Rafferty J.P. (ed.) Minerals

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7 Minerals 7

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Chert and ﬂint occur as individual nodules or layers

of nodules in limestone or dolomite; they are common

in rocks of all ages (notably in the Cretaceous chalk of

England). Hard and chemically resistant, the nodules

become concentrated in residual soils as the surround-

ing carbonate rock weathers away. In places, chert forms

massive beds several hundred metres thick with a lateral

extent of hundreds of kilometres. Chert also occurs as a

ﬁne powder disseminated throughout carbonate rock; it

impregnates shale and, rarely, forms cement in sandstone.

It also develops in the vicinity of some metalliferous

veins, precipitated by hot ore-depositing (hydrother-

mal) solutions. Erosion of chert beds or chert-bearing

limestone produces chert pebbles, which are abundant

in river and beach gravel.

Most chert and ﬂint has formed by replacement of the

enclosing carbonate sediment after burial beneath the sea-

ﬂoor. This replacement origin (similar to the petriﬁcation

of wood) is substantiated by preservation in chert of the

minute textural details of the enclosing carbonate rocks.

Bedded chert, also referred to as ribbon chert, is

made up of layers of chert interbedded with thin layers

of shale. Many bedded cherts are made up of the remains

of siliceous organisms such as diatoms, radiolarians, or

sponge spicules.

High Quartz (β-Quartz)

High quartz, or β-quartz, is the more symmetrical form

quartz takes at sufﬁciently high temperatures (about 573

°C [about 1,060 °F] at one atmosphere of pressure), but

the relationship is pressure-sensitive. High quartz may be

either left- or right-handed, and its c axis is one of sixfold

symmetry rather than threefold; thus, many twin laws of

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ordinary quartz cannot occur. High quartz twins typi-

cally involve inclined sets of axes. High quartz can form

directly from silicate magma or from high-temperature

gases or solutions. It invariably undergoes the transition

to ordinary quartz (low quartz) on cooling, and all ordinary

quartz, when heated above the transition temperature, is

transformed into high quartz. The transformation involves

displacement of the linkage between the tetrahedrons; no

bonds are broken.

tridymite

Tridymite may occur as a primary magmatic phase (i.e., as

a direct result of crystallization from a silicate melt) in sili-

ceous rocks but is most abundant in voids in volcanic rocks

where it probably was deposited metastably from hydrous

gases. Tridymite also forms in contact-metamorphosed

rocks. It has been found in meteorites and is common in

lunar basalts. It occurs in quantity in ﬁrebricks and other

siliceous refractories. Natural tridymite has no speciﬁc

commercial use.

cristobalite

Cristobalite is probably more abundant in nature than

tridymite, although it seldom forms as distinctive crys-

tals. The devitriﬁcation (transformation from the glassy

to the crystalline state) of siliceous volcanic glasses

yields abundant tiny crystallites of cristobalite, and the

mineral is also deposited metastably from hot hydrous

gases in cavities and cracks of many volcanic rocks. It

has been found in lunar basalts and in meteorites and

is common in silica refractories exposed to very high

temperatures.

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244

opal

Opal is poorly crystalline or amorphous hydrous silica

that is compact and vitreous and most commonly translu-

cent white to colourless. Precious opal reﬂects light with

a play of brilliant colours across the visible spectrum, red

being the most valued. Opal forms by precipitation from

silica-bearing solutions near the Earth’s surface. Electron

microscopy has shown that many opals are composed of

spheres of tens to a few thousand angstroms in size that

are arranged in either hexagonal or cubic close packing.

The spheres are composed of hydrous silica that may be

either almost cristobalite-like, tridymite-like, mixtures of

both, or random and nondiffracting. The speciﬁc gravity

and refractive index are lower than those of pure silica

minerals. The play of colours in precious opal arises from

the diffraction of light from submicroscopic layers of

regularly oriented silica spheres. When heated, opal may

lose as much as 20 percent of its weight of water, frac-

ture, and then crystallize to one of the silica minerals

described above.

Opal usually contains 4 to 9 percent water, but lower

and much higher values have been observed. The con-

tents of alumina, ferric oxide, and alkalis are variable

but may amount to several percent in light-coloured

opals and more if pigmenting minerals are also present.

Precious opal has been synthesized. Opaline silica is a fri-

able hydrous silica found near hot springs and geysers.

Vitreous Silica

Vitreous silica, lechatelierite, is supercooled liquid

silica. It has been observed in nature as the result of

fusion of quartz by lightning strikes (fulgurites) or by

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shock associated with large meteorite impacts and may

approach artiﬁcial, very pure silica glass in composition

and physical properties.

Melanophlogite

Melanophlogite is a tetragonal or cubic silica mineral with

a gas-hydrate structure containing many large voids. In

nature these are ﬁlled with 6 to 12 percent by weight of

compounds of hydrogen, carbon, and sulfur, which may

be necessary for mineral growth. If these compounds

are destroyed by heating, they do not cause the crystal

to collapse, but the free carbon formed does darken it.

Melanophlogite occurs with bitumen and forms at tem-

peratures below 112 °C (about 234 °F). It has been found on

native sulfur crystals in Sicily and Santa Clara county, Calif.

Keatite

Keatite is a tetragonal form of silica known only from

the laboratory, where it can be synthesized metastably in

the presence of steam over a temperature range of 300

to 600 °C (about 570 to 1,100 °F) and a pressure range

of 400 to 4,000 bars (standard atmospheric pressure

at sea level is 1,013.3 millibars, or slightly more than 1

bar, which equals 760 mm of mercury). It has negative

thermal expansion along the a axis and positive thermal

expansion along the c axis, so that the overall expansion

is very low or negative.

coesite and Stishovite

Coesite and stishovite are rare dense forms of silica.

They are observed in nature only where quartz-bearing

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246

rocks have been severely shocked by a large meteorite

impact, such as Meteor Crater in Arizona, U.S. Coesite

is found in ultrahigh-pressure metamorphic rocks

such as in Dora Maira, Italy, and the Dabie Mountains,

China. Coesite is made up of tetrahedrons arranged

like those in feldspars. Stishovite is the densest form of

silica and consists of silicon that is octahedrally coor-

dinated with oxygen. Both coesite and stishovite have

been synthesized and found to be stable only at high

pressures.

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ny member of a family of minerals that contain the

carbonate ion, CO

, as the basic structural and com-

positional unit is referred to as a carbonate mineral. The

carbonates are among the most widely distributed miner-

als in the Earth’s crust.

The crystal structure of many carbonate minerals

reﬂects the trigonal symmetry of the carbonate ion, which

is composed of a carbon atom centrally located in an equi-

lateral triangle of oxygen atoms. This anion group usually

occurs in combination with calcium, sodium, uranium,

iron, aluminum, manganese, barium, zinc, copper, lead, or

the rare-earth elements. The carbonates tend to be soft,

soluble in hydrochloric acid, and have a marked anisot-

ropy in many physical properties (e.g., high birefringence)

as a result of the planar structure of the carbonate ion.

Most such rock-forming carbonates belong to one of

two structure groups—either calcite or aragonite. The

calcite structure is usually described with reference to

the sodium chloride structure in which the sodium and

chloride of halite are replaced by calcium atoms and CO

groups, respectively. The unit cell of halite is distorted by

compression along a three-fold axis, resulting in a rhom-

bohedral cell. In calcite all CO

groups are parallel and lie

in horizontal layers; CO

groups in adjacent layers, how-

ever, point in opposite directions. The calcium atoms are

bonded to six oxygen atoms, one each from three CO

groups in a layer above and three from CO

groups in a

chapter 7

carbonates and

other MInerals

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248

layer below. The structure of dolomite, CaMg(CO

)

, is

similar to that of calcite, CaCO

, except that there is reg-

ular alternation of calcium and magnesium, and a lower

symmetry, though still rhombohedral, results. The second

structure group, that of aragonite, is orthorhombic. Like

the calcite structure, the cation in the aragonite structure

is surrounded by six carbonate groups; the CO

groups,

however, are rotated about an axis perpendicular to their

plane and the cation is coordinated to nine oxygen atoms

instead of six.

Carbonate minerals other than simple carbonates

include hydrated carbonates, bicarbonates, and com-

pound carbonates containing other anions in addition to

carbonate. The ﬁrst two groups include nahcolite, trona,

natron, and shortite; they typically occur in sedimentary

evaporite deposits and as low-temperature hydrother-

mal alteration products. The members of the third group

generally contain rare-earth elements and almost always

result from hydrothermal alteration at low temperatures.

Examples of these carbonate minerals are bastnäsite,

doverite, malachite, and azurite.

A number of other, smaller mineral groups are also

included in this chapter. Many of these groups form bonds

with oxygen in a manner similar to the silicates. Others,

however, produce salts.

THE CARBONATES

There are approximately 80 known carbonate minerals,

but most of them are rare. The commonest varieties, cal-

cite, dolomite, and aragonite, are prominent constituents

of certain rocks: calcite is the principal mineral of lime-

stones and marbles; dolomite occurs as a replacement

for calcite in limestones, and when this is extensive the

rock is termed dolomite; and aragonite occurs in some

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recent sediments and in the shells of organisms that

have calcareous skeletons. Other relatively common car-

bonate minerals serve as metal ores: siderite, for iron;

rhodochrosite, for manganese; strontianite, for strontium;

smithsonite, for zinc; witherite, for barium; and cerussite,

for lead. Aragonite, calcite, and dolomite are described in

greater detail below.

Aragonite

Aragonite is a widespread mineral, the stable form of

calcium carbonate (CaCO

) at high pressures. It may be

distinguished from calcite, the commoner form of calcium

carbonate, by its greater hardness and speciﬁc grav-

ity. Aragonite is always found in deposits formed at low

temperatures near the surface of the Earth, as in caves as

stalactites, in the oxidized zone of ore minerals (with lead

substituting for calcium), in serpentine and other basic

rocks, in sediments, and in iron-ore deposits. Aragonite is

the mineral normally found in pearls. It is polymorphous

(same chemical formula but different crystal structure)

with calcite and vaterite, and, with geologic time, prob-

ably inverts to calcite even under normal conditions.

Aragonite is an important element in the shells and

tests of many marine invertebrates. These animals can

secrete the mineral from waters that would ordinarily

yield only calcite; they do so by physiological mechanisms

that are not fully understood.

calcite

Calcite is the most common form of natural calcium car-

bonate (CaCO

), a widely distributed mineral known for

the beautiful development and great variety of its crystals.

It is polymorphous (same chemical formula but different

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250

crystal structure) with the minerals aragonite and vaterite

and with several forms that apparently exist only under

rather extreme experimental conditions.

The carbonate minerals calcite, aragonite, and dolo-

mite have been calculated to make up approximately 15

percent of the Earth’s sediments and sedimentary rocks

and about 2 percent of the terrestrial crust. A large

percentage of the calcite, the most abundant of these

carbonate minerals, occurs in limestones, which con-

stitute noteworthy proportions of many sequences of

marine sediments. Calcite is also the chief component of

marls, travertines, calcite veins, most speleothems (cave

deposits), many marbles and carbonatites, and some ore-

bearing veins.

Calcite is the stable form of CaCO

at most tem-

peratures and pressures. The orthorhombic polymorph

of CaCO

, aragonite, though frequently deposited in

nature, is metastable at room temperature and pres-

sure and readily inverts to calcite; the inversion has

been shown experimentally to be spontaneous when

aragonite is heated to 400 °C (about 750 °F) in dry air

and at lower temperatures when it is in contact with

water. Hexagonal vaterite, the other natural polymorph

of CaCO

, is extremely rare and has been shown in the

laboratory to transform into calcite or aragonite or

both penecontemporaneously with its formation—i.e.,

it appears to be metastable under essentially all known

natural conditions.

chemical composition

Some natural calcites are essentially pure CaCO

; others

contain noteworthy percentages of other cations (e.g.,

Mg, Mn, Fe, boron [B], bromine [Br], Sr, and/or Y), sub-

stituting for some of their calcium. In general, however,

only minor substitution occurs. Only manganese and

251

magnesium are known to substitute extensively for the

calcium: manganocalcite and calcian rhodochrosite (i.e.,

calcium-bearing MnCO

) have been identiﬁed, and solid

solution has been shown to be possible from pure CaCO

to 40 percent MnCO

and from pure MnCO

to about

25 percent CaCO

. Metastable magnesian calcites con-

taining from approximately 5 to 18 percent MgCO

occur

rather widely as biogenic skeletal material and as cement

in some modern marine sediments. Magnesian calcites at

the lower end of this range of MgCO

contents constitute

some marine oöids and calcareous tufas.

Between 60 and 70 elements have been recorded in

minor or trace amounts in limestone analyses. Some of

these elements may occur as substitutions within calcite;

others seem more likely to represent minor constituents,

such as clay minerals, within the analyzed rocks.

The chemical composition of calcite is responsible

for the test that is widely used to identify it, especially in

the field. This test is based on the fact that calcite reacts

with dilute hydrochloric acid (HCl), and the reaction is

manifested by vigorous effervescence. (The dilution of

the HCl usually used is about 90:10 [water:concentrated

HCl].) The reactions involved are

The effervescence is due to the spontaneous break-

down of the carbonic acid (H

) to produce carbon

dioxide gas, CO

crystal Structure

The structure of calcite—one of the ﬁrst mineral struc-

tures to be determined by X-ray methods—has been

described on three different bases. The two most

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