A study of the influence of muscovite on the thermal transformations of kaolinite from room temperature up to 1,100 °C

Global thermal transformation of kaolinite–muscovite mixtures containing at most 25 mass % of muscovite was studied up to 1,100 °C. Quantitative and qualitative DTA analysis reveals a great intereaction between the two phylosilicates above 900 °C, which is characterized by an enhancement of spinodal demixion contribution during the structural reorganization of metakaolinite. The corresponding enthalpy variation are respectively −32 ± 1 kJ/mol and −15 ± 1 kJ/mol of kaolinite for pure kaolinite and for the mixture containing 15 mass % of muscovite. This interaction is mainly correlated with the diffusion of potassium ions from muscovite platelets into metakaolinite structure. The resulting consequence is an enhancement of mullite crystallization and grain growth at 1,050 °C (earlier than in the case of pure kaolinite mineral).