Abstract. Pentlandite has a wide compositional range in the Fe-Ni-Co-S system and the Fe-Ni-S system. The metal : sulfur atomic ratio is approximately 9 : 8. The Co content of pentlandite from the Kamaislii ore deposit and one from the Outokumpu ore deposit (Knop et al. 1965) varies from a Co-free to nearly a Co9S8 composition. Pentlandite forms a complete solid solution between (Fe, Ni)9_+xS 8 and Co9+_xS8 in the 600°-300 °C temperature range. On the other hand, it is presumed that the pentlandite solid solution decomposes into two fields toward the (Fe, Ni)9S 8 and Co9S8 members at 200 °C. The field of solid solution at 500°C is the most extensive and includes all of the other solid-solution fields. The metal : sulfur ratio of the solid solution varies systematically, i.e., the Fe-rich field sinks toward the Fe-Ni-Co plane away from the M9S 8 section, and contrarily the Ni-rich field shifts toward the sulfur apex. The d044 value decreases with increasing S, Ni, or Co contents in pentlandite. The Kainaishi pentlandites, Iwate Prefecture, Japan, lie within or close to the solidsolution field at 300 °C, showing a very good correlation with the Co : Ni ratio of the homogeneous natural pyrrhotite phase.
Rodingite containing vesuvianite with the world's highest Cr 2 O 3 and TiO 2 contents was found to occur in the ultramafic body of the Early to Middle Devonian Darbut -Sartuohai ophiolite zone belonging to the Sartuohai alpine -type ophiolitic melange, west Junggar district, Xinjiang, China. Many blocks of massive and vein rodingite were located at the Sartuohai chromium deposit in the serpentinized ultramafic body along the suture zone near Karamay. The rodingite is mainly composed of vesuvianite, grossular -andradite series garnet, diopside, and chlorite. The Cr 2 O 3 content of the rodingite masses and veins is up to 0.2 wt% and 0.1 wt%, respectively. While the Cr 2 O 3 content (wt%) of each mineral species in rodingite is up to 0.04 for vesuvianite, 0.4 for garnet and 0.4 for diopside, respectively. Tiny sporadic inclusions of vesuvianite and garnet of 5 µm to 35 µm in diameter also disseminate along the cleavage or in the grain of diopside of the massive rodingite. The vesuvianite inclusions are characterized by a high concentration in chromium of Cr 2 O 3 = 6.21 wt% and titanium of TiO 2 = 7.43 wt% of all the reported data, in contrast to the low concentration in the host diopside. The garnet inclusions belong to the grandite series. The morimotoite and uvarovite components are 18% and 11% at most, respectively. The rodingite was considered to have been formed by two stages of metasomatism between gabbroic rock and a calcium -rich solution, which was probably produced in the process of serpentinization of peridotite under Cr -and Ti -rich, and Cr -and Ti -free environments.
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