The transformation of mackinawite into greigite studied by Raman spectroscopy

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_1−3xFe^III_2xS. The oxidation in air finally led to Fe(III) oxyhydroxides and elemental sulfur α‐S₈, but greigite (Fe₃S₄) 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. Copyright © 2010 John Wiley & Sons, Ltd.