Phase relations and phase transformations between the low-temperature iron sulfides mackinawite, greigite, and smythite

Abstract

Studies on natural smythite (Fe 13 S 16 ), greigite (Fe 3 S 4 ), and mackinawite (FeS 1−x ) from the low-temperature hydrothermal Moschellandsberg mercury deposit (SW-Germany) revealed oriented replacements and over-growths of greigite and mackinawite by smythite. Microprobe analyses show that the iron sulfides are essentially pure iron end-members. Thus smythite is a phase in the pure Fe-S system, and no minor elements in solid 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 and by probable metastability of greigite and mackinawite. The observed reactions involve minor changes in the Fe/S ratios which were probably facilitated by iron exchange with the siderite matrix. As a consequence of the observed phase transformations it is suggested that smythite is stable, and that pyrrhotites (except troilite) may be metastable at temperatures below 65°C