Coesite

Coesite^[p] is a form (polymorph) of silicon dioxide SiO₂ that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953. In 1960, coesite was found by Edward C. T. Chao, in collaboration with Eugene Shoemaker, to naturally occur in the Barringer Crater, which was evidence that the crater must have been formed by an impact.

The presence of coesite in unmetamorphosed rocks may be evidence of a meteorite impact event or of an atomic bomb explosion. In metamorphic rocks, coesite commonly is one of the best mineral indicators of metamorphism at very high pressures (UHP, or ultrahigh-pressure metamorphism). Such UHP metamorphic rocks record subduction or continental collisions in which crustal rocks are carried to depths of 70 km or more. Coesite also has been identified in eclogite xenoliths from the mantle of the earth that were carried up by ascending magmas; kimberlite is the most common host of such xenoliths.

The molecular structure of coesite consists of four silicon dioxide tetrahedra arranged in Si₄O₈ and Si₈O₁₆ rings. The rings are further arranged into a chain. This structure is metastable within the stability field of quartz: coesite will eventually decay back into quartz with a consequent volume increase, although the metamorphic reaction is very slow at the low temperatures of the Earth's surface. The crystal symmetry is monoclinic C2/c, No.15, Pearson symbol mS48.

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