An unusual kaolinite-calcite interaction

In a CO₂ atmosphere, one particular sample of kaolinite from Cornwall, England, when mixed with calcite in the proportions 4:1 or greater gave a very small, sharp endotherm at 840-845°C on curves obtained on three differential thermal analysis instruments of different design. Scanning electron microscopy suggests that this endotherm is associated with surface melting or sintering of the calcite rhombs, but X-ray diffraction and infrared absorption spectrometry reveal that other changes also occur. It is suggested that volatiles evolved from the kaolinite initiate a series of complex reactions.