Subsolidus Phase Relations in the System Fe–Ni–S at 450°C and 6 GPa

Subsolidus phase relations in the Fe–Ni–S system were studied in a multianvil press at 6 GPa, 450°C, and a duration of 17 h using ceramic capsules. The nine intermediate compounds were established: FeS2 pyrite (Ni# ≤ 7), NiS2 vaesite (Ni# ≥ 96), (Fe, Ni)1 – xS (Ni# 0–100), (Ni, Fe)4S3 (S# 42–44, Ni# ≥ 60), Ni3S2 heazlewoodite (S# 40, Ni# ≥ 97), Ni3 – xS (S# 26, Ni# 100), Ni3.5Fe0.5S (S# 21, Ni# 87), and metal phase (Ni# 64–68), where Ni# = Ni/(Ni + Fe) × 100 mol % and S# = S/(S + Ni + Fe) × 100 mol %. A wide solubility gap between disulfides results in two large three-phase fields: (Fe0.93Ni0.07)S2 + S + (Ni0.96Fe0.04)S2 and (Fe0.93Ni0.07)S2 + (Ni0.96Fe0.04)1 – xS + (Ni0.96Fe0.04)S2. A narrow field of monosulfide solid-solution is sandwiched between FeS2 + (Fe, Ni)1 – xS and (Fe, Ni)S + (Ni, Fe)4S3 two-phase fields. Large three-phase fields: Fe0.88Ni0.12 Fe0.78Ni0.22S + Fe0.65Ni0.35, Ni063Fe0.37 + Fe0.78Ni0.22S + (Ni0.6Fe0.4)4S3, and Fe0.32Ni068 + (Fe0.3Ni0.7)4S3 + Fe0.05Ni0.95 appear at bulk S# < 50. Stabilization of a new phase, Ni3.5Fe0.5S, yields two large three-phase fields: Ni3.5Fe0.5S + (Fe0.3Ni0.7)4S3 + Ni3S2 and Ni3.5Fe0.5S + Ni3S2 + Ni.