Silica Structure Studies: V, The Variable Inversion in Cristobalite

It is demonstrated experimentally that even in the purest laboratory silicas available the temperature of the α-β inversion in cristobalite is variable and depends on the structure of the starting material and on the temperature and length of heat-treatment. It is shown that this variability is an index of the order achieved in the cristobalite structure quite independent of impurities. There is no specific order characteristic of a particular temperature; the completely ordered 3 C stacking of cristobalite is the most stable cristobalite throughout the temperature range and has an α-β inversion temperature of 267°± 2°C. All disordered cristobalites will tend toward the 3C 267° cristobalite with time. The conversion of cristobalite to tridymite involving the 3 C → 2 H stacking change does not alter the cristobalite inversion temperature, nor have any regular polytypes other than those recognized as tridymite been encountered.