quartz and related minerals

quartz and related minerals α-Quartz, β-quartz, tridymite, cristobalite, coesite, and stishovite are all polymorphs of silicon dioxide or silica (SiO₂); they have different structures and therefore different symmetries and physical properties and are stable under different conditions of temperature and pressure. α-Quartz is by far the most common and is the form that is stable at the lowest temperature. If heated at atmospheric pressure, α-quartz inverts to β-quartz at 573 °C; β-quartz to tridymite at 867 °C; tridymite to cristobalite at 1470 °C. Cristobalite melts at 1713 °C. Coesite and stishovite form only at very high pressures equivalent to depths in the Earth of 60 km and 250 km, respectively.

α-Quartz is colourless when pure but minute amounts of impurities or lattice imperfections give rise to varieties such as amethyst, cairngorm, rose quartz, and smoky quartz. Silica precipitated from aqueous solution at low temperatures gives cryptocrystalline varieties such as opal, jasper, chalcedony, agate, carnelian, onyx, flint, and chert.

α-Quartz is one of the most common minerals in the Earth's crust and is stable over a wide range of geological conditions. It is an important constituent of granites and related acid igneous rocks, and of a range of metamorphic rocks such as quartzites, schists, and gneisses. α-Quartz is a common constituent of hydrothermal veins where, in cavities, it may grow into large, well-formed crystals. Because of its hardness, lack of cleavage, and resistance to chemical attack it is the most common detrital mineral and is the main constituent of sandstones, greywackes, and related sedimentary rocks.

Quartz is an important industrial mineral component of aggregates, sands, glass, and abrasives. Its piezoelectric properties are of vital importance to the electronic industry and, in order to supply an ever-increasing demand, large quantities are synthesized in the laboratory.

R. Bradshaw