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Rock crystal

Quartz
Quartz, Tibet.jpg
Quartz crystal cluster from Tibet
General
Category oxide mineral
Formula
(repeating unit)
SiO2
Strunz classification 4.DA.05 (Oxides)
Dana classification 75.01.03.01 (tectosilicates)
Crystal system α-quartz: trigonal
β-quartz: hexagonal
Crystal class α-quartz: trapezohedral (class 3 2); β-quartz: trapezohedral (class 6 2 2)
Unit cell a = 4.9133 Å, c = 5.4053 Å; Z=3
Identification
Color Colorless through various colors to black
Crystal habit 6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive
Twinning Common Dauphine law, Brazil law and Japan law
Cleavage {0110} Indistinct
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 7 – lower in impure varieties (defining mineral)
Luster Vitreous – waxy to dull when massive
Streak White
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)
Pleochroism None
Melting point 1670 °C (β tridymite) 1713 °C (β cristobalite)
Solubility Insoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2
Other characteristics Piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)
References

Quartz is the second most abundant mineral in Earth's continental crust, after feldspar. Its crystal structure is a continuous framework of SiO4siliconoxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2.

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

The word "quartz" is derived from the German word "Quarz" and its Middle High German ancestor "twarc", which probably originated in Slavic, cf. Czech tvrdý ("hard"), Polish twardy ("hard"), Serbian and Croatian tvrd ("hard").

The Ancient Greeks referred to quartz as κρύσταλλος (krustallos) derived from the Ancient Greek κρύος (kruos) meaning "icy cold", because some philosophers (including Theophrastus) apparently believed the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for the purest form of quartz.

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; usually the crystals are attached at the other end to a matrix and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance within gypsum. A quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward.


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