Reaction of coarse-grained, allogenic layer silicate minerals with the hot, hypersaline brine of the Salton Sea geothermal system has resulted in the formation of a series of metastable intermediate mineral phases that were created within the system, have a finite temperature range over which they exist, and react with the system in a regular but incomplete manner. Intense calcite and dolomite/ankerite cementation allowed a suite of allogenic biotite, chlorite, and muscovite grains to be preserved as unstable mineral phases to temperatures near 200 degrees C. At this stage removal of significant portions of the cement and access of the fluid phase to these minerals initiated a series of complex reactions. A significant degree of the compositional scatter observed in low grade metamorphic layer silicate minerals may be due to the existence of metastable mineral phases which have preserved structural elements of the mineral they originally replaced. The data suggests that some sites within minerals can remain inert while others continue to react with the fluid phase, creating partially equilibrated mineral phases and mineral reactions that must be dealt with on a site-by-site basis.--Modified journal abstract.

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