A petrogenetic grid for the Fe-Mg silicates of pelitic schists

The topology of the reaction surfaces for the phases-garnet, chloritoid, staurolite, kyanite, cordierite, chlorite, and biotite-is shown for two limiting sections of Pₛ, T, PH₂O space for the part of the system-SiO₂-Al₂O₃-FeO-MgO-K₂O-H₂O in which quartz and muscovite are stable. The basic assumption made is that the course of progressive metamorphism is primarily one of progressive dehydration with a progressive restriction of the fields of the most hydrous minerals, notably chlorite. The resulting petrogenetic grids for the surfaces Pₛ ≈ PH₂O and Ps ≈ ρs/ρH₂O are predominantly theoretical and should serve as a useful check in assigning natural metamorphic sequences to their relative pressure-temperature relationships.