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Roud, S. C.; Gilder, S. A.; Park, S.-H. (19. July 2022): Greigite (Fe3S4) formation in artificial sediments via solid-state transformation of lepidocrocite. In: Geochemistry, Geophysics, Geosystems, Vol. 23, e2022GC010376
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Greigite (Fe3S4) is a ferrimagnetic iron-sulfide mineral that forms in sediments during diagenesis. Greigite growth can occur diachronously within a stratigraphic profile, complicating or overprinting environmental and paleomagnetic records. An important objective for paleo- and rock-magnetic studies is to identify the presence of greigite and to discern its formation conditions. Greigite detection remains, however, challenging and its magnetic properties obscure due to the lack of pure, stable material of well-defined grain size. To overcome these limitations, we report a new method to selectively transform lepidocrocite to greigite via the intermediate phase mackinawite (FeS). In-situ magnetic characterization was performed on discrete samples with different sediment substrates. Susceptibility and chemical remanent magnetization increased proportionally over time, defining two distinct greigite growth regimes. Temperature dependent and constant initial growth rates indicate a solid-state FeS to greigite transformation with an activation energy of 78-90 kJ/mol. Low and room temperature magnetic remanence and coercivity ratios match with calculated mixing curves for superparamagnetic (SP) and single domain (SD) greigite and suggest ∼25% and ∼50% SD proportions at 300 K and 100 K, respectively. The trend coincides with empirical data reported for natural greigite-bearing sediments, suggesting a common SP endmember size of 5-10 nm that is likely inherited from mackinawite crystallites. The average particle size of 20-50 nm determined by X-ray powder diffraction and electron microscopy accords with theoretical predictions of the SP/SD threshold size in greigite. The method constitutes a novel approach to synthesize greigite and to investigate its formation in sediments.