Alphaquartz

Alphaquartz is the low-temperature polymorph of quartz, the crystalline form of silicon dioxide (SiO2) that is stable at Earth surface conditions. In alphaquartz, silicon atoms are tetrahedrally coordinated to oxygen, forming a three-dimensional framework of SiO4 tetrahedra that shares oxygen atoms. The crystal system is trigonal, often described as hexagonal with enantiomorphic forms, and natural crystals can occur as well-formed prisms, scalenohedra, or as common grains and aggregates in rocks. Alphaquartz is one of two polymorphs of SiO2; at temperatures above 573°C at one atmosphere, it transforms to beta-quartz, a high-temperature form. The alpha to beta transition is reversible on cooling.

Natural alphaquartz occurs in a wide range of rock types, including igneous rocks such as granites and rhyolites, metamorphic rocks like schists and quartzites, and sedimentary formations such as cherts. It is commonly found in hydrothermal veins and geodes, often with inclusions of other minerals.

Physically, alphaquartz is uniaxial, piezoelectric, and optically active; these properties underpin its use in precise timing devices and electronic resonators. Industrially, synthetic alphaquartz crystals are grown for use in oscillators, filters, and optical components, while natural crystals are valued by collectors and in geology. See also beta-quartz for the high-temperature polymorph.