Polysulfides revealed as drivers of pyrite formation in marine sediments

Pyrite (FeS₂) forms in sedimentary environments where it plays a central role in the global iron and sulfur cycles and in sedimentary redox cycling. Yet the mechanisms that govern its formation under natural conditions have remained poorly constrained, because most of what scientists know comes from laboratory models rather than direct observations of real sediments.

A new study of sediments from Saanich Inlet, British Columbia, addresses that gap. Using Mössbauer spectroscopy and X-ray photoelectron spectroscopy, researchers report direct evidence that polysulfides facilitate the transformation of FeSx phases into pyrite in natural sediments. The work provides direct spectroscopic evidence consistent with polysulfide-mediated iron-sulfur transformations in low-temperature natural sediments, bridging a long-standing gap between experimental models and environmental observations.

The team identified two nanoparticulate FeS1+x phases in the sediments: a mackinawite-like phase, designated np-FeS1+x−1, and a greigite-like phase, designated np-FeS1+x−2. These metastable iron sulfides are considered stepping stones on the pathway to pyrite. Notably, the researchers found that under persistently sulfidic conditions, the greigite-like phase can compete with pyrite as the prevailing iron-sulfur mineral, meaning pyrite is not always the dominant end product even where conditions favor its formation.

The study also uncovered a measurement problem with practical consequences. Sequential chemical extractions, a widely used method for quantifying reactive iron phases in sediments, were shown to underestimate highly reactive nanoparticulate iron phases. The researchers attribute this partly to pyrite framboids encapsulated within diatom frustules, which can shield the mineral from the extraction reagents.

By capturing the intermediate stages of pyrite formation directly in natural sediments, the findings refine current understanding of early diagenetic pyrite formation. The article was published in Communications Earth & Environment on 28 October 2026 as an open-access paper.

Subject of Research: Polysulfide-mediated pyrite formation pathways in marine sediments

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

Article References: Figueroa, M. C., Kukkadapu, R. K., Gregory, D. D., Chappaz, A., Engelhard, M. H., & Lyons, T. W. (2026). Pathways to pyrite through polysulfides and metastable iron sulfides in marine sediments. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-04161-z

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04161-z

Keywords: pyrite, polysulfides, mackinawite, greigite, marine sediments, Mössbauer spectroscopy, XPS, iron-sulfur cycle, early diagenesis, Saanich Inlet, sequential chemical extractions, Pathways