Spectroscopic studies of iron sulfide formation and phase relations at low temperatures

Alistair R. Lennie, David Vaughan

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The iron mono sulfides mackinawite (Fe1+xS) and "amorphous FeS," the first formed iron sulfides in aqueous systems at low temperatures, have been studied using X-ray photoelectron and X-ray absorption spectroscopies. The divalent oxidation state of Fe and the local structure environment of Fe and S in mackinawite have been confirmed. Comparisons of the Fe K-edge X-ray absorption spectra of synthetic mackinawite with those of "amorphous FeS" precipitates subjected to ageing for periods ranging from 1 to 1800 seconds, and precipitated under pH conditions between pH 3.9 and pH 7.4, show the precipitates to have local structure consistent with that found in crystalline mackinawite. Combining these data with results in the literature leads to the proposal of a model which accounts for the formation from aqueous solution of Cubic FeS, troilite, and mackinawite. We use this model, which takes account of the effect of electron withdrawal by H+ at the crystal surface, to propose that these three phases can all be formed from Fe in tetrahedral coordination with S. This model facilitates understanding of the phase relationships in the Fe-S system at low temperatures, whereby initial formation is of iron "monosulfides" with either cubic closepacked (ccp) or hexagonal close-packed (hcp) S sublattices. Subsequent sulfidation of either sublattice type will form pyrite (or marcasite). However, the predominant sulfidation sequence at low temperatures is:
[Cubic FeSj/amorphous FeS ---> mackinawite ---> greigite ---> marcasite/pyrite. Both the nucleation of hcp S sub-lattice phases in low temperature aqueous iron sulfide systems, and the formation of hcp phases by transformation from the cubic monosulfides, are inhibited by kinetic factors.
Original languageEnglish
Pages (from-to)117-131
Number of pages15
JournalGeochemical Society. Special Publication
Publication statusPublished - 1996


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