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coesite
Coesite 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.〔The word "coesite" is pronounced as "Coze-ite" after chemist Loring Coes, Jr. 〕
==Occurrences==
In 1960, a natural occurrence of coesite was reported by Edward C. T. Chao, in collaboration with Eugene Shoemaker, from Barringer Crater, in Arizona, USA which was evidence that the crater must have been formed by an impact. After this report, the presence of coesite in unmetamorphosed rocks was taken as evidence of a meteorite impact event or of an atomic bomb explosion. It was not expected that coesite would survive in high pressure metamorphic rocks.
In metamorphic rocks, coesite was initially described 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. In metamorphic rocks, coesite is now recognized as 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 is formed at pressures above about 2.5 GPa and temperature above about 700 °C. This corresponds to a depth of about 70 km in the Earth. It can be preserved as mineral inclusions in other phases because as it partially reverts to quartz, the quartz rim exerts pressure on the core of the grain, preserving the metastable grain as tectonic forces uplift and expose these rock at the surface. As a result, the grains have a characteristic texture of a polycrystalline quartz rim (see infobox figure).
Coesite has been identified in UHP metamorphic rocks around the world, including the western Alps of Italy at Dora Maira,〔 the Erzgebirge of Germany, the Lanterman Range of Antarctica, in the Kokchetav Massif of Kazakhstan, in the Western Gneiss region of Norway, the Dabie-Shan Range in Eastern China, and the Himalayas of Eastern Pakistan.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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