At the Overlook gold mine in northeastern Washington, a fayalite-bearing assemblage of iron silicates, magnetite, and pyrrhotite, plus quartz and calcite, formed during alteration of a Permian volcanogenic, carbonate-rich, massive magnetite-sulfide seafloor deposit. The temperature of fluid inclusion homogenization, oxygen isotope ratios, and phase equilibrium calculations constrain the temperature of the fayalite-forming environment to approximately 300 degrees C. The fayalite is close to end-member composition, Fa (sub 95-98) , and the greenalite and minnesotaite are correspondingly Fe-rich. No grunerite has been found. The paragenetic sequence fayalite-->greenalite-->minnesotaite-->quartz represents infiltration at approximately 300 degrees C of an initially very reducing, Fe-rich hydrothermal fluid that became progressively more SiO 2 -rich. Provisional thermodynamic data for greenalite are presented, and these are used to construct P-T-a(SiO 2 ) phase diagrams for the system FeO-SiO 2 -H 2 O. Phase relations in this system differ topologically from those in the analogue system MgO-SiO 2 -H 2 O by virtue of the H 2 O-conserved reaction: talc + olivine = MgFe-amphibole + serpentine, which proceeds from left to right with an increase in Fe:Mg ratio of the system. One sample contains in addition to greenalite an iron-dominant serpentine with ideal serpentine stoichiometry, Fe:Si = 3:2.