Dynamics of disordered mackinawite (FeS m ) 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.

Keywords

Mackinawite / Hydrogen production / Anoxic corrosion / Particle growth

CRediT authorship contribution statement

Hoon Young Jeong: Investigation, Funding acquisition, Data curation, Conceptualization. Hyun Hwi Lee: Investigation, Formal analysis, Data curation. Minji Park: Methodology, Formal analysis, Data curation. Sookyung Kim: Methodology, Formal analysis. Kim Ford Hayes: Validation, Resources, Investigation. So-Jeong Kim: Resources, Methodology, Data curation. Young-Soo Han: Writing - review & editing, Validation, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Dr. Jun Hyuk Moon for his invaluable support in Cryo-TEM analysis. This study was supported by the National Research Foundation of Korea [NRF-2021R1A2C1004455, RS-2021-NR066106].

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gsf.2025.102160.

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