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 Na20, at 883' to 902'C. with K20, and above 1O0SoC. with LizO. Cristobalite occurred as an intermediate phase above 893OC. with Li,O, above 898'C. with Na20, and above 902'C. with K20. 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 stricffy 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.

The solubility of sulfur trioxide in sodium silicate melts was determined from 1150° to 1250°C by equilibrating melts in gas mixtures of known contents of sulfur dioxide and oxygen. Sulfate forms according to the reactions: O2‐+ SO2+ 1/2O2= O2‐+ SO3= SO42‐. The data obtained at 1200°C were interpreted by the linear equation: log(SO42‐) = log(Pso2½Po21/2) + log Y in which Y is a function of the soda/silica ratio. A series of parallel lines was obtained. Relative free oxygen ion activities calculated for 1200°C were in good agreement with theoretical values calculated from the thermodynamic model of Toop and Samis.

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