quartz: Definition and Much More from Answers.com
- ️Wed Jul 01 2015
The most common oxide on the Earth's surface, constituting 12% of the crust by volume. Quartz is a crystalline form of silicon dioxide (SiO2). Among the igneous rocks, quartz is especially common within granites, granodiorites, pegmatites, and rhyolites. In addition, quartz can be observed in low- to high-grade metamorphic rocks, including phyllites, quartzites, schists, granulites, and eclogites. Because hydrothermal fluids are enriched in dissolved silica, the passage of fluids through rock fractures results in the emplacement of quartz veins. See also Granite; Granodiorite; Igneous rocks; Metamorphic rocks; Pegmatite; Rhyolite.
Once quartz has formed, it persists through erosional reworking because of its low solubility in water (parts per million) and its high mechanical hardness (7 on Mohs scale). Consequently, quartz becomes increasingly concentrated in beach sands as they mature, and it is a major component of sandstone. In sedimentary environments, quartz also forms as the final crystallization product during silica diagenesis; amorphous silica on the sea floor that derives from the skeletons of diatoms, radiolarians, and sponges will transform to quartz upon prolonged exposure to increased temperatures (≤300°C or 572°F) and pressures (≤2 kilobars or 200 pascals) after burial. See also Diagenesis; Hardness scales; Sandstone.
As with virtually all silicates, the atomic framework of the quartz structure consists of Si4+ cations that are tetrahedrally coordinated by oxygen anions (O2−). Every oxygen anion is bonded to two silicon cations, so that the tetrahedral units are corner-linked to form continuous chains. In low-temperature quartz (or α-quartz), two distinct tetrahedral chains spiral about the crystallographic c axis.
Although the silica tetrahedra can be depicted as spirals about the c axis in a left-handed sense, right-handed quartz crystals are found in nature as abundantly as are left-handed crystals. These enantiomorphic varieties are known as the Brazil twins of quartz, and they may be distinguished by crystal shape (corresponding crystal faces occur in different orientations) and by opposite optical activities. See also Crystal optics.
Impurity concentrations in natural α-quartz crystals usually fall below 1000 parts per million. The violet and yellow hues observed in amethyst and citrine are associated with Fe, and black smoky quartz contains Al. The white coloration of milky quartz reflects light scattering off minute fluid inclusions, and the pink tint in rose quartz is believed to arise from fine-scale intergrowths of a pegmatitic mineral called dumortierite [Al27B4Si12O69(OH)3]. See also Amethyst; Dumortierite.
Quartz is used predominantly by the construction industry as gravel and as aggregate in concrete. In addition, quartz is important in advanced technologies. Quartz is piezoelectric and has an extremely high quality factor. The high quality factor means that a bell made of quartz would resonate (ring) for a very long time. This property, combined with its piezoelectric behavior, makes quartz the perfect crystal for oscillators in watches.
Compression of α-quartz perpendicular to the c axis creates an electrostatic charge, and this property is exploited in oscillator plates in electronic components. Large flawless crystals of quartz are routinely synthesized for oscillators and for prisms in laser optic systems. Quartz also is employed in abrasives, fluxes, porcelains, and paints. See also Concrete; Oscillator; Piezoelectricity.
SiO
Hexagonal -- Trigonal trapezohedral
Environment
Commonest of minerals, found in every class of rocks and forming in every conceivable condition.
Crystal description
Quartz can be divided into two groups on the basis of its appearance: crystallized and microcrystalline. The microcrystalline group can in turn be subdivided into a parallel, fibrous crystal arrangement and a heterogeneous, finely granular type.
1. Crystallized quartz often occurs in large, well-formed, colorless crystals or crystal crusts, often transparent and gemmy. Colorless crystals are known as rock crystal. Violet crystals are known as amethyst. Yellow-brown crystals are known as citrine. Gray to black crystals are known as smoky quartz. Also forms veins or central cores in pegmatites of coarsely crystallized material, which may be milky quartz or rose quartz.
2a. Chalcedony is a microscopically crystallized, translucent variety of the same mineral, in which layers of microscopic individual crystals have arranged themselves as layers of slender upright fibers arranged in parallel bands. The upper surface of a chalcedony-lined pocket tends to be botryoidal or mammillary, and is often uniformly smooth. White or cloudy banding irregularities commonly develop on cross-fractured weathering surfaces. In agate, the banding is colored and penetrating, often making a series of concentric rings lining geodes.
2b. The chert, flint, and jasper group are also tight and hard, microscopically grained quartz, but in these there is no definite banding nor translucency. They form dull-surfaced opaque masses. Usually chert and jasper have appreciable quantities of impurities, with lusters ranging from earthy to sub-glassy (flint) to matte. Jasper is often colorful, tinted by ocher or hematite.
Massive white quartz chunks, sometimes with gold, can originate in solid veins that slash older rocks. Massive quartz, quartz sand, and disseminated grains of quartz in other rocks, or in pebbles of quartz or quartzite, are very common, and usually are the most important constituents of any gravel or sand beach.
Free-growing quartz crystals vary in habit from long slender prisms to crusts or "points." Prism faces are usually horizontally striated; six terminal faces usually exhibit alternating development of larger and smaller faces (indicating the rhombohedral rather than a hexagonal symmetry). More than a thousand differently indexed faces have been described (on alphaquartz, see below).
Because of the composition, quartz and the other SiO 2 minerals, cristobalite, tridymite, and coesite were long grouped with the oxides, but their physical properties and crystal structure are more in accord with those of the silicate group, so it now seems appropriate to group them as silicates. Quartz itself has several crystal-class modifications, and although it has only right- or left-hand rhombohedral symmetry at normal temperatures, it is fully developed or paired rhombohedral at temperatures above 573°C. This higher symmetry form is known as beta-quartz (ȣ-quartz), and only forms when quartz crystallizes from truly hot solutions. However, after crystallizing -- as its surroundings cool below 573°C -- the structure then changes to alpha-quartz (ᅣ-quartz), and all quartz that we find in our temperate surroundings is of course ᅣ-quartz. Quartz has been useful as a geological thermometer, because the habit it assumes sometimes indicates whether it formed above or below 573°C.
Two chemically identical minerals to be described following quartz carry geological thermometry higher. SiO 2 crystallizing above 870°C forms platy orthorhombic crystals of a mineral known as tridymite (TRID ih mite) . Crystallizing above 1470°C, SiO 2 forms in white cubic-system crystals and is known as cristobalite. Hence, from the series of differently crystallizing compounds of this fortunately common substance, we can deduce the temperature of the formation of many rocks. The two high-temperature SiO 2 compounds are rarer than might be expected, for in solidifying, cooling rocks stay liquid down to far lower temperatures expected if we were to judge by the heat that is required to remelt them, once they have recrystallized. Under some conditions it is thought that tridymite and cristobalite can form at temperatures slightly lower than those at which they are really stable.
Physical properties
Colorless, white, smoky, rose, violet, green, yellow and brown; also translucent and tinted any hue by impurities. Luster glassy; hardness 7; specific gravity 2.6; fracture conchoidal (irregular for microcrystalline); cleavage rhombohedral, sometimes observable. Large crystals brittle, microcrystalline varieties tough to very tough; transparent to subtranslucent from impurities. Rock crystal often triboluminescent; electrically responsive to pressure (piezoelectric) and current; irradiation will make aluminous rock crystal smoky, and subsequent heating will then turn some examples yellow.
Composition
Silicon dioxide (46.7% Si, 53.3% O).
Tests
Infusible (before the blowpipe; it melts at 1720°C), insoluble. The hardness of 7 is important. Powder mixed with sodium carbonate fuses to a clear glass.
Distinguishing characteristics
The luster and fracture are typical, together with a hardness greater than that of most similar minerals. Crystals are easy to recognize if the hexagonal pattern (of usually alternating larger and smaller triangular faces) or the typical points can be seen. Horizontal striations on the prism are very helpful. Specific gravity is a useful test for this mineral. A crystal flake held in gas flame flies to pieces (as soon as it reaches 573°C). White beryl just frosts but stays intact, white topaz has a ready cleavage. On a diamond saw, a groove being cut in rock crystal shows an orange glow of constant sparking.
Occurrence
Quartz can occur almost anywhere. High-temperature veins are usually coarsely crystallized, whereas low-temperature veins and geodes in sedimentary rocks may be lined with crystals or show one of the finer-grained varieties.
Veins with colorless crystals of "rock crystal" are abundant everywhere: in Arkansas in the Hot Springs area; at Little Falls, New York, in small, brilliant, doubly terminated crystals ("Herkimer diamonds"); in Ontario near Lyndhurst. The smoky crystals of the Pikes Peak area of Colorado are often spectacular, and some fine crystals have come from the Maine and the California pegmatite areas. Foreign localities include the famous Alpine crystal-lined pocket occurrences (it seems that the higher their discovery elevation, the smokier their crystals) and the commercially important Madagascar and Brazilian rock crystals.
Beautiful amethyst is just too common to be in the precious category. It is found in Maine, Pennsylvania, Virginia, North Carolina, along the north shore of Lake Superior (Thunder Bay), Arizona, and elsewhere in the U.S. The purple variety is more often in points than in tall prismatic crystals, but Mexico is the home of two fine prismatic sources, one in Guerrero (veins) with steep rhombohedron faces, and a second at Las Vegas, in the state of Vera Cruz (pockets in andesitic lava). Bahia and Rio Grande do Sul are the two important amethyst-producing states in Brazil; also in neighboring Uruguay and Bolivia. For many years an occurrence in Kapnik, Hungary, was about the only known site for elongated amethysts; almost all the South American specimens were groups of points.
Citrine quartz is naturally yellow, but rather uncommon. An occurrence of internally gemmy, though superficially ugly, frosted prisms in veins in granite at Villasbuenos, Salamanca, Spain was the original site for "Spanish Topaz," long a popular jewelers' name for citrine. With heat of around 400-500°C, Brazilian amethysts bleach, and above 500°C (but under 573°C) turn golden to brown; others show green after such treatment. A combination, now called ametrine, which has citrine and amethyst sections in the same crystal, has been found in southern Bolivia. Irradiation turns heated amethyst-citrines back to amethyst, but ametrine appears to be natural
Slightly cloudy rose quartz, perhaps with microcrystalline rutile, is a pegmatite mineral often occupying the quartz core of a pegmatite. Pink crystals are scarce; token ones were found long ago at Newry, Maine, and more spectacular ones at two Brazilian localities. The failure to find large, well-formed crystals of rose quartz is a geological mystery, and even now there seem to be some possibly basic differences between the common solid rose quartz of pegmatite cores and the three occurrences of pink-hued, slightly cloudy, late-formed crystals often associated with pink chalcedony. Sapucaia and Taquaral in Minas Gerais, Brazil, have produced some beautiful specimens of pink crystals. Glass-clear pink bands have been noted in some Brazilian rock crystals but have not yet been studied.
Smoky quartz appears to have been naturally irradiated aluminum-bearing material. It is common in the Alps; there seems a relationship between the elevation of the source and the blackness of the crystals.
The best agate forms in cavities in basaltic rock, as in the w. U.S., northern Mexico, and in Rio Grande do Sul, Brazil, the chief source of commercial agate. The famous Idar gemcutting industry of Germany owes its start to the occurrence there of agate concretions in neighboring lava flows. Queensland, Australia has fine small agate nodules and much of the jadelike green chalcedony called chrysoprase.
While we prefer to designate as agate the abundant Brazilian banded material, the word is loosely used and often applied to nonbanded material, as in the dendritic black-stained chalcedony known as Montana agate, the green and red "moss"-saturated chalcedony known as moss agate, and the jasperized (inappropriately called agatized) Arizona petrified wood. There are various opaque microcrystalline examples. Bloodstone, for example, seems to be a moss agate so full of green and hematite specks, that it approaches jasper's opacity. Microcrystalline quartz that forms pseudomorphs after asbestos fibers can be cut into chatoyant stones known as "tiger eye." Similar material rich in red hematite is found in Australia and sold as "tiger iron."
Whole books have been written on quartz and its relatives. Volume 3 of the seventh edition of Dana is devoted to SiO 2 .
Remarks
An important industrial material from many standpoints. Sand is used in glass manufacture or to make fused silica. The clear rock crystal is of great value in electronic equipment, as in oscillators for controlling radio frequencies and in watches. The beautifully colored varieties have gem value. Rose quartz often shows asterism when cut in a sphere or hemisphere. White quartz veins are common guides to gold in some regions. Chalcedony and agate, which can be dyed in rainbow hues for decorative use, are also of value for bearings and in mortars. Flint, for which the Dover Cliffs are famous, is a gray to black, subvitreous and compact variety once valued for the sparks it gave when struck by steel. Chrysoprase is a natural, nickel-stained green chalcedony but it is perfectly duplicated with dyes in Idar Oberstein, Germany. Simple "onyx" is a dyed-black chalcedony (not to be confused with banded calcite onyx of cave and hot spring deposits). There is a rainbow of other dyed chalcedony-onyxes. Chert, the common light-colored nodule quartz in Midwest limestones, was used for arrowheads. Jasper is usually darker and more opaque and may contain as much as 20 percent impurities such as hematite and goethite. When fairly pure it can be very, very tough and has been used in ball mills for crushing sulfide ores. Cryptocrystalline (microcrystalline) quartz is a favorite medium for the gem carvers of Idar Oberstein, which owes its preeminence to the once-abundant agate mined in the hills and contained in stream cobbles.