FORMATION OF MULLITE FROM SOL-GEL MIXTURES AND KAOLINITE

Critical analyses of published reaction studies on kaolinite, and sol-gel and powder mixtures equivalent to 3Al2O3.2SiO2 mullite, were made. Supplementary experiments on reactions of kaolinite at different rates of heating were analyzed. Complete atomic homogeneity of precursors results in formation of tetragonal mullite. Colloidal precursors initially form spinel which reacts with the remaining silica by a diffusion-nucleation mechanism to form orthorhombic mullite. Precursors with intermediate atomic homogeneity follow both reaction paths. Kaolinite with a layer lattice structure has two-dimensional atomic homogeneity. Both reaction routes are followed and their degree of interaction is dependent on the crystallinity and impurities of the kaolinite.