Pathways to pyrite through polysulfides and metastable iron sulfides in marine sediments

The formation of pyrite (FeS₂) is a critical process in sedimentary environments, central to the global iron and sulfur cycles and sedimentary redox cycling. However, the mechanisms governing pyrite formation under natural conditions remain poorly constrained. Using Mössbauer spectroscopy and X-ray photoelectron spectroscopy (XPS), we present direct evidence of polysulfides facilitating the transformation of FeS x phases into pyrite in natural sediments from Saanich Inlet, British Columbia. We identify two nanoparticulate FeS 1+x phases: a mackinawite-like phase (np-FeS 1+x −1) and a greigite-like phase (np-FeS 1+x −2). Under persistently sulfidic conditions, the greigite-like phase can compete with pyrite as the prevailing Fe-S mineral. We also demonstrate that sequential chemical extractions underestimate highly reactive nanoparticulate Fe phases owing, in part, to pyrite framboids encapsulated within diatom frustules. Notably, these findings provide direct spectroscopic evidence consistent with polysulfide-mediated Fe-S transformations in low-temperature natural sediments, bridging a long-standing gap between experimental models and environmental observations and refining our understanding of early diagenetic pyrite formation.