Disordered mackinawite (FeSm), an initial iron sulfide forming under ambient, anoxic conditions, plays a central role in sedimentary iron and sulfur cycling and may have contributed to early biochemical processes relevant to the origin of life. However, its structural variability complicates the assessments of its geochemical behavior and environmental impacts. Here, we demonstrate that FeSm undergoes anoxic corrosion at 25 °C, generating H2 even in the absence of traditional oxidants such as hydrogen sulfide (H2S) or elemental sulfur (S0). This abiotic H2 production provides a potential reductant for early Earth carbon fixation and may support modern oligotrophic ecosystems by influencing carbon cycling. The pH-dependent H2 production kinetics suggests that protons (H+) likely act as the primary oxidant in FeSm corrosion. The formation of Fe(III)-rich surface layers during this process passivates further corrosion and modulates surface reactivity—potentially facilitating the oxidation of H2S to S0 and intermediate species, thus driving FeSm transformation into greigite (Fe3S4) and pyrite (FeS2). Particle growth mechanisms vary with pH: Ostwald ripening dominates under acidic conditions, while oriented attachment is favored at neutral to alkaline pH. Instead, with prolonged aging, FeSm becomes stabilized through less-oriented attachment, producing polycrystalline particles. Both surface passivation and particle growth contribute to the resilience and dynamic behavior of FeSm under diverse geochemical conditions, reinforcing its role in sustaining iron and sulfur biogeochemical cycles. This study offers mechanistic insights into the structural evolution of FeSm, with implications for both early Earth environments and modern sedimentary systems.
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