Porewater chemistry, not lattice diffusion, controls pyritization of stable greigite

Greigite (Fe₃S₄) is the ferrimagnetic iron thiospinel that carries magnetic remanence in sulfidic marine and lacustrine sediments and is precipitated by magnetotactic bacteria as a potential biosignature. Its geological interpretation has rested for decades on the assumption that greigite is metastable, so its presence records arrested conversion to pyrite rather than equilibrium mineralogy. Calorimetry now shows greigite to be thermodynamically stable with respect to its decomposition to pyrite and pyrrhotite, which removes that premise. Incorporating the measured properties places greigite within the stable low temperature Fe–S assemblage and gives it a broad equilibrium field in reducing porewater that persists across the calorimetric uncertainty. An independent kinetic test shows that bulk sulfur diffusion does not control its conversion to pyrite: for a 1 µm crystal, measured sulfur self-diffusion predicts conversion times of ~10¹⁷ years at 25 °C and ~7 × 10⁸ years at 200 °C, although pyritization is observed within hours. Greigite persistence is controlled not by a slow intracrystalline conversion but by the aqueous chemistry that enables faster pyritization pathways. Greigite can consequently record diagenetic redox conditions rather than simply an arrested step toward pyrite, with implications for paleomagnetic records, magnetofossil preservation and sulfidic sediment formation on Earth and Mars.