Sedimentary greigite formation and stability

Since the classic work of R.A. Berner, greigite (Fe3S4) has been considered a metastable iron sulfide that forms as a precursor to pyrite (FeS2). However, revised thermodynamic data now indicate that greigite is a stable sedimentary Fe-S phase. Equilibrium computations with the revised data explain key aspects of sedimentary greigite occurrences. Greigite has a large stability area in pE-pH space relative to pyrite. It dominates in low pE regimes, which is consistent with its widespread occurrence in methanic sediments. It also has a small but significant stability field near the sulfate-sulfide stability boundary. Its significance increases in environments with relatively high dissolved Fe:S ratios, which explains its occurrence in freshwater sediments and iron-enriched marine sediments. It is also a paleoenvironmental marker for transitional environments, especially between freshwater and marine systems. Widespread sedimentary greigite occurrence is consistent with its stability. It is stable with respect to pyrite and monoclinic pyrrhotite to over 200 °C, and its presence indicates that enclosing rocks have not exceeded this temperature for prolonged periods since greigite formation. However, greigite is sensitive to oxidation and its long-term geological preservation depends mostly on protection from oxidation by low sediment permeability or enclosure in other materials. Also, most sedimentary and biological greigite forms via equilibrium reactions involving mackinawite-like precursors, but greigite is not a precursor to pyrite; these minerals form independently during sedimentary diagenesis.