COESITE

a high-pressure polymorph (crystal form) of silica, silicon dioxide (SiO2). It has essentially the same composition as the minerals cristobalite, keatite, lechatelierite, quartz, and tridymite but a different crystal structure. Because of the very high pressure necessary for its formation, it does not occur naturally in the Earth's crust. Artificially produced in 1953 by the American chemist Loring Coes, Jr., it was discovered in nature in 1960 in the sandstone on the floor of Meteor Crater, near Winslow, Ariz. There coesite was formed from quartz under the high temperature and pressure generated by the large meteorite's impact. It has since been found in the craters of other large meteorites. On theoretical grounds, quartz should change to coesite at depths of 60 to 100 km (40 to 60 miles) below the Earth's surface. For detailed physical properties, see silica mineral (table).