Chemistry and Iron-magnesium Distribution among Phyllosilicates

An important feature of the chemistry of natural chlorites is their 

A review of some 200 chlorite analyses from the literature (Velde, 1973) indicated that Fe-Mg-Al ranges for 7 and 14 8 chlorites are similar. 

However, both 7 and 14 8 chlorites are considered to be iron-rich when found in low temperature environments. Why are the diagenetic or authi-genic chlorites found in sedimentary rocks ferrous The answer can be found in the phase relations of the minerals common in sedimentary rocks. Basically, 14 8 chlorite is formed either through the destabilization of the montmorillonite-illite mixed layered mineral or kaolinite in the majority of argillaceous sedimentary rocks (Dunoyer de Segonzac, 1969 van Moort, 1971 Perry and Hower, 1970 Muffler and White, 1969). The increase in chlorite content is frequently observed in the presence of illite or a mixed layered mineral with a high non-expandable layer content. 

It is well known that two silicate minerals, stable at the same time, will have a specific distribution of homologous ions between them, depending upon the temperature and pressure conditions prevalent. This is 

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