HERFD-XAS evidence for an octahedrally coordinated CoSn-polysulfide precursor as a probe for the mechanism of pyrite formation

Trace element contents in authigenic pyrite (FeS₂) are often considered as a reliable geochemical archive of past marine conditions. For instance, cobalt (Co) abundance in marine sedimentary pyrite may track back the extent of past ocean anoxia and is considered as a reverse proxy for the rise of atmospheric oxygen. However, the molecular-scale route of Co incorporation in pyrite at low temperature is not well documented. Thus, any insight on this aspect are expected to help better assess the actual role of pyrite in Co cycling in modern and past subsurface environments. In this study, a series of pyrites were synthesized in solution via the polysulfide pathway under anoxic conditions and at ambient temperature, with various initial aqueous Co concentrations (Co:Fe = 0.13–5). Rietveld refinement analysis of the powder X-ray diffraction (XRD) patterns shows that pyrite is the principal component (69(5) wt%) of the final solid products, with small fractions of marcasite (17(4) wt%) and FeS (10(2) wt%). High Energy Resolution Fluorescence Detected (HERFD) Co K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray absorption fine structure (EXAFS) analysis indicate that a minor fraction (20–36 %) of Co substitutes for Fe in the pyrite structure as compared with a theoretical spectrum of a Co-substituted pyrite supercell calculated using Density Functional Theory (DFT). Besides, in the final products, the major part (64–80 %) of Co persists in the form of an amorphous CoS_n-polysulfide precursor phase that represents the whole Co speciation before pyrite nucleation. In this precursor observed at the monosulfide FeS pre-pyrite stage, Co early adopts an octahedral coordination as attested by Co pre-edge data, whereas Co is in tetrahedral coordination in our Co-doped mackinawite FeS reference compound. The local structure of the CoS_n-polysulfide precursor is further elucidated by EXAFS shell-by-shell analysis that points to monomeric units (< 1 nm), where Co is octahedrally coordinated to first neighboring S atoms with at least three of these S neighbors belonging to a polysulfide chain of undetermined length. Aggregation of such monomeric units into an amorphous CoS_n-polysulfide phase is supported by X-ray scattering-pair distribution function analysis (XRD-PDF) of an analogous amorphous CoS_n compound. These results could have important implications on our understanding of pyrite nucleation mechanisms via the polysulfide pathway since the observed octahedral CoS_n-polysulfide precursor differs from the generally proposed models of tetrahedral FeS precursors. In addition, the persistence of these peculiar species and the observed delay of Co incorporation in pyrite highlights the importance of trace elements in pyrite formation kinetics at low temperature. Lastly, our results illustrate the high affinity of Co for polysulfides and raise questions on the possible presence and evolution of this non-pyrite CoS_n phase in sedimentary archives. In this regard, this study may provide new mechanistic insights that could help explaining the moderate affinity for authigenic pyrite generally reported for Co, in particular in a sedimentary context where other possible bearing phases such as clay minerals and Mn oxides are involved.