Stability Region of Topaz [Al<sub>2</sub>SiO<sub>4</sub>F<sub><i>x</i></sub>(OH)<sub>2-<i>x</i></sub>

Abstract

This research was carried on as a part of the study on the synthesis of topaz [Al2SiO4Fx(OH)2-x]. Mixtures of SiO2 and Al2O3 gels and AlF3 specimens corresponding to compositions Al2SiO4Fx(OH)2-x, where x=0.0, 1.0, 1.5 and 2.0 were treated at temperature ranging from 300° to 2200°C at pressures ranging from the atmospheric pressure to 70kb using squeezer and girdle type high pressure apparatus. The heat treatment temperature was measured by a platinum-rhodium thermocouple inserted in the specimen chamber or was estimated from the relations between the temperature and heating power imput at several pressures. Minerals in the treated specimens were identified by the X-ray diffraction method, the infrared spectroscopy and the microscopic observations.No topaz was found in the heat-pressure treated products of the mixture of x=0.0. Under 700°C, pyrophyllite, corundum, and α-quartz etc. were formed as the crystalline phases at arbitary pressure. Kyanite was stable at pressure over about 30kb at 1000°C and above 35kb at 1500°C. Under these conditions, sillimanite, mullite and silica were obtained. Above 1900°C at 40kb, liquid and vapor phases were found.In the specimen of x=1.0, topaz was stable at pressures above 10kb at 1000°C and above 30kb at 1600°C. Under these pressures at the respective temperatures, mixture of mullite and silica was obtained as the stable crystalline phases. It seems that topaz melts incongruently to corundum, liquid and vapor at temperatures from 1700°C to 1900°C at 40kb. Liquid and vapor phases were found above 1900°C at same pressure.In the specimen of x=2.0, it was obtained nearly similar result in comparison with a case of x=1.0, but a stability region of topaz was expanded wider.Maximum size observed of synthesized topaz was about 0.2mm long. The color of topaz was slightly transparent.