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

<p>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 Fe<span class="inline-formula">₃</span>S<span class="inline-formula">₄</span> greigite and FeS<span class="inline-formula">₂</span> 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 <span class="inline-formula"><i>K</i></span>-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 <span class="inline-formula"><i>K</i></span>-edge XANES spectra of mackinawite (FeS) and pyrite (FeS<span class="inline-formula">₂</span>) model compounds via first-principle calculations. Our results show that, after FeS has precipitated from aqueous Fe(II) and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">H</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">S</mi><mo>/</mo><msup><mi mathvariant="normal">HS</mi><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4c2e61bebf3aa9ce1218ba765298f1ba"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00001.svg" width="50pt" height="14pt" src="ejm-38-135-2026-ie00001.png"/></svg:svg></span></span> in a saline medium at pH 7.1, aqueous aging at the same pH over 47 d results in the formation of nanocrystalline mackinawite (MCD<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mrow><mi>a</mi><mi>b</mi></mrow></msub><mo>=</mo><mn mathvariant="normal">11.5</mn><mo>±</mo><mn mathvariant="normal">0.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="2118cfa36d0a1e9457e8bd91777ce44c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00002.svg" width="73pt" height="12pt" src="ejm-38-135-2026-ie00002.png"/></svg:svg></span></span> nm; MCD<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mi>c</mi></msub><mo>=</mo><mn mathvariant="normal">7.1</mn><mo>±</mo><mn mathvariant="normal">0.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="ac2221155b0d94eb38546465fb7c0007"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00003.svg" width="62pt" height="12pt" src="ejm-38-135-2026-ie00003.png"/></svg:svg></span></span> nm). When Na<span class="inline-formula">₂</span>S 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 (MCD<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mrow><mi>a</mi><mi>b</mi></mrow></msub><mo>=</mo><mn mathvariant="normal">7.9</mn><mo>±</mo><mn mathvariant="normal">0.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="fa999fd907eb689fcbdb8c12680b6237"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00004.svg" width="67pt" height="12pt" src="ejm-38-135-2026-ie00004.png"/></svg:svg></span></span> nm; MCD<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mi>c</mi></msub><mo>=</mo><mn mathvariant="normal">4.6</mn><mo>±</mo><mn mathvariant="normal">0.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="31f119bd6c4630f13e88f111abe3dff8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00005.svg" width="62pt" height="12pt" src="ejm-38-135-2026-ie00005.png"/></svg:svg></span></span> nm). In this sample, an additional weak and broad peak appears at <span class="inline-formula"><i>d</i>=10.5</span> Å that could be interpreted as being due to either lattice expansion at the particle boundaries or a double-cell super-structure. When H<span class="inline-formula">⁺</span> is added as HCl to reach pH 5.1 before the aging phase, the size of mackinawite particles increases (MCD<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mrow><mi>a</mi><mi>b</mi></mrow></msub><mo>=</mo><mn mathvariant="normal">13.0</mn><mo>±</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="73pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="8aadd77d956dbcf7a2e1acff85e27c33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00006.svg" width="73pt" height="12pt" src="ejm-38-135-2026-ie00006.png"/></svg:svg></span></span> nm; MCD<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mi>c</mi></msub><mo>=</mo><mn mathvariant="normal">8.1</mn><mo>±</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="9e74d8dc994f8d90fbcdf8b67208ec4c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-38-135-2026-ie00007.svg" width="62pt" height="12pt" src="ejm-38-135-2026-ie00007.png"/></svg:svg></span></span> nm), and a fraction transforms into greigite (Fe<span class="inline-formula">₃</span>S<span class="inline-formula">₄</span>). This reaction is accompanied by a pH increase to 6.4, likely because of H<span class="inline-formula">⁺</span> consumption, which suggests that Fe(II) in FeS would serve as an electron donor and that H<span class="inline-formula">⁺</span> would serve as an electron acceptor. The calculated electronic structure of mackinawite shows partly filled Fe-3<span class="inline-formula"><i>d</i></span> 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 H<span class="inline-formula">₂</span> 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 Fe<span class="inline-formula">₃</span>S<span class="inline-formula">₄</span> and H<span class="inline-formula">₂</span> 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<span class="inline-formula">⁺</span> in this mineral species could proceed without the need for another oxidizing agent. Although the possible formation of pyrite from greigite<span id="page136"/> 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<span class="inline-formula">₂</span>S pathway to pyrite.</p>