THE TOPAZ TO MULLITE TRANSFORMATION ON HEATING

The decomposition of topaz to mullite and other siliceous phases on he ating above about 1100 degrees C was found to depend on sample size an d 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 r esonance. In relatively large samples, the transformation to mullite e vidently takes place by a vapor phase mechanism within the bulk. The s urface reaction that took place for samples heated in a wet atmosphere allowed the formation of high-silica glass, as well as mullite. The u se of a hydrogenous heating atmosphere resulted in the sublimation and reformation of mullite whiskers, well outside the boundary of the ori ginal topaz.