The ratio of cation to hydrogen-ion concentration is used as a basis for coordinating many observed varieties of silicate rock alteration. Low ratios drive alteration from feldspar through sericite, montmorillonite, or paragonite, toward kaolinite or pyrophyllite. Principal controls of ratio are reactions with wall rock, pressure-temperature changes, mixing of solutions, and oxidation of H 2 S; their interactions govern the assemblage attained.

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