29 Si- and 27 Al-MAS/NMR study of the thermal transformations of kaolinite

Thermal transformations of kaolinite were studied by 29 Si- and 27 Al-MAS/NMR and ESR techniques. In metakaolin, Si is still dominantly in the Q 3 state (three Si atoms bonded to an Si-O 4 tetrahedron) and Al detectable by NMR is in both 4-coordination and 6-coordination. Coordination polyhedra around Al or Fe replacing Al are much distorted. Metakaolin crystallizes into γ-alumina and mullite exothermically at ∼980°C, this crystallization being preceded by a faint endothermic reaction. The latter is due to segregation of SiO 2 and Al 2 O 3 , which results in an increase of Si in the Q 4 state and an increase of 6-coordinated Al 2 O 3 .