Crystallization of cristobalite and tridymite in the presence of vanadium

The formation of cristobalite and tridymite compounds is described in the s ystem SiO2-V2O5-M2CO3 (M = Na, K). Syntheses were performed with various va nadium concentrations at annealing temperatures between room temperature an d 1000 degrees C. Crystallization of cristobalite occurs at much lower temp eratures (about 800 degrees C) in the presence of vanadium than observed in the pure silica system, which remains amorphous in this temperature range. With increasing amounts of vanadium, the crystallization of NaVO3 or K3V5O 14 is observed along with the formation of cristobalite and tridymite. XRD, IR, and NMR indicate that cristobalite is formed by catalytic processes in the presence of vanadium without measurable V-substitution in its structur e. Variations in the thermal behavior of cristobalite are attributed to dif ferent states of crystallinity.