Conversion of Quartz to Tridymite

The conditions under which tridymite becomes a stable phase have been reinvestigated. When quartz was heated with 2% alkali oxide, tridymite formed directly at 872° to 898°C. with Na 2 O, at 883° to 902°C. with K 2 O, and above 1005°C. with Li 2 O. Cristobalite occurred as an intermediate phase above 893°C. with Li 2 O, above 898°C. with Na 2 O, and above 902°C. with K 2 O. When quartz plus sodium chloride was heated in vacuum, tridymite did not form but cristobalite started to appear at 1050°C. The results showed that the formation of tridymite can be strictly a solid‐state process. New schematic tentative diagrams for the high‐silica region of binary systems are suggested. Quartz and cristobalite are regarded as the only stable crystalline phases of pure silica. Tridymite is pictured as a binary incongruently melting phase.