Quartz

Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.[10]

Quartz
General
Categorysilicate mineral[1]
Formula
(repeating unit)
SiO2
IMA symbolQz[2]
Strunz classification4.DA.05 (oxides)
Dana classification75.01.03.01 (tectosilicates)
Crystal systemα-quartz: trigonal
β-quartz: hexagonal
Crystal classα-quartz: trapezohedral (class 3 2)
β-quartz: trapezohedral (class 6 2 2)[3]
Space groupα-quartz: P3221 (no. 154)[4]
β-quartz: P6222 (no. 180) or P6422 (no. 181)[5]
Unit cella = 4.9133 Å, c = 5.4053 Å; Z=3
Identification
Formula mass60.083 g·mol−1
ColorColorless through various colors to black
Crystal habit6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive
TwinningCommon Dauphine law, Brazil law and Japan law
Cleavage{0110} Indistinct
FractureConchoidal
TenacityBrittle
Mohs scale hardness7 – lower in impure varieties (defining mineral)
LusterVitreous – waxy to dull when massive
StreakWhite
DiaphaneityTransparent to nearly opaque
Specific gravity2.65; variable 2.59–2.63 in impure varieties
Optical propertiesUniaxial (+)
Refractive indexnω = 1.543–1.545
nε = 1.552–1.554
Birefringence+0.009 (B-G interval)
PleochroismNone
Melting point1670 °C (β tridymite) 1713 °C (β cristobalite)[3]
SolubilityInsoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2[3]
Other characteristicslattice: hexagonal, Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)
References[6][7][8][9]

Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold.

There are many different varieties of quartz, several of which are classified as gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia.

Quartz is the mineral defining the value of 7 on the Mohs scale of hardness, a qualitative scratch method for determining the hardness of a material to abrasion.


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