Sedimentary Greigite formation

Rickard, David ORCID: https://orcid.org/0000-0002-4632-5711, Roberts, Andrew P. and Navrotsky, Alexandra 2024. Sedimentary Greigite formation. American Journal of Science 324 , 13. 10.2475/001c.121855

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Abstract

Revised thermodynamic data for greigite (Fe3S4) indicate that it is a stable sedimentary Fe-S phase. Greigite was previously regarded as metastable. Equilibrium computations using revised data explain apparently contradictory observations regarding greigite occurrences in sediments and sedimentary rocks. Greigite has a large stability area in pe-pH space relative to pyrite. It dominates in low pe regimes especially near the lower water stability boundary, which is consistent with its widespread occurrence in methanic sediments. It also has a small but significant stability zone near the sulfate-sulfide stability boundary. Its significance increases in regimes with relatively high dissolved Fe:S ratios, which explains its occurrence in freshwater sediments and iron-enriched marine sediments. It is also a paleoenvironmental marker for transitional environments, especially between freshwater and marine systems. It is stable relative to pyrrhotite and smythite, although their formation together with greigite in low pe environments may be facilitated by catalytic processes. The greigite-smythite (pyrrhotite)-siderite association is a potential marker for ancient methanogenesis. Greigite is relatively sensitive to oxidation and its long-term geological preservation depends mostly on protection from oxidation by low sediment permeability or enclosure in other minerals or organic remains. Most sedimentary and biological greigite forms via equilibrium reactions involving mackinawite-like precursors, with no direct coupling of greigite with pyrite; these minerals form independently during sedimentary diagenesis. Magnetosomal greigite production by magnetotactic bacteria is a consequence of relative greigite stability, its decoupling from pyrite, and its protection from oxidation by cell membranes.

Item Type:	Article
Date Type:	Published Online
Status:	Published
Schools:	Earth and Environmental Sciences
Publisher:	American Journal of Science
ISSN:	0002-9599
Date of First Compliant Deposit:	9 October 2024
Last Modified:	09 Oct 2024 14:05
URI:	https://orca.cardiff.ac.uk/id/eprint/172762

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