570 MINERALS/Quartz Figure Phase diagram for the silica system (after Klein and Hurlbut, (1973) Manual of Mineralogy, 21st edn, p 527 ß John Wiley and Sons (Reprinted with permission) !29 kbar; it is found as inclusions, generally partially replaced by quartz, in zircon, garnet, monazite, and other phases formed in low-temperature–highpressure conditions and is used as an indicator of such ultrahigh-grade metamorphic conditions in rocks which have generally been exhumed relatively rapidly in orogenic belts (as in the Himalaya, the Alps and in SE China) Stishovite requires even higher pressures (of around 130 kbar) for its formation and is typical found in meteorites or meteorite impact craters, also in tektites Microscopic fluid inclusions often occur in quartz, and may contain a gas phase as well as a liquid and one or more microscopic crystals of such minerals as halite, sylvite, calcite, etc., believed to have been precipitated from the same source as the quartz The development of sophisticated microanalytical techniques has allowed the composition of these liquids and solids to be determined and to the determination of the temperature at which the original precipitation and crystallization occurred This has important applications in unravelling the petrogenetic history of both igneous and metamorphic rocks as well as in the geochemistry and conditions of deposition of ore deposits and the provenance of sedimentary rocks (see Sedimentary Rocks: Mineralogy and Classification) Chalcedony is a group name for the compact varieties of silica composed of minute crystals of quartz with submicroscopic pores The colour and texture vary considerably according to the impurities present, but in general such materials may be subdivided into chalcedony in which the colour is fairly uniform, and agate in which the colour may be arranged in bands or concentric zones The terms ‘chert (see Sedimentary Rocks: Chert)’ and ‘flint’ are used for opaque dull-coloured or black chalcedony, and in common usage chert is taken as the name for this material when it occurs in stratified or massive form in rocks, while the term flint is generally restricted to dark chalcedony occurring in nodular form in a rock matrix, particularly in the chalk Jasper is a red opaque massive form of chalcedony Natural hydrous silica may be divided into three well-defined structural groups (see Minerals: Other Silicates): opal-C (well-ordered g-cristobalite), opalCT (disordered a-cristobalite, a-tridymite), and opalA (highly disordered, nearly amorphous) Opal may be colourless, milky white, yellow, red, green, blue, or black In precious opal a play of delicate colours is seen, due not to chemical impurities but to optical diffraction from a randomly faulted close-packed structure of minute transparent silica spheres (around 0.25 mm) Colourless opal is a major constituent in hot-spring deposits and deep-sea precipitates Quartz is an extremely important in many industrial processes At the simplest level it is raw material for the production of glasses, ceramics, and foundry moulds Most quartz sands are a golden yellow due to the quartz particles being encased by a thin coating of iron oxides and/or hydroxides, and the iron would cause any glass to be dark green In certain geological situations, however, the beach sands may be rolled to and fro by the tides for several million years, leading to the erosion of the iron coating and forming silver sands, which are much sought after for glass manufacture For the more technical applications requiring an even higher degree of purity, colourless vein quartz is used, or nowadays reliance is placed on the manufacture of synthetic single crystals of quartz in an autoclave One of the main technical aspects of quartz lies in its piezoelectric properties, whereby the crystal develops an electric polarization when mechanically stressed in an appropriate direction In the construction of quartz oscillators, thin plates of quartz are cut in a particular direction specified to generate a desired resonant vibrational frequency Although most quartz is colourless or white, wellknown coloured varieties include amethyst (violet), citrine (yellow to brown), and smoky quartz Each of these contains either a substitutional or interstitial component other than SiO2 For amethyst and citrine, iron is an important constituent (Fe 25–280 ppm) In smoky quartz the brownish or black colour is due to natural radiation acting upon Al3ỵ-bearing quartz Quartz is an essential constituent in many igneous, sedimentary, and metamorphic rocks, and also occurs as secondary material, often forming a cementing