Solid solutions in I/S mixed-layer minerals and illite

Chemographic analysis of suites of I/S minerals using M⁺-4Si-R²⁺ coordinates reveals the existence of independent mineral series. The interlayered smectite is a montmorillonite whose composition and charge depends upon the P, T, and chemical conditions of formation. The charge can vary from near 0.33 to 0.66 depending upon the series. In each of the I/S series the montmorillonite is interlayered with an illite that has nearly the same composition for all I/S phases. The illite layer has a charge of 0.87. It appears that the members of the I/S mineral series are interlayered from a fully smectitic composition to a bulk composition with a charge of 0.75. Thus the interlayering does not reach the illite end-member with a charge of 0.87. This series can be treated as an incomplete solid-solution.

I/S mixed-layer minerals can be considered as a partial solid solution with three components: high-charge ill_0.87, low-charge mnt_0.33, and high-charge mnt_0.66. It is probable that the high-charge ill_0.87 component cannot replace the montmorillonite in all proportions from 0 to l00% because of crystallographic incompatibilities.