PHASE-RELATIONS AND PHASE-TRANSFORMATIONS BETWEEN THE LOW-TEMPERATUREIRON SULFIDES MACKINAWITE, GREIGITE, AND SMYTHITE

Studies on natural smythite (Fe13S16), greigite (Fe3S4), and mackinawi te (FeS1-x) from the low-temperature hydrothermal Moschellandsberg mer cury deposit (SW-Germany) revealed oriented replacements and overgrowt hs of greigite and mackinawite by smythite. Microprobe analyses show t hat the iron sulfides are essentially pure iron end-members. Thus smyt hite is a phase in the pure Fe-S system, and no minor elements in soli d solution are required to stabilize it. The oriented replacements can be interpreted in part as a recrystallization process and in part as solid-state transformations, driven by disequilibrium between phases a nd by probable metastability of greigite and mackinawite. The observed reactions involve minor changes in the Fe/S ratios which were probabl y facilitated by iron exchange with the siderite matrix. As a conseque nce of the observed phase transformations it is suggested that smythit e is stable, and that pyrrhotites (except troilite) may be metastable at temperatures below 65-degrees-C.