Mackinawite transformation into greigite at room temperature under anoxic and acidic conditions: a corrosion pathway?

Abstract. In surface soils and sediments, iron monosulfide (FeS) species, including nanocrystalline mackinawite, tend to quickly form in the presence of iron and sulfide in anoxic conditions. As such, FeS species are the main precursors for the formation of other iron sulfides such as Fe3S4 greigite and FeS2 pyrite, which are ubiquitous in surface sedimentary environments. It is known that, under prolonged aging under reducing conditions in a sulfidic aqueous medium, FeS species can evolve into crystalline mackinawite. However, the possible influence of pH on the evolution of mackinawite under such anoxic low-temperature conditions relevant to sedimentary (sub)surface environments has not been investigated yet. In this study, we used Rietveld refinement and pair distribution function analysis (PDF) of synchrotron-based X-ray powder diffraction (XRD) patterns to derive the mean coherent domain (MCD) size of mackinawite after aging under various pH conditions and X-ray absorption near-edge structure (XANES) spectroscopy at the S and Fe K-edges to study the structural and electronic properties. Moreover, in order to strengthen our interpretations, we confirmed the shape and relative energy of pre-edge features in the S K-edge XANES spectra of mackinawite (FeS) and pyrite (FeS2) model compounds via first-principle calculations. Our results show that, after FeS has precipitated from aqueous Fe(II) and H2S/HS- in a saline medium at pH 7.1, aqueous aging at the same pH over 47 d results in the formation of nanocrystalline mackinawite (MCDab=11.5±0.1 nm; MCDc=7.1±0.1 nm). When Na2S is added into the solution to reach pH 9.7 after FeS has precipitated at pH 7.1, no other Fe sulfide is observed during the aging phase, and mackinawite particles are of smaller size (MCDab=7.9±0.1 nm; MCDc=4.6±0.1 nm). In this sample, an additional weak and broad peak appears at d=10.5 Å that could be interpreted as being due to either lattice expansion at the particle boundaries or a double-cell super-structure. When H+ is added as HCl to reach pH 5.1 before the aging phase, the size of mackinawite particles increases (MCDab=13.0±0.2 nm; MCDc=8.1±0.2 nm), and a fraction transforms into greigite (Fe3S4). This reaction is accompanied by a pH increase to 6.4, likely because of H+ consumption, which suggests that Fe(II) in FeS would serve as an electron donor and that H+ would serve as an electron acceptor. The calculated electronic structure of mackinawite shows partly filled Fe-3d states, which supports the fact that acidic aging conditions are favorable for Fe(II) to act as an electron donor. We propose and further discuss the fact that the formation of greigite from nanocrystalline mackinawite could result in H2 production as, for instance, observed for anoxic corrosion of zero-valent Fe at higher temperatures. Greigite has been designated in the literature either as an intermediate towards pyrite formation or as a mineralogical endmember in another reaction route. Our observations raise the question of the existence of such a reaction producing Fe3S4 and H2 in reducing sedimentary (micro)environments across geological times. In addition, the metallic character of mackinawite suggests that Fe(II) oxidation to Fe(III) by H+ in this mineral species could proceed without the need for another oxidizing agent. Although the possible formation of pyrite from greigite would require further studies on extended aging time and/or under more acid-sulfidic conditions, our findings could have implications for the understanding of the initial steps of the H2S pathway to pyrite.