Stoichiometry of Fluorotopaz and of Mullite Made from Fluorotopaz

Mixtures of alumina and silica containing 68–78 wt% alumina react above 600°C with 0.5 mol of SiF 4 per mole of alumina to form fluorotopaz. High‐resolution X‐ray powder diffraction data were used to determine very accurate cell parameters of fluorotopaz as a function of 1 wt% compositional increments. The samples containing 69–76 wt% A1 2 O 3 were found to have a linear cell parameter relationship. Compositions outside that range show discontinuities in the cell parameters, indicating solid solution behavior between 69 and 76 wt% alumina. Within this range the composition of fluorotopaz may be written 2A1 2 O 3 · x SiO 2 · SiF 4 where 1.07 x 1.53. Pyrolysis of all compositions of fluorotopaz solid solution yields mullite whiskers containing 76.1 wt% alumina (65.2 mol%).