Monoclinic Pyrrhotite

Abstract

Recent literature has produced much information on the thermal and compositional stability and on the natural occurrences and mineral associations of monoclinic pyrrhotite. These data together with those obtained from new field and laboratory studies have made it possible to derive the phase relations from 600° to about 200°C in the portion of the Fe-S-O system which involves the minerals hexagonal pyrrhotite, pyrite, monoclinic pyrrhotite and magnetite. Monoclinic pyrrhotite is stable below 310 ± 5°C. Near the upper limit of its thermal stability range it can only be synthesized at rather low, closely controlled oxygen pressures, but over a fairly large variation in Fe/S ratios. Monoclinic pyrrhotite has an Fe/S + O ratio of, or near, 7/8. Monoclinic pyrrhotite in the ternary system is stable with hexagonal pyrrhotite and pyrite below 310 ± 5°C. At about 220°C an invariant reaction involving hexagonal pyrrhotite, pyrite, monoclinic pyrrhotite, magnetite and vapor, takes place. Below this temperature monoclinic pyrrhotite and magnetite form a stable mineral pair. The maximum concentration of oxygen in solid solution in monoclinic pyrrhotite occurs at this invariant temperature. The monoclinic pyrrhotite solid solution composition may reach its closest proximity to the Fe-S boundary at the temperature where smythite becomes stable (about 75°C). Hexagonal pyrrhotite takes a small amount of oxygen in solid solution. This oxygen may be responsible for the formation of hexagonal superstructures and may be the cause of the metastable behavior of supersaturated hexagonal pyrrhotite.

Keywords

• monoclinic pyrrhotite
• phase relations
• Fe-S-O system
• oxygen in sulfides