The thermal stability of kaolinite and the microstructure of its thermal products strongly depend on random defects (R-2) rather than crystalline defects (141). Kaolinite with a lower random defect density is more stable than that with higher defect density during dehydroxylation and the derived metakaolinite can be directly transformed into orthorhombic mullite (3/2-multite). However, for kaolinite with higher random defect density, there is a cubic phase occurring in the transformation from metakaolinite to primary mullite. Primary mullite will be transformed into orthorhombic mullite as temperature increases. Al-V is universally present in the metakaolinite and the relative amounts of Al-VI, Al-V, and Al-IV vary with the random defect density of the parent kaolinite.