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rock crystal:see quartzquartz,
one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO2.
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a colorless, transparent variant of a low-temperature modification of quartz (SiO2) that is usually chemically pure, with almost no impurities, and that crystallizes in a trigonal system. It is found in the form of prismati-cally hexagonal crystals, either single or combined into druses, that sometimes may weigh a ton or more.
The internal structure of rock crystal is often complicated by so-called interpenetration twins, which significantly decrease its piezoelectric homogeneity. Large pure monocrys-tals are found rarely, mainly in the hollows and cracks of metamorphic shales, in the hollows of hydrothermal veins of various types, and also in chamber pegmatites. Homogeneous transparent monocrystals are extremely valuable raw material for optical instruments (prisms of spectrographs and lenses for ultraviolet optics) and for piezoelectric articles in electrical and radio engineering. Rock crystal is also used for making quartz glass (low quality raw material), in artistic stone-cutting, and for articles of jewelry. Deposits of rock crystal in the USSR are concentrated mainly in the Urals, the Altai, the Ukraine, the Pamirs, and the southern part of the Yakut ASSR. Abroad they are found, among other places, in China, in Brazil, on Madagascar and Ceylon, in Burma, and in the Swiss Alps. Crystals of rock crystal of high technical quality are also obtained artificially.
G. P. BARSANOV