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In this study, Raman spectroscopy was used to precisely understand the mechanisms of oxidation of mackinawite (FeS). Two experimental conditions were considered: (1) oxidation in air at room temperature and (2) oxidation in acidic anoxic solutions at 80 • C. In both cases, the oxidation process began by the in situ oxidation of Fe(II) cations inside the crystal structure of mackinawite and led to Fe(III)-containing mackinawite, Fe II 1−3x Fe III 2x S. The oxidation in air finally led to Fe(III) oxyhydroxides and elemental sulfur α-S 8 , but greigite (Fe 3 S 4 ) was observed as an intermediate compound. In anoxic acidic solutions, the product of the oxidation was proved to depend on the Fe/S concentration ratio. For Fe/S = 3/4, greigite was the only product obtained, and this allowed us to determine unambiguously the Raman spectrum of this compound. For Fe/S = 1/2, the reaction led to pyrite FeS 2 .

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