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)[2]. 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 a ring. 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.
^ E. C. T. Chao, E. M. Shoemaker, and B. M. Madsen (1960) First natural occurrence of coesite. Science vol. 132. no. 3421, pp. 220-222
^ Chopin C. (1984): Coesite and pure pyrope in high-grade blueschists of the western Alps: a first record and some consequences. Contrib. Mineral. Petrol. 86, 107-118
