Phase Diagrams for Dioctahedral Montmorillonites

As we have seen in the previous section, the bulk chemical compositions of montmorillonites taken from the literature are dispersed over the field of fully expandable, mixed layered and even extreme illite compositions. Just what the limits of true montmorillonite composition are cannot be established at present. We can, nevertheless, as a basis for discussion, assume that the ideal composition of beidellite with 0.25 charge per 10 oxygens and of montmorillonite with the same structural charge do exist in nature and that they form the end-members of montmorillonite solid solutions. Using this assumption one can suppose either solid solution between these two points or intimate mixtures of these two theoretical end-member fully expandable minerals. In either case the observable phase relations will be similar, since it is very difficult if not impossible to distinguish between the two species by physical or chemical methods should they be mixed together. As the bulk chemistry of the expandable phases suggests a mixture of two phases, we will use this hypothesis and it will be assumed here that the two montmorillonite 

The two series of phase relations deduced above result in, at a first approximation, two facies for the expandable dioctahedral minerals— that of low temperature where fully expandable minerals exist and where the tie-line or association beidellite-montmorillonite persists. More elevated conditions produce a kaolinite-illite tie-line characteristic of sequences of buried rocks. 

The stability of dioctahedral montmorillonites is, of course, not uniquely a function of P-T conditions acting upon a given silicate mineral assemblage. Studies in the system Na H A O -Sit - O-Cl (Hemley, jet al.. 1961) shows that a high activity of sodium ion at given silica and hydrogen activities can destabilize beidellite. Hess (1966) extrapolates this hydrothermal study to atmospheric conditions where the range of H+, Na+ and SiC activities can determine the presence of an expandable phase. 

It is evident that such theoretical constructions find form in real situations, since the transformation of montmorillonites to mica (potassic) or 

We have seen that experimental data suggest high temperatures for dioctahedral montmorillonite stabilities, especially the Na-Ca types with beidellitic substitution. Yet in most studies of montmorillonite stabilities under natural conditions, the fully expandable phase is lost rather early, near 100°C. This phase appears to be succeeded by an interlayered expandable-non-expandable mineral. Apparently two sets of information do not agree. 

---