Microstructures, formation mechanisms, and depth-zoning of phyllosilicates in geothermally altered shales, Salton Sea, California

SEM, TEM and analytical EM studies of the shales revealed that phyllosilicates progress through zones of illite-muscovite, chlorite and biotite. They occur principally as discrete, euhedral to subhedral crystals partially filling pore space; structural and chemical heterogeneity is generally absent. Textures and microstructures suggest that dissolution of detrital phases, mass transport through inter-connecting pore space and direct crystallization of phyllosilicates from solution were involved. The zoning could be due to increase in T, or changing ion concentrations in solutions with depth. Data indicate that the mineral assemblages and interstitial fluids approach equilibrium relative to the original detrital suites. Alteration processes may have occurred in a single brief hydrothermal event.