Reexamination of the Kaolinite‐to‐Mullite Reaction Series

The kaolinite‐to‐mullite reaction series was reexamined with special attention to the nature of the metaphase, the eontroversial spinel phase, and the cause of the exothermic peak at 980°C. Amorphous SiO 2 forms during the exothermic reaction; it can be leached by alkali extraction. When the residual cubic phase is heated further, it forms mullite only. This result indicates that the cubic phase is an Al‐Si spinel and that metakaolinite is an AI 2 O 3 ‐SiO 2 compound. It was established that the exotherm exhibited by kaolinite at 980°C represents the sudden transformation of metakaolinite to Al‐Si spinel, the crystallization of mullite, and the liberation of amorphous SiO 2 . The AI‐Si spinel has the same composition as mullite, containing both AI(IV) and AI(VI). This spinel transforms into mullite at the second exothermic peak; no amorphous SiO 2 is liberated.