Coordinates for Clay Mineral Assemblages

The basic representation of the clay mineral compositions has been + 3 3 2 

Fe2+ R3 - Fe3+, Al3+, Ti4+ M+ = Na+, K+ and 2 x Ca2+. An M+R3 pole represents the bulk composition of feldspars, commonly found associated with clay minerals. They are the most alkali-rich minerals present (excluding zeolites). Calcium is considered to fulfill a chemical role similar to the alkali ions in most clay minerals. It is notably not present in micas stable at low temperatures (Velde, 1971, Hemley, ejt al.,  

1971) and hence does not compare directly to Na-K systems. It is also possible that the calcium concentration in a given aggregate of phyllo-silicates could be largely controlled independently of them through its activity in aqueous solution in equilibrium with carbonates. Therefore its presence is difficult to interpret in a purely silicate framework. 

As a result, calcium is largely ignored in the analysis of clay minerals here. It is believed that high calcium concentration will not provoke 

The poles 2R and 3R were chosen because such ions are present in 

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