Rafferty J.P. (ed.) Minerals

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Mammoth Hot Springs in Yellowstone National Park in

the western United States.

Quartz is mechanically resistant and relatively inert

chemically during rock weathering in temperate and cold

climates. Thus, it becomes enriched in river, lake, and

beach sediments, which commonly contain more than one-

half quartz by weight. Some strata consist almost entirely

of quartz over large lateral distances and tens or hundreds

of metres in thickness. Known as glass sands, these strata

are important economic sources of silica for glass and

chemical industries. Quartz-bearing strata are abundant in

metamorphic terrains. The reincorporation of free silica

into complex silicates and the solution and redeposition of

silica into veins is characteristic of such terrains.

the Silica Phase diagram

In diagrams of pressure-temperature ﬁelds of stability of

silica minerals, stability ﬁelds are not shown for keatite,

melanophlogite, opal, or the low forms of tridymite and

cristobalite because they have not been demonstrated.

Quartz is the stable phase of silica under the physical

conditions that prevail over most of the Earth’s crust.

Coesite occurs at depths of about 100 km (60 miles) in

the Earth’s mantle. Stishovite would require even greater

depths of burial, and no rocks that occur on the terres-

trial surface have been buried so deeply. Stishovite is

reported only in a few localities that were subjected to

very high pressures from meteorite impact events.

USES

Quartz is the only natural silica mineral used in signiﬁ-

cant quantities; millions of tons are consumed annually

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by many industries. The sand that is an essential ingre-

dient of concrete and mortar is largely quartz, as are

the sandstone and quartzite used as building stones.

Crushed sandstone and quartzite are used for road and

railway construction, rooﬁng granules, and riprap—

erosion-control linings of river channels. Quartz is hard

(7 on the Mohs scale) and resists fracture because it lacks

easy cleavage. These properties, combined with its ready

availability, lead to its use as a sandpaper abrasive and in

sandblasting; for polishing and cutting glass, stone, and

metal; and for providing traction on stairs, streets, and

rails. Large amounts of relatively pure quartz are used

in refractory products, such as insulation and ﬁrebricks,

foundry molds, and electrical insulators, because of the

combination of its high melting temperatures, low coef-

ﬁcients of expansion, inertness of the high-temperature

forms of silica, and low costs.

Relatively pure quartz is required in large tonnages

as an ingredient for glass and porcelain manufacture.

High purity quartz is fused to make premium grades of

chemical and optical glass for which one or more of its

desirable properties of low thermal expansion, high-

shape stability, elasticity, low solubility, and transparency

to various kinds of light can justify the greatly increased

costs involved. Fibres of vitreous silica are essential for

precision instruments, such as balances, galvanometers,

and gravimeters. Tons of quartz of various qualities are

used as raw materials for processes in which silica is not

the ﬁnal product. These include the production of water

glass, or sodium silicate, various sols—very ﬁne disper-

sions of solids in liquids—that are used as hydrophobic

(water-repelling) coatings, organic silicates and silicones,

silicon carbide, silicon metal, smelting ﬂux, and alloying

in metallurgy.

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Quartz and its varieties have been used since antiquity

as semiprecious gems, ornamental stones, and collector’s

items. Precious opal, a hydrous form of silica, has been a

gemstone since Roman times.

iNDiViDUA l SiliCA MiNERAlS

Quartz is by far the most commonly occurring form.

Tridymite, cristobalite, and the hydrous silica mineral

opal are uncommon, and vitreous (glassy) silica, coesite,

and stishovite have been reported from only a few locali-

ties. Several other forms have been produced in the

laboratory but have not been found in nature. Keatite is

one example of a silica mineral that has been produced

synthetically.

Quartz

Quartz occurs in many varieties in almost all types of

igneous, sedimentary, and metamorphic rocks. It has

also been found in meteorites and in some lunar rocks.

Quartz is a widely distributed mineral that occurs in

many varieties. It consists primarily of silica, or sili-

con dioxide (SiO

). Minor impurities such as lithium,

sodium, potassium, and titanium may be present.

Quartz has attracted attention from the earliest times;

water-clear crystals were known to the ancient Greeks

as krystallos —hence the name crystal , or more commonly

rock crystal , applied to this variety. The name quartz is

an old German word of uncertain origin ﬁrst used by

Georgius Agricola in 1530.

Quartz is the second most abundant mineral in the

Earth’s crust after feldspar. It occurs in nearly all acid

igneous, metamorphic, and sedimentary rocks. It is an

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essential mineral in such silica-rich felsic rocks as gran-

ites, granodiorites, and rhyolites. It is highly resistant to

weathering and tends to concentrate in sandstones and

other detrital rocks. Secondary quartz serves as a cement

in sedimentary rocks of this kind, forming overgrowths

on detrital grains. Microcrystalline varieties of silica

known as chert, ﬂint, agate, and jasper consist of a ﬁne

network of quartz. Metamorphism of quartz-bearing

igneous and sedimentary rocks typically increases the

amount of quartz and its grain size.

Quartz crystals lack a centre of symmetry or planes

of symmetry and have one crystallographic axis (c) per-

pendicular to three polar axes (a) that are 120° apart.

One end of a polar axis is different from its other end;

when mechanical stress is applied on such an axis, oppo-

site electrical charges develop on each end. This leads to

important applications in electronics as a frequency con-

trol and in pressure gauges and other devices. The lack

of symmetry planes parallel to the vertical axis allows

quartz crystals to occur as two types: left-handed or right-

handed (enantiomorphism). Left-handed quartz is less

than 1 percent more abundant than right-handed quartz.

The structural tetrahedrons spiral upward through the

crystal in the sense of the handedness parallel to the c

axis. Similarly, if polarized light is transmitted by a quartz

crystal along the c-axis direction, the plane is rotated in

the direction of the handedness by tens of degrees per

millimetre, the amount depending on the wavelength of

the light. This property is used in optical instruments

such as monochromators.

Quartz is piezoelectric: a crystal develops positive and

negative charges on alternate prism edges when it is sub-

jected to pressure or tension. The charges are proportional

to the change in pressure. Because of its piezoelectric

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property, a quartz plate can be used as a pressure gauge, as

in depth-sounding apparatus.

Just as compression and tension produce oppo-

site charges, the converse effect is that alternating

opposite charges will cause alternating expansion and

contraction. A section cut from a quartz crystal with deﬁ-

nite orientation and dimensions has a natural frequency

of this expansion and contraction (i.e., vibration) that

is very high, measured in millions of vibrations per sec-

ond. Properly cut plates of quartz are used for frequency

control in radios, televisions, and other electronic com-

munications equipment and for crystal-controlled clocks

and watches.

Quartz shows less range in chemical composition than

do most other minerals, but it commonly contains tens to

hundreds of parts per million of aluminum atoms substi-

tuting for silicon atoms, with charge balance maintained

by the incorporation of small atoms, such as hydrogen,

lithium, or sodium. Titanium, magnesium, or iron atoms

substituting for silicon atoms also have been reported,

but anionic substitution (i.e., substitution for the negative

ion, oxygen) is limited because the linkage of the tetrahe-

drons is disrupted.

Quartz exists in two forms: (1) alpha-, or low, quartz,

which is stable up to 573° C (1,063° F), and (2) beta-, or high,

quartz, stable above 573° C. The two are closely related,

with only small movements of their constituent atoms

during the alpha-beta transition. The structure of beta-

quartz is hexagonal, with either a left- or right-handed

symmetry group equally populated in crystals. The

structure of alpha-quartz is trigonal, again with either a

right- or left-handed symmetry group. At the transition

temperature the tetrahedral framework of beta-quartz

twists, resulting in the symmetry of alpha-quartz; atoms

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move from special space group positions to more gen-

eral positions. At temperatures above 867° C (1,593° F),

beta-quartz changes into tridymite, but the transfor-

mation is very slow because bond breaking takes place

to form a more open structure. At very high pressures

alpha-quartz transforms into coesite and, at still higher

pressures, stishovite. Such phases have been observed in

impact craters.

Quartz may contain inclusions of other minerals,

such as rutile (rutilated quartz), tourmaline, asbestiform

amphiboles, or platy minerals, such as mica, iron oxides,

or chlorite (aventurine).

Coloured varieties of quartz are numerous and have

many causes. Most colours result from mechanically

incorporated admixtures within fine-crystallized or

granular quartz, but some coarse-crystallized varieties,

quartz agates from Mexico. Courtesy of Joseph and Helen Guetterman

collection;photograph, John H. Gerard—EB Inc.

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such as amethyst (vio-

let), citrine (yellow),

milky quartz, smoky

quartz or morion

(black), or rose quartz,

may be coloured by

ions other than silicon

and oxygen that occur

within the crystal struc-

ture. Small fractions of

1 percent by weight of

iron, aluminum, man-

ganese, titanium, hydrogen, and small alkali atoms,

such as lithium and sodium, have been shown to be

the cause of different colours. Heat treatment or vari-

ous irradiation treatments under oxidizing or reducing

atmospheres are used to change one coloured vari-

ety to another. Citrine is commonly produced by

heat-treating amethyst at 250–400 °C (482–752 °F),

for example.

Quartz has great economic importance. Many vari-

eties are gemstones, including amethyst, citrine, smoky

quartz, and rose quartz. Sandstone, composed mainly of

quartz, is an important building stone. Large amounts

of quartz sand (also known as silica sand) are used in the

manufacture of glass and ceramics and for foundry molds

in metal casting. Crushed quartz is used as an abrasive

in sandpaper, silica sand is employed in sandblasting, and

sandstone is still used whole to make whetstones, mill-

stones, and grindstones. Silica glass (also called fused

quartz) is used in optics to transmit ultraviolet light.

Tubing and various vessels of fused quartz have important

laboratory applications, and quartz ﬁbres are employed

in extremely sensitive weighing devices.

Rose quartz. B.M. Shaub

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chalcedony

Chalcedony is a white, buff, or light tan, ﬁnely crystal-

lized or ﬁbrous quartz that forms rounded crusts, rinds,

or stalactites (mineral deposits suspended from the

roofs of caverns) in volcanic and sedimentary rocks as a

precipitate from moving solutions. If chalcedony is con-

spicuously colour-banded, it may be called agate; onyx

is agate with alternate bands of white and black or dark

brown. Some concentrically banded “eye” agate nodules

contain cores of coarsely crystalline quartz, and other

agates are mottled or variegated in colour. Arborescent

or dendritic (branching) dark-coloured patterns set in

a lighter ﬁeld are called moss agate or Mocha stone.

Translucent red chalcedony is called carnelian, and trans-

lucent brown shades are referred to as sard; both are

pigmented by admixed iron oxides.

Chrysoprase, plasma, and prase are names for green

varieties of chalcedony coloured by admixed green miner-

als, such as chlorite, ﬁbrous amphiboles, or hydrous nickel

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SARD AND SARDONyX

Sard and sardonyx are translucent, light- to dark-brown varieties of

the silica mineral chalcedony, historically two of the most widely

used semiprecious stones. Sard and its close relative carnelian have

been used in engraved jewelry for centuries. Sard (from Sardis,

the ancient capital of Lydia) was originally called sardion, which

included both sard and carnelian until the Middle Ages. Except for

crystal, it is the oldest known name for a silica mineral. One locality

famous as a source of sard is Ratnapura, Sri Lanka. Bands of sard

and white chalcedony are called sardonyx, which at one time was

more precious than gold, silver, or sapphire. Sardonyx is widely used

in cameos and intaglios. Its properties are those of quartz.

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silicates. Bloodstone and heliotrope are green chalcedony

with red spots.

Jasper, chert, and Flint

Jasper is opaque red, brown, or yellow quartz that is pig-

mented by admixed iron oxides. Chert and ﬂint are ﬁnely

crystallized varieties of gray to black quartz that occur as

nodules or bands in sedimentary rocks.

Jasper

Jasper is an opaque, ﬁne-grained or dense variety of the

silica mineral chert that exhibits various colours. Chieﬂy

brick red to brownish red, it owes its colour to admixed

hematite; but when it occurs with clay admixed, the colour

is a yellowish white or gray, or with goethite a brown or

yellow. Jasper, long used for jewelry and ornamentation,

has a dull lustre but takes a ﬁne polish, and its hardness

and other physical properties are those of quartz.

The name jasper is from the Greek iaspis, of Semitic

origin; in ancient writings the term was chieﬂy applied

to translucent and brightly coloured stones, particularly

chalcedony, but also was applied to the opaque jasper.

Medicinal values were long attributed to jasper, including

a belief that wearing it strengthened the stomach.

Jasper is common and widely distributed, occurring

chieﬂy as veinlets, concretions, and replacements in sedi-

mentary and metamorphic rocks, as in the Urals, North

Africa, Sicily, Germany, and elsewhere. Some varieties are

colour-banded, and beautiful examples of jasperized fossil

wood are found in Arizona, U.S. Jasper is also common as

detrital pebbles.

For thousands of years, black jasper (and also black

slate) was used to test gold-silver alloys for their gold

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content. Rubbing the alloys on the stone, called a touch-

stone, produces a streak the colour of which determines

the gold content within one part in one hundred.

chert and Flint

Chert and ﬂint are varieties of very ﬁne-grained quartz.

Several varieties are included under the general term

chert: jasper, chalcedony, agate, ﬂint, porcelanite, and

novaculite.

Flint is gray to black and nearly opaque (translucent

brown in thin splinters) because of included carbonaceous

matter. Opaque, dull, whitish to pale-brown or gray speci-

mens are simply called chert; the light colour and opacity

are caused by abundant, extremely minute inclusions of

water or air. The physical properties are those of quartz.

Chert and ﬂint provided the main source of tools and

weapons for Stone Age man. The uniform ﬁne grain, brit-

tleness, and conchoidal fracture made it relatively easy to

shape arrowheads by ﬂaking off chips, and the edges pro-

duced were quite sharp. Quarrying and manufacture of

ﬂint weapons were among humankind’s earliest business

ventures, and it is sometimes possible to trace ancient

trade routes by knowing where a particular type of ﬂint

was obtained. From the 17th through the early 19th cen-

tury, ﬂints again found extensive military use in ﬂintlock

riﬂes. Crushed ﬂint is still used as the abrasive agent on

sandpapers for the ﬁnishing of wood and leather. In addi-

tion, ﬂint pebbles are used in mills that grind raw materials

for the ceramic and paint industries; the use of ﬂint peb-

bles instead of steel balls as a grinding agent is desirable

in order to avoid contaminating the product with iron.

Considerable amounts of chert are also used in road con-

struction and as concrete aggregate. Some chert takes an

excellent polish and serves as semiprecious jewelry.

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