Quartz belongs to the trigonal crystal system. The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. Well-formed crystals typically form in a 'bed' that has unconstrained growth into a void, but because the crystals must be attached at the other end to a matrix, only one termination pyramid is present. There are exceptions as doubly terminated crystals do occur. An occurrence in Herkimer County, New York is noted for these Herkimer diamonds with terminations at both ends. A quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward.
Crystal structure of α-quartzβ-quartzα-quartz crystallizes in the trigonal crystal system, space group P3121 or P3221. β-quartz belongs to the hexagonal system, space group P6221 or P6421. These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO4 tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without change in the way they are linked, this process is called the quartz inversion.
Quartz is an essential constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is also a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Because of its resistance to weathering it is very common in stream sediments and in residual soils. Quartz, therefore, occupies the lowest potential to weather in the Goldich dissolution series.
Quartz occurs in hydrothermal veins as gangue along with ore minerals. Large crystals of quartz are found in pegmatites. Well-formed crystals may reach several meters in length and weigh as much as 1,400 pounds (640 kg).
Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare. A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.
Tridymite and cristobalite are high-temperature polymorphs of SiO2 that occur in high-silica volcanic rocks. Coesite is a denser polymorph of quartz found in some meteorite impact sites and in metamorphic rocks formed at pressures greater than those typical of the Earth's crust. Stishovite and Seifertite are yet denser and higher-pressure polymorphs of quartz found in some meteorite impact sites. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.
Most quartz used in microelectronics is produced synthetically. Large, flawless and untwinned crystals are produced in an autoclave via the hydrothermal process. The process involves treating crushed natural quartz with hot aqueous solution of a base such as sodium hydroxide. The hydroxide serves as a "mineralizer", i.e. it helps dissolve the "nutrient" quartz. High temperatures are required, often around 675 °C. The dissolved quartz then recrystallizes at a seed crystal at slightly lower temperatures. Approximately 200 tons of quartz were produced in the US in 2005; large synthesis facilities exist throughout the world. Synthetic quartz is often evaluated on the basis of its Q factor, a measure of its piezoelectric response and an indicator of the purity of the crystal.
Quartz crystal cluster from Tibet
|Chemical formula||Silica (silicon dioxide, SiO2)|
|Crystal symmetry||Trigonal H–M Symbol 32|
|Unit cell||a = 4.9133 Å, c = 5.4053 Å; Z = 3|
|Color||From colorless to black, through various colors|
|Crystal habit||6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive|
|Crystal system||α-quartz: trigonal trapezohedral class 3 2; β-quartz: hexagonal 622|
|Twinning||Common Dauphine law, Brazil law and Japan law|
|Mohs scale hardness||7 – lower in impure varieties (defining mineral)|
|Luster||Vitreous – waxy to dull when massive|
|Diaphaneity||Transparent to nearly opaque|
|Specific gravity||2.65; variable 2.59–2.63 in impure varieties|
|Optical properties||Uniaxial (+)|
|Refractive index||nω = 1.543–1.545|
nε = 1.552–1.554
|Birefringence||+0.009 (B-G interval)|
|Melting point||1670 °C (β tridymite)|
1717 °C (β cristobalite)
|Solubility||Insoluble at STP; 1 ppmmass at 400 °C and 34 bar to 2600 ppmmass at 500 °C and 103 bar|
|Other characteristics||Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)|