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°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.

Muscovite reacted to very fine grained interlayered illite/smectite through a phengitic muscovite intermediate phase that persists in the geothermal system for less than 50°C. The overall reaction of muscovite occurred in two steps, the first involving change within the 2:1 layer via Mg addition and Al loss but little change in the interlayer sheet, and the second involving significant K loss in the interlayer site as expandable smectite layers with exchangable Mg formed. Allogenic Ti-bearing biotite reacted completely at temperatures near 200°C to a metastable optically anomalous titaniferous chlorite phase by a reaction mechanism that apparently allowed the 2:1 octahedral sheet of the reactant biotite to be preserved intact in the product chlorite. The metastable chlorite persists throughout the entire chlorite zone, shows some systematic compositional variation with temperature, but was steadily reduced in amount by reaction to fine-grained Ti-free equilibrated authigenic chlorite. At the biotite isograd at 325°C, the remaining metastable titaniferous chlorite reacted completely and rapidly to a metastable titaniferous biotite in a reaction that involved all structural sites in the minerals. This biotite reacted, within 10° above the biotite isograd, to Ti-free equilibrium authigenic biotite.

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.

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