Stability of pentlandite in the Fe-Ni-Co-S system

Abstract

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.

Sulfur geochemistry of peridotite-hosted hydrothermal systems: Comparing the Ligurian ophiolites with oceanic serpentinites

“…Stability of pentlandite in the ternary system Fe 9 S 8 -Ni 9 S 8 -Co 9 S 8 . Pentlandite occupies the widest range in composition at 500°C and separates into two stability fields at formation temperatures below 200°C (after Kaneda et al, 1986). Most of the measured pentlandite assemblages do not clearly reveal the formation temperature and fall in a range that is stable down to temperatures of 200°C.…”

“…The pentlandites show a relatively uniform Co-content and mostly vary in their Fe-and Ni-composition. Experimental studies by Kaneda et al (1986) have shown that pentlandite compositions are temperature-dependent. While pentlandite covers a wide compositional range at temperatures between 300°a nd 600°C, forming a complete solid solution between (Fe,Ni) 9 S 8 and the Co-end member Co 9 S 8 , an immiscibility gap is present at lower temperatures ( Fig.…”

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: II. Sulfides

“…The results (see Table 6) shows gradual pressure decrease from 4 kbar in marginal (apatite)-forsterite phoscorite to 2 kbar in axial carbonate-rich phoscorite and phoscorite-related carbonatite, and then again growth to 4 kbar in vein calcite carbonatite, and decrease to 2 kbar in vein dolomite carbonatite (i.e., decrease by 2 kbar from earlier to the latest rock within both pipe and vein series). It is necessary to note similar behavior of oxygen fugacity [26] On the M9S8 plane of the Fe-Ni-Co-S tetrahedron (Figure 12), chemical compositions of pentlandite are concentrated within the field of the solid-solution stability at ≥200 °C [58]. Chemical compositions of cobaltpentlandite correspond to the Fe4.5Ni4.5S-Co9S8 trend, whereas most samples from host silicate rocks, marginal forsterite-dominant and intermediate low-carbonate magnetite-rich phoscorite are within the stability field at 300-400 °C, and most samples from axial carbonate rich phoscorite and carbonatites are within the stability field at 200-400 °C.…”

“…In the sequence of rock formation, there is a gradual increase of Co content in pentlandite-cobaltpentlandite due to Ni and Fe (see Figure 4c), which causes corresponding spatial zonation of the phoscorite-carbonatite complex (see Figure 5c). On the M9S8 plane of the Fe-Ni-Co-S tetrahedron (Figure 12), chemical compositions of pentlandite are concentrated within the field of the solid-solution stability at ≥200 °C [58]. Chemical compositions of cobaltpentlandite correspond to the Fe4.5Ni4.5S-Co9S8 trend, whereas most samples from host silicate rocks, marginal forsterite-dominant and intermediate low-carbonate magnetite-rich phoscorite are within the stability field at 300-400 °C, and most samples from axial carbonate rich phoscorite and carbonatites are within the stability field at 200-400 °C.…”

“…Chemical compositions of cobaltpentlandite correspond to the Fe4.5Ni4.5S-Co9S8 trend, whereas most samples from host silicate rocks, marginal forsterite-dominant and intermediate low-carbonate magnetite-rich phoscorite are within the stability field at 300-400 °C, and most samples from axial carbonate rich phoscorite and carbonatites are within the stability field at 200-400 °C. Relationship between composition of the Kovdor's pentlandite-cobaltpentlandite and fields of pentlandite-cobaltpentlandite stability after [58].…”