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Tridymite
Chemical formula: SiO2
Modifications: Several silica modifications are known. The following phases are stable at ordinary pressure: quartz - high quartz - tridymite - cristobalite - melt.
Melting point: 1670oC
Stability range: 870-1470oC
Transformation temperature ranges between the four tridymite modifications: 100-150, 140-180, 180-220oC
Density: 2.26-2.39 g/cm3
Microhardnes: cca 1600 kp/mm2 (low tridymite)
Hardness: 9
Luminescence: Tridymite often shows a light-blue fluorescence in EPMA.
Crystal system: Triclinic
Cell dimensions: a=9.932 A, b=17.216 A, c=81.864 A
PDF number: 01-071-0261
ICSD number: Data not found
Note: The tridymite phase in steel inclusions often crystallizes as rather dark, thin plates which appear in polished sections as laths, often slightly split at the ends.