Dynamics of disordered mackinawite (FeSm) at low temperatures and its geochemical implications

• FeS m produces H 2 abiotically at 25°C without H 2 S or S 0 under anoxic conditions. • Fe(III)-rich surface layers retard the corrosion of FeS m , enhancing its stability. • FeS m undergoes particle growth, with the mechanism dependent on pH and aging time. Disordered mackinawite (FeS m ), 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 FeS m undergoes anoxic corrosion at 25 o C, generating H 2 even in the absence of traditional oxidants such as hydrogen sulfide (H 2 S) or elemental sulfur (S 0 ). This abiotic H 2 production provides a potential reductant for early Earth carbon fixation and may support modern oligotrophic ecosystems by influencing carbon cycling. The pH-dependent H 2 production kinetics suggests that protons (H + ) likely act as the primary oxidant in FeS m corrosion. The formation of Fe(III)-rich surface layers during this process passivates further corrosion and modulates surface reactivity—potentially facilitating the oxidation of H 2 S to S 0 and intermediate species, thus driving FeS m transformation into greigite (Fe 3 S 4 ) and pyrite (FeS 2 ). 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, FeS m becomes stabilized through less-oriented attachment, producing polycrystalline particles. Both surface passivation and particle growth contribute to the resilience and dynamic behavior of FeS m under diverse geochemical conditions, reinforcing its role in sustaining iron and sulfur biogeochemical cycles. This study offers mechanistic insights into the structural evolution of FeS m , with implications for both early Earth environments and modern sedimentary systems.