Phyllosilicate minerals equilibria

In the seawater example (Table 6.6), the saturation indices are inflated somewhat by the choice of a rather alkaline pH, reflecting equilibrium with atmospheric CO2. If we had chosen a more acidic pH within the range observed in seawater, the indices would be smaller. The choice of large formula units for the phyllosilicate minerals, as discussed later in this section, also serves to inflate the saturation indices reported for these minerals. 

This possibility is due to the non-equivalence of Mg and Fe which segregate into corrensite and chlorite respectively. This effect is discussed in the chlorite chapter. Thus four major phyllosilicate phases could be present in an equilibrium situation. It should be noted that the expanding trioctahedral phase is or can be more aluminous than chlorite. This might lead one to think that some of the layers might in fact be dioctahedral such as those in sudoite. The importance of the differentiation of the two types of mixed layered minerals lies in the segregation of alumina and potassium in one (the dioctahedral mixed layered mineral)... 

There are zeolite-bearing rocks in which one mineral is apparently being replaced by another mineral under constant P-T conditions. This indicates a system in which certain chemical components appear to be perfectly mobile a system in which the total number of phases that can coexist at equilibrium is reduced as a function of the number of chemical components which ar e internal variables of the system. Two examples of this type of equilibrium concerning zeolites can be cited saline lakes and analcite-bearing soil profiles (Hay, 1966 Hay and Moiola, 1963 Jones, 1965 and Frankart and Herbillon, 1970). In both cases a montmorillonite-bearing assemblage becomes analcite or zeolite-bearing at the expense of the expandable phyllosilicate. Other phases remain constantly present. 

When an expanding mineral is no longer stable, the iron content of the chlorite in equilibrium with illite will become more variable (Figure 49b). If chlorite is present due to a relatively high Fe/Fe + Mg content of a rock, it can occur with three other aluminous phases such as illite-montmorillonite and kaolinite. Thus the four-phase phyllosilicate assemblage common to argillaceous rocks can be accounted for by dividing the... 

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