Thermal analysis of topaz synthesis from kaolinite

Abstract

The present work represents a thermal analysis study of topaz synthesis from kaolinite by its sintering with aluminium fluoride using a derivatograph. Also, it includes the study of the influence of aluminium fluoride on the thermal behaviour of kaolinite. The products of sintering were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA and X-ray powder diffraction data show the beginning of the appearance of both topaz and mullite at 600°C and indicate that the reaction of kaolinite and aluminium fluoride of different amounts takes place in two distinct steps. The first is marked by a medium endothermic peak at 750–790°C, representing the intensive formation of topaz, and the second by a large and sharp or wide endothermic peak at 930–950°C, representing its subsequent dissociation with the formation of either mullite or (corundum and mullite) or corundum, depending on the amount of aluminium fluoride. This process is accompanied by a sharp decrease in weight, (TG curve), due to the volatilization of silicon tetrafluoride, water vapour and other gases. The synthesized topaz is colourless in thin sections and crystallizes in orthorhombic system in the form of elongated prismatic crystals, with perfect basal (0 0 1) cleavage; it is optically positive.