Mackinawite transformation to greigite at room temperature under anoxic and acidic conditions: A corrosion pathway ?

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In surface soils and sediments, iron monosulfides species (FeS) 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 Fe 3 S 4 greigite and FeS 2 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 upon various pH conditions, and XANES spectrosocopy at the S and Fe K-edges to study its 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 (FeS 2 ) model compounds via first-principles calculations. Our results show that after FeS has precipitated from aqueous Fe(II) and H 2 S/HS -in a saline medium at pH 7.1, aqueous aging at the same pH during 47 days results in the formation of nanocrystalline mackinawite (MCD ab =11.5±0.1 nm; MCD c =7.1±0.1 nm). When Na 2 S is added in 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 (MCD ab =7.9±0.1 nm; MCD c =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 due to either lattice expansion at the particle boundaries or to 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 (MCD ab =13.0±0.2 nm; MCD c =8.1±0.2 nm) and a fraction transforms into greigite (Fe 3 S 4 ). This reaction is accompanied by pH increase to 6.4, likely because of H + comsumption, which suggests a reaction similar to a corrosion reaction in which Fe(II) would serve as an electron donor and H + would serve as an electron acceptor. The calculated electronic structure of mackinawite shows partly filled Fe-3d states, which supports that acidic aging conditions are favorable for Fe(II) to act as an electron donnor. The formation of greigite from nanocrystalline mackinawite via this pathway ressembling anoxic corrosion at ambient temperature would likely result in H 2 production as classically observed for 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. This study yields more clues on these reports and shows that greigite can form at low temperature by H + oxidation of crystalline mackinawite due to the particular electronic properties of the latter and may further persist under reducing conditions. This conclusion raises the question of the existence of such a reaction producing Fe 3 S 4 and H 2 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 of 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 H 2 S pathway to pyrite.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.