On heating at temperatures below 600°C kaolinite reacts with salts of alkali metals according to the equation
2SiO2 · Al2O3 · 2H2O + 2nMX → 2SiO2 · Al2O3 · nM2O + 2nHX + (2−n)H2O
The reaction commences with the dehydroxylation of the clay and is favoured by high solubility of the salt and small size of the alkali ion.
It appears that on dehydroxylation the clay becomes reactive and, concurrently, the water liberated dissolves adjacent salt particles and catalyses the reaction.
Infrared spectra indicate that alkali ions are incorporated directly into the aluminosilicate structure, without the formation of metakaolinite as a detectable intermediate. The initial product is disordered, but as n approaches 1, crystalline phases tend to be formed.