Scanning, transmission, and analytical electron microscopy studies of shales from the Salton Sea geothermal field revealed that phyllosilicates progress through zones of illite-muscovite (115°−220°C), chlorite (220°−310°C), and biotite (310°C). These phyllosilicates occur principally as discrete, euhedral to subhedral crystals which partly fill pore space. The structural and chemical heterogeneity, which is typical of phyllosilicates in shales subject to diagenesis, is generally absent. Textures and microstructures indicate that the mineral progression involves dissolution of detrital phases, mass transport through interconnecting pore space, and direct crystallization of phyllosilicates from solution.

Phyllosilicate stability relations indicate that either increase in temperature or changing ion concentrations in solutions with depth are capable of explaining the observed mineral zoning. Textural and compositional data suggest that the observed mineral assemblages and the interstitial fluids approach equilibrium relative to the original detrital suites. The alteration process may have occurred in a single, short-lived, episodic hydrothermal event in which the original detrital phases (smectite, etc.) reacted directly to precipitate illite, chlorite, or biotite at different temperatures (depths) without producing intermediate phases.