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.