Source: Journal of Geophysical Research: Solid Earth
The strongly ferrimagnetic thiospinel of iron, greigite (Fe3S4), forms as a precursor of pyrite (FeS2) in reducing sediments and provides a crucial mineral for paleomagnetic and environmental magnetic studies. The most important factors for greigite formation and preservation is biologically degradable organic matter, reactive iron, and dissolved sulfate that can be biologically reduced to sulfide by sulfate-reducing microorganisms.
Mineral magnetic measurements in combination with electron microscopic observations taken by Yang et al.  revealed the presence of very large greigite grains up to several tens of micrometers in submarine fan sediments from the Sumatra Subduction Margin cored by IODP Expedition 362. Such “giant” crystal sizes of greigite are exceptional compared to grain sizes below a few hundred nanometers, observed normally for this iron sulfide. In the cored sequence, the greigite is confined to multiple organic-rich mudstone horizons with high magnetic susceptibility. The authors propose that formation of greigite is fostered by successive organic-rich turbidite cycles that have been intensively reworked by benthic fauna.
This formation pathway may be an overlooked important iron-sulfur sink in carbon-rich turbidite beds and should be considered when assessing the marine iron-sulfur-carbon cycle.
Citation: Yang, T., Dekkers, M. J., Zhao, X., Petronotis, K. E., & Chou, Y.-M. (2022). Greigite formation modulated by turbidites and bioturbation in deep-sea sediments offshore Sumatra. Journal of Geophysical Research: Solid Earth, 127, e2022JB024734. https://doi.org/10.1029/2022JB024734
—Agnes Kontny, Associate Editor, Journal of Geophysical Research: Solid Earth