The sluggish nature, measured in periods of up to twelve months, of solid state reactions in the Zn-Fe-S system hampered the low-temperature phase studies of earlier workers. New techniques involving the preparation of sulfide starting material by precipitation and fluxing of these sulfides with salts has made it possible to grow (Zn,Fe)S crystals in equilibrium, established in one direction only, with pyrrhotite and pyrite in less than a week at 395 degrees C. As a result it was possible to investigate the sphalerite solvus in the ZnS-FeS-FeS 2 system from 714 degrees C to a new low temperature of 303 degrees C.After reaching an FeS composition maximum at 580 degrees C, the sphalerite solvus does not reverse its slope but instead drops vertically at a constant composition of 20.8 + or - 0.5 mole percent FeS to the hexagonal-monoclinic pyrrhotite inversion temperature. At 303 degrees C hexagonal pyrrhotite, pyrite and sphalerite constitute the equilibrium assemblage on the sphalerite solvus. Monoclinic pyrrhotite was present in a run at 210 degrees C. Therefore, over the temperature range of geological interest, below 600 degrees C, the assemblage pyrite-hexagonal pyrrhotite-sphalerite is unsuitable for geothermometry.

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