Phase relations and phase transformations between the low-temperature iron sulfides mackinawite, greigite, and smythite
Krupp, Ralf E.
European Journal of Mineralogy Volume 6 Number 2 (1994), p. 265 - 278
published: Mar 31, 1994
manuscript accepted: Nov 23, 1993
manuscript received: May 7, 1993
ArtNo. ESP147050602010, Price: 29.00 €
Abstract Studies on natural smythite (Fe13S16), greigite (Fe3S4), and mackinawite (FeSi-x) from the low-temperature hydrothermal Moschellandsberg mercury deposit (SW-Geπnany) revealed oriented replacements and overgrowths 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.