Summary of the Information from Experimental Studies

If we consider the three types of substitutions dioctahedral, trioctahedral and interlayer ion, we see that Na and trioctahedral minerals are stable to higher temperatures than others. This is logical when one considers that sodic trioctahedral magnesian minerals are unknown, paragon-ite is unstable below about 340°C (Chatterjee, 1968) and phlogopite is unstable below 250°C (Velde, unpublished). By contrast muscovite appears stable at low temperatures under high K O concentration (Velde, 1969) or appropriate H+, K+ and SiO activities (Garrels and Howard, 1959). 

Type Elements in 2 1 lattice 1- Reference -2Kb pressure temp, °C 

Compiled data for dioctahedral montmorillonites and mixed layered 

It can be surmised that even though X-ray data indicated only expandable material, there must be significant interlayering with illite or other non-expandable mica-like phases such as glauconite-celadonite in order to give such a high structural charge imbalance. If not, one wonders why illites, with a similar chemical formula, are not expandable as well. 

Basically, little can be said about the montmorillonite data. The compositional dispersion is quite large. Some of this variation can be explained in part by an overpopulation of octahedral sites due to mis-assignment of magnesium ions as proposed by Foster (1951). In such case, a portion of the magnesium present is exchangeable, interlayer ions which if misassigned to octahedral sites will decrease the apparent interlayer 

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