The topaz to mullite transformation on heating
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J. S. Hartman Chemistry Department, Brock University, St. Catherines, Ontario, L2S 3A1, Canada (Received 25 November 1994; accepted 17 July 1995)
The decomposition of topaz to mullite and other siliceous phases on heating above about 1100 °C was found to depend on sample size and the presence of water vapor in the heating atmosphere. The principal experimental technique employed was scanning electron microscopy, but the data were supported by x-ray diffraction, thermal analysis, mass spectroscopy of volatile emissions, and solid-state nuclear magnetic resonance. In relatively large samples, the transformation to mullite evidently takes place by a vapor phase mechanism within the bulk. The surface reaction that took place for samples heated in a wet atmosphere allowed the formation of high-silica glass, as well as mullite. The use of a hydrogenous heating atmosphere resulted in the sublimation and reformation of mullite whiskers, well outside the boundary of the original topaz.
I. INTRODUCTION Topaz [Al 2 Si0 4 (F, OH) 2 ] decomposes on heating above —1100 °C into mullite (nominally 3A12O3 • 2SiO2), but the decomposition may involve a liquid phase, a vapor phase, or both. Hampar and Zussman1 proposed a multistage reaction mechanism to explain the thermal decomposition of single topaz crystals to produce minor glass (liquid), cristobalite, and even corundum, as well as mullite. Moyer and Hughes2 found that synthetic fluorotopaz, formed by the reaction of A1F3 and SiO2 above 600 °C, yields acicular mullite crystals in a two-step process on heating to —1100 °C. They determined that mullite growth took place in a liquid phase after initial evolution of SiF4 and nucleation of mullite crystallites on the topaz surface. In contrast, Okada and Otsuka3 grew mullite whiskers from a vapor phase containing A1OF and SiF 4 . Iwai et al.4 believed that when natural topaz decomposed into mullite and small amounts of cristobalite, SiF4 and A1F3 were volatilized. The present paper reports the results from experiments on natural hydroxyl-bearing topaz in different atmospheres, using scanning and transmission electron microscopy, x-ray diffraction, differential thermal and thermogravimetric analysis, mass spectroscopy, and magic-angle spinning nuclear magnetic resonance to characterize the decomposition products. II. EXPERIMENTAL A. Samples and experimental conditions Naturally occurring hydroxyl-bearing Australian topaz was used as the starting material in all of the J. Mater. Res., Vol. 10, No. 11, Nov 1995 http://journals.cambridge.org
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