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The phase transformation from kaolinite to mullite was examined with new electron diffraction data obtained using an energy‐filtering transmission electron microscope. Kaolinite was finally transformed to mullite and cristobalite through several steps of endothermic and exothermic reactions, which resulted in metakaolinite, a microcrystalline spinel‐type phase and amorphous silica. Metakaolinite maintained its short‐range order even at 920°C. The spinel‐type phase resulted from a topotactic transformation of metakaolinite. Mullite first appeared at around 940°C, showing no clear crystallographic relationships to the parent metakaolinite structure. It coexisted with metakaolinite and the spinel‐type phase produced previously. The first strong exothermic peak on the DTA curve was mainly due to the extraction of amorphous silica from metakaolinite and the gradual nucleation of the mullite phase. The initially formed spinel‐type and mullite phases were suggested to be Al‐rich, but to progressively gain Si in their structures at higher temperatures. The spinel‐type phase decomposed completely through a second weak exothermic reaction, promoting the crystallization of cristobalite from amorphous silica, and the growth of mullite.
Lee et al. (Fri,) studied this question.