Quartz has a formula unit composition of SiO2 and is classified as a Tectosilicate. Alpha-quartz is the most common polymorph of the silica minerals. Other polymorphs include beta-quartz, tridymite, cristobalite, and silica glass, which has a short-range ordered structure.
All four oxygens in quartz are polymerized to additional silica groups; all silica tetrahedra are polymerized to four other tetrahedra. In its characteristic crystal form, quartz has a hexagonal structure.
Highlighting FeaturesSilicon (Si) atoms Oxygen (O) atoms Tetrahedrally-coordinated Si atom Single unit cell 3x3x3 lattice
Quartz is resistant to weathering because of its chemical composition and atomic bonds; it therefore accumulates as the other minerals in the rock weather. The major portion of the sand- and silt-sized fractions of soil is composed of quartz, and clay-sized quartz is a major component of the clay fraction in soils. Quartz is the most abundant mineral in loess deposits and is also an important constituent of glacial till. Rounding of quartz grains has been used as an indication of an eolian source for soil parent materials in landscapes where quartz is common. Even in Hawaii, where soils have formed over quartz-free rocks, the A-horizon of some soils are as much as 1/3 quartz – the result of atmospheric deposition.
Quartz is the most stable and most common form of silica and is the most common silicate mineral in the Earth’s crust. It has also been found in some lunar rocks and meteorites. In pure form, quartz is transparent, clear, and colorless, but it is often colored by impurities to produce crystals of white (milky quartz), pink (rose quartz), amber, gray (smoky quartz), purple (amethyst), and other colors. There are more variety names given to quartz than any other mineral. Quartz is a common and diagnostic mineral in light-colored igneous rocks, such as granite, diorite, andesite, and rhyolite. Quartz is also common in metamorphic rocks, such as gneiss. Quartz is an important mineral with numerous uses. Sand, which consists mostly of quartz, is the primary ingredient for the manufacture of glass, and quartz crystals are used in the study of optics. Quartz has many electronic uses. It is used as oscillators in radios, watches, and pressure gauges, and silicon semiconductors are essential in computers. Quartz is used as an abrasive and is important in the production of soaps and ceramics.
Levien, L., C.T. Prewitt, and D.J. Weidner. 1980. Structure and elastic properties of quartz at pressure P=1 atm. American Mineralogist. 65:920-930.