The system Fe-Co-Ni-As-S.: I.: Phase relations in the (Fe,Co,Ni)As0.5S1.5 section at 650° and 500°C

The phase relations in the sulfur-rich regions of the system Fe-Co-Ni-As-S were investigated with mineral syntheses, and characterization of the products by electron-microprobe analysis and powder X-ray diffraction. The experimental charges were weighed with an atomic ratio (Fe + Co + Ni) /(As + S) of 1:2 and a S:As ratio of 3:1. Two isothermal sections were investigated, 500degrees and 650degreesC. The 500degreesC experiments resulted in the formation of arsenopyrite, cobaltite, cattierite, gersdorffite, linnaeite, monosulfide solid-solution, pyrite, siegenite and vaesite. Pyrite, the most common phase, is found in all Fe-bearing assemblages. The phase assemblages formed at 650degreesC involve cattierite, vaesite, cobaltite and gersdorffite in Fe-poor samples. In Fe-rich samples, the phase assemblage is dominated by monosulfide solid-solution and As1-xSx melt. The formation of this melt made it possible to estimate the activity of sulfur, using the constraints of Barton (1969). Cobaltite, cattierite and gersdorffite all sihow extensive solid-solution with respect to Co-Ni and As-S. All the Me(As,S)(2) phases have a pyrite-type structure with a disordered distribution of As and S (space group Pa3); the unit-cell parameter a was determined.