Experiments with natural and synthetic hexagonal FeS q-hexagonal Fe 1-xS assemblages indicate that FeS remains essentially stoichiometric in composition at all temperatures of heating, whereas the composition of co-existing hexagonal Fe 1-xS varies with temperature of heating as follows: at 25, 60, 105, 125, 140° C its composition is 47.85, 48.17, 48.57, 49.18, 49.67 atomic % Fe, respectively. A comparison of the compositions of natural co-existing FeS and hexagonal F 1-xS with the compositions of these phases determined experimentally, suggests that the majority of the natural phases have attained equilibrium compositions at earth surface temperatures. Experiments involving hexagonal Fe 1-xS, monoclinic Fe 1-xS and pyrite indicate that monoclinic Fe 1-xS forms in charges containing <47.20 - 0.10 atomic % Fe heated at ≤<304 + 6° C.. The anomalous behavior of monoclinic Fe 1-xS and the development of the non-equilibrium assemblage hexagonal Fe 1-xS q-monoclinic Fe 1-xS q-pyrite q-vapor in charges heated at a variety of temperatures has led to the interpretation that monoclinic Fe 1-xS is metastable with respect to hexagonal Fex-xS q-pyrite. The compositions of co-existing hexagonal Fe 1-xS and monoclinic Fe 1-xS at <304 + 6° C are judged to be 47.20 + 0.10 and 46.75 q-0.05 atomic % Fe, respectively. The development of natural pyrrhotite assemblages in terms of the experimental results is briefly discussed. © 1969 Society of Economic Geologists, Inc.
