe, Al) (Al, Si)2O6.

Amphibole: The mostcommon amphibole ishornblende,NaCa2(Mg, Fe, Al)5(Si, Al)8O22(OH)2.

Mica, clay minerals, chlorite,e.g.: muscovite KAl2(Si3Al)O10(OH)2

Quartz: SiO2Feldspar: As anexample, potassiumfeldspar is KAlSi3O8.

3. To make silicate minerals electrically neutral, othercations must combine with the silicate tetrahedra tobalance their negative charges. (The lone exceptionis quartz, in which the positive charges on the sili-cons exactly balance the negative ones on the oxy-gens. How this occurs is described later.)

4. Silicate tetrahedra commonly link together by shar-ing oxygens. Thus, two tetrahedra may share a sin-gle oxygen, bonding the tetrahedra together.

Rock-Forming Silicate Minerals

The rock-forming silicates (and most other silicate min-erals) fall into five classes, based on five ways in whichtetrahedra share oxygens (Fig. 3–18). Each class con-tains at least one of the rock-forming mineral groups.

1. In independent tetrahedra silicates, adjacent tetrahe-dra do not share oxygens (Fig. 3–18A). Olivine isan independent tetrahedra mineral that occurs insmall quantities in basalt of both continental and

oceanic crust (Fig. 3–19a). However, rocks com-posed mostly of olivine and pyroxene are thought to make up most of the mantle.

2. In the single-chain silicates, each tetrahedron linksto two others by sharing oxygens, forming a contin-uous chain of tetrahedra (Fig. 3–18B). The pyrox-enes are a group of similar minerals with singlechain structures (Fig. 3–19b). Pyroxenes are a majorcomponent of both oceanic crust and the mantle andare also abundant in some continental rocks.

3. The double-chain silicates consist of two singlechains crosslinked by the sharing of additional oxygens between them (Fig. 3–18C). The amphi-boles (Fig. 3–19c) are a group of double-chain sili-cates with similar properties. They occur commonlyin many continental rocks. One variety of amphibolegrows as sharply pointed needles and is a type ofasbestos.

Pyroxene and amphibole can resemble each other so closely that they are difficult to tell apart.Both groups have similar chain structures and simi-


(a)

Figure 3–19 The seven rock-forming silicate mineral groups. (a) Olivine. (GeoffreySutton) (b) Pyroxene. (� Jeffrey A. Scovil) (c) Amphibole. (� Jeffrey A. Scovil) (d) Black biotiteis one common type of mica. White muscovite (Fig. 3–8) is the other. (e) Clay. (f) Feldspar,represented here by orthoclase feldspar. (Breck P. Kent) (g) Quartz. (� Jeffrey A. Scovil)

(b) (c)

(d) (e) (f) (g)

Summary 53

lar chemical compositions. In addition, both com-monly grow as pencil-shaped crystals.

4. In the sheet silicates, each tetrahedron shares oxy-gens with three others in the same plane, forming acontinuous sheet (Fig. 3–18D). All of the atomswithin each sheet are strongly bonded, but eachsheet is only weakly bonded to those above and be-low. Therefore, sheet silicates have excellent cleav-age. The micas are sheet silicates and typicallygrow as plate-shaped crystals, with flat surfaces(Fig. 3–19d). Mica is common in continental rocks.The clay minerals (Fig. 3–19e) are similar to micain structure, composition, and platy habit. Individualclay crystals are so small that they can barely beseen with a good optical microscope. Most clayforms when other minerals weather at the Earth’ssurface. Thus, clay minerals are abundant near theEarth’s surface and are an important component ofsoil and of sedimentary rocks.

5. In the framework silicates, each tetrahedron sharesall four of its oxygens with adjacent tetrahedra (Fig.3–18E). Because tetrahedra share oxygens in all di-rections, minerals with the framework structure tendto grow blocky crystals that have similar dimensionsin all directions. Feldspar and quartz have frame-work structures.

The feldspars (Fig. 3–19f) make up more than 50percent of the Earth’s crust. The different varieties offeldspar are named according to whether potassium or amixture of sodium and calcium is present in the mineral.Orthoclase is a common feldspar containing potassium.Feldspar containing calcium and/or sodium is called pla-gioclase. Plagioclase and orthoclase often look alike andcan be difficult to tell apart.

Quartz is the only common silicate mineral that contains no cations other than silicon; it is pure SiO2

(Fig. 3–19g). It has a ratio of one 4� silicon for everytwo 2� oxygens, so the positive and negative chargesneutralize each other perfectly. Quartz is widespread andabundant in continental rocks but rare in oceanic crustand the mantle.

� 3.7 IONIC SUBSTITUTION

Ionic substitution is the replacement of one ion by an-other in the crystal structure of a mineral. Generally, oneion can substitute for another if the ions are of similarsize and if their charges are within 1� or 1�.

Many minerals show variations in composition be-cause of ionic substitution, although the variation is restricted to well-defined limits for each mineral. Forexample, olivine can be pure Mg2SiO4 or pure Fe2SiO4,and it can also have any proportion of magnesium (Mg)to iron (Fe) between the two extremes. As we mentionedearlier in this chapter, ionic substitution of small amountsof chromium for aluminum in the mineral corundum isresponsible for the characteristic red color of ruby, andsubstitution of iron or titanium produces the blue of sapphire.

In the tetrahedral framework structure of potassiumfeldspar, one Al3� ion substitutes for every fourth Si4�.Because aluminum has only a 3� charge and silicon is4�, a charge of 1� develops for every fourth tetrahe-dron. One 1� potassium ion enters the feldspar structureto maintain electrical neutrality. Similar substitution ofaluminum for silicon occurs in all other feldspars and inmany other silicate minerals.

S U M M A R Y

Minerals are the substances that make up rocks. A min-eral is a naturally occurring inorganic solid with a defi-nite chemical composition and a crystalline structure.Each mineral consists of chemical elements bonded together in definite proportions, so that its chemicalcomposition can be given as a chemical formula. Thecrystalline structure of a mineral is the orderly, period-ically repeated arrangement of its atoms. A unit cell is asmall structural and compositional module that repeatsitself throughout a crystal. The shape of a crystal is de-termined by the shape and arrangement of its unit cells.Every mineral is distinguished from others by its chem-ical composition and crystal structure.

Most common minerals are easily recognized andidentified visually. Identification is aided by observing afew physical properties, including crystal habit, cleav-age, fracture, hardness, specific gravity, color, streak,and luster.

Although about 3500 minerals are known in theEarth’s crust, only the nine rock-forming mineralgroups are abundant in most rocks. They are feldspar,quartz, pyroxene, amphibole, mica, the clay minerals,olivine, calcite, and dolomite. The first seven on this listare silicates; their structures and compositions are basedon the silicate tetrahedron, in which a silicon atom issurrounded by four oxygens to form a pyramid-shaped


structure. Silicate tetrahedra link together by sharingoxygens to form the basic structures of the silicateminerals. The silicates are the most abundant mineralsbecause silicon and oxygen are the two most abundantelements in the Earth’s crust and bond together readilyto form the silicate tetrahedron. Two carbonate minerals,calcite and dolomite, are also sufficiently abundant to becalled rock-forming minerals.

K E Y W O R D S

mineral 36element 38atom 38nucleus 38electron 38proton 38neutron 38ion 39

cation 39anion 40compound 40chemical bond 40rock-forming mineral 40crystal 40crystalline structure 40unit cell 40

crystal face 41crystal habit 44cleavage 45fracture 45hardness 45Mohs hardness scale 46specific gravity 47streak 47

luster 47accessory mineral 48gem 48ore mineral 48polymorph 49silicates 50silicate tetrahedron 50ionic substitution 53

Rock-Forming Mineral Groups

Feldspar Amphibole OlivineQuartz Mica CalcitePyroxene Clay minerals Dolomite

R E V I E W Q U E S T I O N S

1. What properties distinguish minerals from other sub-stances?

2. Explain why oil and coal are not minerals.

3. What does the chemical formula for quartz, SiO2, tellyou about its chemical composition? What doesKAlSi3O8 tell you about orthoclase feldspar?

4. What is an atom? An ion? A cation? An anion? Whatroles do they play in minerals?

5. What is a chemical bond? What role do chemical bondsplay in minerals?

6. Every mineral has a crystalline structure. What does thismean?

7. What factors control the shape of a well-formed crystal?

8. What is a crystal face?

9. What conditions allow minerals to grow well-formedcrystals? What conditions prevent their growth?

10. List and explain the physical properties of minerals mostuseful for identification.

11. Why do some minerals have cleavage and others do not?Why do some minerals have more than one set of cleav-age planes?

12. Why is color often an unreliable property for mineralidentification?

13. List the rock-forming mineral groups. Why are theycalled “rock-forming”? Which are silicates? Why are somany of them silicates?

14. Draw a three-dimensional view of a single silicate tetra-hedron. Draw the five different arrangements of tetrahe-dra found in the rock-forming silicate minerals. Howmany oxygen ions are shared between adjacent tetrahedrain each of the five configurations?

15. Make a table with two columns. In the left column listthe basic silicate structures. In the right column list oneor more rock-forming minerals with each structure.

Accessory minerals are commonly found, but insmall amounts. Ore minerals, industrial minerals, andgems are important for economic reasons. Many mineralsshow compositional variation because of ionic substitu-tion. In general, one element can substitute for another ifthe two are similar in charge and size.

Discussion Questions 55

D I S C U S S I O N Q U E S T I O N S

1. Diamond and graphite are two minerals with identicalchemical compositions, pure carbon (C). Diamond is thehardest of all minerals, and graphite is one of the softest.If their compositions are identical, why do they have suchprofound differences in physical properties?

2. List the eight most abundant chemical elements in theEarth’s crust. Are any unfamiliar to you? List familiar elements that are not among the eight. Why are they familiar?

3. Table 3–1 shows that silicon and oxygen together make up

nearly 75 percent by weight of the Earth’s crust. But sili-cate minerals make up more than 95 percent of the crust.Explain the apparent discrepancy.

4. Quartz is SiO2. Why does no mineral exist with the com-position SiO3?

5. If you were given a crystal of diamond and another ofquartz, how would you tell which is diamond?

6. Would you expect minerals found on the Moon, Mars, orVenus to be different from those of the Earth’s crust?Explain your answer.

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