Composition of Natural Trioctahedral Montmorillonites

Despite the small number of analyses, it is apparent that the range of solid solution for expandable trioctahedral minerals is very great. 

Experiments at atmospheric and room temperature conditions by Kittrick (1973) demonstrate that vermiculite is not stable compared to the 

Roy and Romo (1957) and Boettcher (1966) performed high pressure experiments on natural vermiculites. They observed the production of a 14 X chlorite between 300 and 550°C, talc + enstatite and an unidentified phase above 650°C. The experiments on natural minerals indicate that vermiculite will occur when alkali content or activity in solution is low. This trioctahedral expanding phase is relatively stable at high pressures and temperatures as are interlayered minerals which are composed in part by such layers. It is not stable relative to montmorillonite at low emperature. 

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