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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 .

Archaeological objects are exposed to the action of micro-organisms when they lay in a biologically active environment. The presence of iron sulfides in the corrosion system testifies in most cases that the degradation was influenced by sulfate-reducing bacteria. Iron sulfides and other iron/sulfur-containing compounds were detected by micro-Raman spectroscopy in rust layers of archaeological ferrous objects and in wet wooden fragments contaminated by iron, extracted from ancient wrecks. Although mackinawite (FeS) is very reactive towards oxygen, this phase was observed in each sample. Its crystallisation levels and oxidation states could be differentiated. Greigite (Fe 3 S 4 ) was also identified by means of X-ray diffraction, used when possible as a complementary analytical tool. Known as the result of the oxidation of mackinawite, greigite was likely to form during the experiments carried out without any protection against air. It was then possible to describe the formation and oxidation processes of iron sulfides in archaeological iron artefacts or in organic materials contaminated by iron.

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