Decomposition of Mullite by Silica Volatilization

Thermal decomposition of mullite into corundum was investigated using a high‐temperature X‐ray single‐crystal camera equipped with a gas‐flame furnace and by scanning electron microscopy and electron probe microanalysis (EPMA). When heated to ∼1750°C, mullite decomposed to corundum by the liberation of the SiO2 component with topotaxial relations of: 1 (310)mull∥(001)cor; [001]mull∥[110]cor 2 (130)mull∥(001)cor; [001]mull∥[110]cor 3 (110)mull∥(001)cor; [001]mull∥[110]cor Thus, it was considered that, when mullite decomposed into corundum, their oxygen close‐packed planes were almost preserved. The SEM photographs showed that the crystals of the developed corundum are prismatic and ∼5 μm wide. The EPMA showed that the phase boundary between mullite and developed corundum is discontinuous.