12.1.4: Sulfides

Sulfides are a group of minerals that include the sulfide ion \(\left(\text{S}^{2-}\right)\) as an essential structural unit (Deer et al., 1992). Sulfide minerals can impact water quality by serving as a source of acid where they are oxidatively weathering (Rice and Herman, 2012) and by helping regulate the mobility of numerous trace elements (Dellwig et al., 2002; Kirk et al., 2010; Raiswell and Plant, 1980). They are a large reservoir of sulfur in the global sulfur cycle and one of the links between the sulfur and iron cycles. Moreover, their burial in sediments is a major control on both the oxygen levels of the atmosphere (Berner, 1989) and the concentration of sulfate in seawater (Holland, 1978) over geological time scales.

Most of the sulfide precipitation on Earth is driven by microbial activity in low-temperature anoxic environments (Picard et al., 2016). Microorganisms can induce formation of sulfide minerals by catalyzing sulfate reduction and causing the minerals to become supersaturated. Numerous microbial groups can use sulfate as their electron acceptor in dissimilatory metabolism (Section 5.4.1). The sulfide produced by sulfate reduction can react with several divalent cations and precipitate. Examples of sulfide minerals that have been observed in microbial systems include mackinawite \(\left(\text{FeS}\right)\), greigite \(\left(\text{Fe}_{3} \text{S}_{4}\right)\), pyrite \(\left(\text{FeS}_{2}\right)\), cinnabar \((\text{HgS})\), rambergite \((\text{MnS})\), orpiment \(\left(\text{As}_{2} \text{S}_{3}\right)\), and sphalerite \((\text{ZnS})\) (Duverger et al., 2020; Labrenz et al., 2000; Lee et al., 2011; Mann et al., 1990; Newman et al., 1997; Thiel et al., 2019). Other groups of microorganisms can contribute by supplying those divalent cations. For example, microbial iron reduction can supply ferrous iron \((\text{Fe(II)})\), which can react with sulfide to form mackinawite. Ferrous iron can also be supplied by abiotic reaction of sulfide with iron (oxyhydr)oxide minerals (Sections 5.3.1 and 10.2.4).

Like each of the other three groups of minerals discussed above in this section, microorganisms also appear to provide nucleation surfaces for sulfide mineralization. Labrenz et al. (2000), for example, observed micron-scale sphalerite aggregates forming on biofilms dominated by sulfate-reducing bacteria of the family Desulfobacteriaceae. Picard et al. (2018), as a second example, examined sulfide minerals precipitated in the presence and absence of Desulfovibrio hydrothermalis AM13 cells. Their results demonstrated that the surfaces of cells and extracellular polymeric substances (EPS) provided templates for mackinawite nucleation. Moreover, the presence of live cells also influenced the morphology of the minerals and whether greigite could also form during the incubations.

Among the iron-sulfide minerals that form in near-surface environments, pyrite is the most thermodynamically stable and abundant (Rickard and Luther, 2007). Pyrite is unlikely to directly precipitate from a simple precipitation reaction because the sulfur in pyrite \(\left(S_{2}^{-2}\right)\) is not stable as a dissolved ion in low-temperature aqueous solutions (Kamyshny et al., 2004). Instead, pyrite is thought to form through multi-step reaction paths that begin with mackinawite.

Mackinawite is typically the first precipitate to form during sulfate reduction in iron-bearing systems (Berner, 1970) (Fig. \(12.7\text{A}\)). Mackinawite has low solubility in water with near-neutral pH (Benning et al., 2000). Under such conditions, extensive sulfate reduction is not necessary cause the mineral to supersaturate even if ferrous iron concentrations are relatively low (Kirk et al., 2010). Moreover, mackinawite can rapidly nucleate (Schoonen and Barnes, 1991). Mackinawite was previously thought to initially precipitate as an amorphous iron-sulfide phase (e.g., amorphous \(\text{FeS}\)) and then crystalize over time. However, a detailed analysis by Michel et al. (2005) indicates that the initial precipitates are nanoscale mackinawite crystallites, not amorphous solids.

Pyrite forms from mackinawite through the intermediate phase greigite (Hunger and Benning, 2007) (Fig. \(12.7\text{B}\) and \(12.7\text{C}\)). One pathway by which these reactions occur is referred to as the polysulfide pathway (Berner, 1970). Through this pathway, mackinawite forms greigite by reacting with zerovalent sulfur \(\left(\text{S}^{0}\right)\) dissolved as polysulfide molecules: \[3 \ \text{FeS} + \text{S}^{0} \longleftrightarrow \text{Fe}_{3} \text{S}_{4}\]

Polysulfides are chains of sulfur atoms that can form as a result of sulfide oxidation (Kamyshny et al., 2004; Steudel, 1996). The greigite produced from reaction \(\PageIndex{1}\) can then react with additional sulfur to form pyrite: \[\text{Fe}_{3} \text{S}_{4} + 2 \ \text{S}^{0} \longleftrightarrow 3 \ \text{FeS}_{2}\]

A second potential pathway of pyrite formation is known as the \(\text{H}_{2}\text{S}\) pathway (Rickard, 1997, 1975). Reactions describing this pathway follow, again with greigite as an intermediate between mackinawite and pyrite: \[\begin{align} & 3 \ \text{FeS} + \text{H}_{2} \text{S} \longleftrightarrow \text{Fe}_{3} \text{S}_{4} + \text{H}_{2} \\ & \text{Fe}_{3} \text{S}_{4} + 2 \ \text{H}_{2} \text{S} \longleftrightarrow \text{FeS}_{2} + 2 \ \text{H}_{2} \end{align}\]

Sulfate reducers and potentially iron reducers can feed both of these pathways by supplying mackinawite, as discussed above. Sulfur-oxidizing microorganisms have the potential to contribute to the polysulfide pathway by supplying zerovalent sulfur. Moreover, hydrogenotrophic microorganisms can help facilitate the \(\text{H}_{2} \text{S}\) pathway by consuming dihydrogen (Thiel et al., 2019). Beyond those contributions, the reactions that form pyrite may be largely abiotic. However, the boundaries between biotic and abiotic processes in these pathways are not clear (Picard et al., 2016).

Lastly, as noted above in the discussion of iron (oxyhydr)oxides, magnetotactic bacteria sometimes form greigite magnetosomes as an alternative to magnetite. At least one bacterium can produce both magnetite and greigite, but most strains of magnetotactic bacteria produce one or the other (Bazylinski and Frankel, 2004). Iron-sulfide minerals other than greigite have been observed in magnetotactic bacteria, including mackinawite (tetragonal \(\text{FeS}\)) and cubic \(\text{FeS}\). However, they are thought to be precursors to greigite (Bazylinski and Frankel, 2004).

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