Solid Solution Range and Microstructures of Melt‐Grown Mullite

The solid solution range of melt‐grown mullite was examined by crystal‐chemical methods. The maximum Al 2 O 3 content as determined by EDX was ∼83.6 wt%, 75 mol%, or the nominal composition 3Al 2 O 3 .SiO 2 . For samples of overall composition 81 to 83 wt% Al 2 O 3 , extra lines indicating crystallographic superstructure appeared in Guinier X‐ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 μm wide, with oriented exsolution of α‐A1 2 O 3 , often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO 2 ‐Al 2 O 3 metastable phase equilibria are discussed.