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a natural silica glass (silicon dioxide, SiO2) that has the same chemical composition as coesite, cristobalite, stishovite, quartz, and tridymite but has a different crystal structure. Two varieties are included: meteoritic silica glass, produced when terrestrial silica is fused in the intense heat and pressure created by the impact of large meteorites; and fulgurite (q.v.), glass produced when silica is fused in the heat generated by a lightning strike. Tektites, tear-shaped meteoritic glass, the silica content of which is usually between 68 and 82 percent, often contain strings or grains of lechatelierite. Meteoritic silica glass has been found in and near large meteorite craters, as at Winslow, Ariz., in the United States, and not associated with particular craters but still with meteoritic impact, as in the Libyan Desert.