Thermal changes in tridymite were examined at high temperatures by X-ray single-crystal and optical methods, using a hydrothermally-synthesized crystal of monoclinic modification. Five phases were confirmed on heating and cooling. They were MC, OP, OS, OC, and HP phases, in the temperature ranges from room temperature to 110°, 110°–150°, 150°–190°, 190°–380°, and above 380°C respectively: MC, monoclinic Cc, a = 18.49, b = 4.991, c = 25.83A, β = 117.75°; OP, orthorhombic P212121a = 26.65, b = 5.02, c = 8.15A; OS, metrically orthorhombic with a non-integral superstructure, a = variable from 95 to 65, b = 5.02, c = 8.18A; OC, orthorhombic C2221, a = 8.73, b = 5.04, c = 8.28A; HP, hexagonal P63/mmc, a = 5.05, c = 8.28A.

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