The kaolinite-to-mullite reaction series was reexamined with special attention to the nature of the metaphase, the controversial spinel phase, and the cause of the exothermic peak at 980° C. Amorphous SiO₂ 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 Al₂O₃-SiO₂ compound. It was established that the exotherm exhibited by kaolinite at 980° C represents the sudden transformation of metakolinite to Al-Si spinel, the crystallization of mullite, and the liberation of amorphous SiO₂. The Al-Si spinel has the same composition as mullite, containing both Al(IV) and AL(VI). This spinel transforms into mullite at the second exothermic peak; no amorphous SiO₂ is liberated.