HEXAGONAL AND MONOCLINIC PYRRHOTITES

The sulfur-rich limit of hexagonal pyrrhotite below 290° C is 47.45 atomic percent iron and is independent of temperature. Monoclinic pyrrhotite is metastable above 290° C. This observation, combined with the reported conversion of a natural monoclinic pyrrhotite to the hexagonal form at 260° C, suggests that monoclinic pyrrhotite may be metastable at lower temperature as well. The composition of monoclinic pyrrhotite seems to be a function of temperature, but experimental determination of its composition also depends on the decomposition rate if it is metastable. The dry and aqueous synthesis of monoclinic pyrrhotite at low temperature (not inversion of hexagonal pyrrhotite on cooling) and its occurrence in low-temperature environments, indicates that the use of the composition of monoclinic pyrrhotite and the hexagonal pyrrhotite solvus for estimating formation temperatures is not justified unless the thermal history of the specimen is known. © 1969 Society of Economic Geologists, Inc.