Montana montmorillonites

Dahl [1965] has studied Permian bentonites from Texas and found illite-montmorillonite mixed layers, as well as mixtures of these minerals with chlorite. A study has been made of illites and mixed-layer illite-montmorillonites (in Montana, Illinois, Indiana, Colorado, Oklahoma) by Hower and Mowatt [1966]. These authors conclude that the nonexpandable layers are of lower change than true mica and contain lenses of trapped water. 

Reynolds and Hower [1970] have studied a range of illite-montmorillonites (from Colorado, Montana, and Wyoming), using Reynold s method for calculating the diffracted intensity. They find that three main types of interstratification are present (1) random, (2) allevarditelike, (3) four-layer units of type IMII. (1) occurs generally with 40 to 100% ex-pandibility, (2) for <40%, and (3) only for < 10%. 

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