Characterisation of fired silicas

The products produced by firing a very pure quartz and a commercial purity quartz, with and without a mineral additive, to temperatures between 1100 degrees C and 1700 degrees C have been studied by DSC. The temperature at which the cristobalite alpha-beta inversion occurred and the degree of hysteresis observed on heating and cooling were both dependent on the sample history. The highest inversion temperature and greatest degree of hysteresis on heating and cooling were obtained with pure quartz samples heated at 1600 degrees C or 1700 degrees C. Products produced by firing finely ground pure quartz had a lower inversion temperature and less hysteresis than coarse grained samples. if a mineral additive was present tridymite inversions were also observed. Firing of the commercial purity quartz at a given temperature led to cristobalites with lower inversion temperatures and less hysteresis. The variations in inversion temperature and hysteresis are probably related to the degree of order in the cristobalite structure with the most highly ordered material having the highest inversion temperature and greatest degree of hysteresis. (C) 1998 Elsevier Science B.V.