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

Abstract

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 °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 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 H₂S to S⁰ and intermediate species, thus driving FeS_m 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, 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.