Pyrite polysulfides

The pyrites and marcasite structures can be thought of as containing 82 units though the variability of the interatomic distance and other properties suggest substantial deviation from a purely ionic description. Numerous higher polysulfides S have been characterized, particularly for the more electropositive elements Na, K, Ba, etc. They are yellow at room temperature, turn dark red on being heated, and may be thought of as salts of the polysulfanes... 

Iron frequently has been postulated to be an important electron acceptor for oxidation of sulfide (58, 84,119, 142, 152). Experimental and theoretical studies have demonstrated that Fe(III) will oxidize pyrite (153-157). Reductive dissolution of iron oxides by sulfide also is well documented. Progressive depletion of iron oxides often is coincident with increases in iron sulfides in marine sediments (94, 158, 159). Low concentrations of sulfide even in zones of rapid sulfide formation were attributed to reactions with iron oxides (94). Pyzik and Sommer (160) and Rickard (161) studied the kinetics of goethite reduction by sulfide thiosulfate and elemental S were the oxidized S species identified. Recent investigations of reductive dissolution of hematite and lepidocrocite found polysulfides, thiosulfate, sulfite, and sulfate as end products (162, 163). 

As Berner (36) pointed out in his classic work, the formation of pyrite is coupled to a process in which free sulfide is oxidized to form polysulfides, which again react with FeS to form pyrite. In this study elemental sulfur was the oxidant. However, elemental sulfur was always less than 1% of the total sulfur content in the study of White et al. (35). The findings of the experimental studies discussed on the interaction between H2S and ferric oxides (20-23), in combination with the field observations, suggest a mechanism in which ferric iron oxides are the oxidants to form polysulfides and subsequently pyrite. 

The importance of polysulfides in the pyrite formation process was outlined by several studies (37, 38). Schoonen and Barnes (37) showed that no precipitation from homogeneous solution can be observed within a reasonable time scale, even in solutions highly supersaturated with respect to pyrite, unless pyrite seeds are already existing. Therefore future studies should address the role of ferric oxide surfaces in promoting the nucleation of pyrite. 

As shown in the following reaction, H3ASO30 may sorb onto pyrite surfaces under anaerobic conditions, which leads to the formation of FeAsS, Fe(III) hydroxides, and polysulfides, such as FeS4 (Bostick and Fendorf, 2003) ... 

The notion that naturally occurring organic polysulfides in coal decompose to form elemental sulfur has also been tested in another way. Buchanan and his associates have shown that the 32S/34S ratios of the elemental sulfur and the pyrite in another Illinois Basin Coal Sample Program coal are similar and different from the 32S/34S ratio for the organic material in the same coal (Buchanan, D., private communication, 1989). This result infers that pyrite is the source of elemental sulfur. Thus, we conclude that oxidative chemical and bacteriological processes convert pyrite to elemental sulfur when pristine coals are exposed to the atmosphere. 

Pyritization can then occur under anoxic conditions relatively quickly by reaction of H2S and S(0) [as Sg or polysulfides (S2-)] with FeS, both aqueous and solid forms (Rickard, 1975,1997 Luther, 1991 Rickard and Luther, 1997 Rozan et al., 2002), in the following equations ... 

Luther III, G.W. (1991) Pyrite synthesis via polysulfide compounds. Geochim. Cosmochim. Acta 55, 2839-2849. 

The evolution in time of the weathering conditions and the composition of the percolating water in the downstream part of the weathering profile is given in Figure 14. As shown by Rickard (21) the FeS-Sg paragenesis is not stable in the presence of polysulfide ions and the reformation of pyrite will occur quite rapidly in the conditions encountered in the percolating water. The process is probably (21) ... 

Elemental sulfur, in the form of highly reactive colloid (28), can be the nucleus of additional pyrite reformation following polysulfide formation on the surface of the colloidal particles and... 

The rates of oxidation of both pyrite and pyrrhotite at 25 °C and standard atmospheric oxygen indicate that pyrrhotite can react 20-100 times faster than pyrite. During oxidation of a particle of pyrrhotite, iron diffuses to the exposed surface, thereby creating a sulfur-enriched inner zone that contains disulfide and polysulfide-hke species (Mycroft et al., 1995). 

Marcasite forms below about pH 5, when the neutral, undissociated polysulfide acids dominate, whereas pyrite is favored at pH values above 6, when polysulfide anions are the major species (Fig. 12.15). Schoonen and Barnes (1991a) offer detailed explanations for such complex behavior. 

Because pyrite is a polysulfide species itself, its oxidation suggests that other polysulfides (S2 ) should undergo oxidation to form thiosulfate (or perhaps sulfite) and eventually sulfate. The length of the polysulfide chain and the amount of available oxidant will dictate whether S8 will form. The reactions of polysulfides to form thiosulfate, sulfite and S8 can be represented by... 

Direct precipitation of pyrite without intermediate iron sulfide precursors was reported for salt marsh sediments, where pore waters were undersaturated with respect to amorphous FeS but oversaturated with respect to pyrite (Howarth 1979 Giblin and Howarth 1984). In these sediments, the oxidizing activity of the roots favored the formation of elemental sulfur and polysulfides which were thought to react directly with Fe +. The direct reaction pathway may proceed within hours, resulting in the formation of single small, euhedral pyrite crystals (Rickard 1975 Luther et al. 1982). Framboidal pyrite, apart from that formed by the mechanism presented by Rickard (1997), is formed slowly (over years) via intermediate iron sulfides (Sweeney and Kaplan 1973 Raiswell... 

Pyrite oxidation by nitrate could not be demonstrated through short-term sediment experiments but FeS is readily oxidized, both by nitrate and MnOj (Aller and Rude 1988 Schippers 2004). The immediate product by FeS oxidation is not thiosulfate but polysulfide which subsequently degrades into elemental sulfur ... 

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