Hydrothermal reactions and crystallographic relationships found during the reactive hot pressing of kaolinites

Water released by dehydroxylation was trapped in clay compacts during reactive hot pressing. The dynamic conditions of water pressure and temperature produced in the compacts modified the high temperature reactions of kaolinite. Kaolinite was converted to pyrophyllite or mullite in the temperature range 600-700°C depending on the applied pressure. Oriented aggregates of clay were used to determine orientational relationships between kaolinite and the high temperature phases. Structural continuity was observed in the reactions through the basal planes of kaolinite and pyrophyllite to the (130) plane of mullite.