Phase separation in (Fe,Co)1-xS monosulfide solid-solution below 450°C, with consequences for coexisting pyrrhotite and pentlandite in magmatic sulfide deposits

Phase relations for (Fe,Co)(l-x)S coexisting with cobalt pentlandite have been studied up to 450degreesC using sealed silica.-lass tube experiments. The (Fe,Co) monosulfide solid-solution with the NiAs-type structure (Fe,Co-mss) unmixes abruptly below 425degreesC, to coexisting Fe-rich (Fe,Co-mss1; 1C structure) and Co-rich (Fe,Co-mss2; 3C structure) phases. For bulk compositions with 52.0 at.% S, the equilibrium Fe,Co-mss1 solvus is located at about 0.83 molar Fe/Fe + Cc) at 400degreesC and progressively diverges toward the Fel-xS end-member composition with decrease in temperature to 0.98 molar Fe/Fe + Cc) at 105degreesC. At 400degreesC, the equilibrium Fe,Co-mss2 solvus is at about 0.37 molar Fe/Fe + Cc) and does not appear to vary significantly with decrease in temperature. There is a metastable solvus within the equilibrium miscibility-gap, with a critical temperature at 425degreesC between 0.45 and 0.50 molar Fe/Fe + Co), as indicated by single-phase products that persist even with prolonged annealing. A narrow field of spontaneous phase-separation is centered at 0.75 molar Fe/Fe + Co), and results in satellite reflections of h0l NiAs-type subcell reflections in products quenched from a high temperature (800degreesC). The miscibility gap in Fe.Co-mss is similar to the phase separation of mss in the system Fe-Ni-S. However, phase separation occurs at a slightly higher temperature in Fe,Co-mss (between 425 and 400degreesC) than in mss (between 400 and 300degreesC), and is initially discontinuous.