Thermodynamically stable greigite (Fe₃S₄) reshapes the Fe–S and Fe–S–O predominance diagrams

Greigite (Fe₃S₄), an iron thiospinel and an important ferrimagnetic mineral in sedimentary environments, has long been regarded as metastable. Recent calorimetry indicates that greigite is thermodynamically stable in the Fe–S system, so the classical Fe–S and Fe–S–O predominance diagrams require reassessment. Using a CALPHAD workflow built on pycalphad and a unified thermodynamic database that incorporates the measured greigite data, we recompute these diagrams and compare them with the classical assessments. Treating greigite as metastable reproduces the classical Fe–S and Fe–S–O topologies, validating the database and method. Introducing stable greigite substantially modifies both systems: in Fe–S, greigite occupies an intermediate position between metallic iron and pyrite, analogous to magnetite in Fe–O, and the equilibrium pyrrhotite field is absent for the mean greigite enthalpy of formation, reappearing only within its experimental uncertainty. In Fe–S–O, greigite borders the iron oxides directly, displacing the classical pyrite–hematite contact. Differential scanning calorimetry and powder X-ray diffraction show that greigite persists to approximately 600 K before transforming to pyrite and pyrrhotite. The results show that closely balanced thermodynamic parameters can yield markedly different yet equally admissible phase diagrams, with implications for low-temperature iron-sulfide equilibria in geochemical systems. All code, databases, and data are openly available.