Reactions forming pyrite and marcasite from solution: II. Via FeS precursors below 100°C

The formation of pyrite and marcasite from a FeS precursor has been examined experimentally. In aging experiments at 65°C, the conversion of precursor amorphous FeS depends on these geologically relevant variables: concentration of sulfur-contributing species, acidity, redox state, time, Fe(II)/S(−II) ratio in solution, and addition of an organic ligand (citrate). The results indicate that pyrite and marcasite formation proceed at a significant rate only if intermediate sulfur species (i.e., polysulfides, polythionates, or thiosulfate) are present in solution. In the absence of any sulfur contributor or with only hydrogen sulfide or bisulfide present, no FeS2 formed within 16 days. Sulfidation of the precursor proceeds through progressively more sulfur-rich, Fe-S phases: am FeS (Fe1.11S-Fe1.09S) → mackinawite (FeS0.93-FeS0.96) → greigite (Fe3S4) → pyrite/marcasite FeS2. Greigite is absent under very reduced environments. The conversion sequence found in this study is in good agreement with iron-sulfide distribution patterns found in modern marine sediments.

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