A tentative phase diagram for the system FeS-FeS 2 shows the pyrite-pyrrhotite solvus termination downward at a nearly isothermal phase boundary near 315 degrees C. Above this boundary, hexagonal pyrrhotite has a supercell in which a=2A, c=7C, not previously recognized. Immediately below this boundary, hexagonal pyrrhotite (supercell with a=2A, c=5C) occupies most of the field between 46.6 and 50 percent Fe. This field decreases downward, between a narrow two-phase field with monoclinic pyrrhotite (stoichiometric Fe (sub ) S 8 ) on one side, and a low-temperature (<140 degrees C) two-phase field with troilite on the other. The numerous processes that may affect the composition of pyrrhotite with pyrite limit the reliability of the pyrrhotite geothermometer.
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Desborough et al. (1965) studied this question.
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