The transformation of natural clay minerals at elevated pressures and temperatures

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Abstract
Hydrothermal experiments have been carried out, using three, mixed-layered, natural, clay minerals as starting material. The results allow of a qualitative comparison being made with experimental work done in a simplified chemical system. Three major parageneses are proposed — one in which fully expandable phases are stable, one in which a more restricted range of expandability is possible, depending upon the temperature, and a third in which the illite —allevardite (about 30 % expandable ordered structure) assemblage is predominant. The trioctahedral equivalents of these minerals appear to be stable at higher temperatures.