Pyrite synthesis via polysulfide compounds

The reactions of Fe(II) and Fe(III) solutions with polysulfide solutions prepared from freshly synthesized Na2Sx (x = 2, 4, 5) were studied at 25 and 100°C over the pH range 5.5 to 8. Direct and instantaneous pyrite formation was not observed in any reaction. High temperature reactions are nearly quantitative over periods of four hours with Fe(II) and polysulfide solutions. The rate of reaction at room temperature is comparable to that found by Rickard (1975), and the observations reported here are in agreement with his mechanism of pyrite formation. Based on the polarographic results of these reactions and previous work, a refinement of the mechanism which includes dissolved iron sulfide complexes is proposed. Every reaction of equimolar amounts of polysulfide and Fe(II) gave the kinetic product “FeS” (an example of the Ostwald step rule). Polarographic results demonstrate that the “FeS” initially formed consists of (1) a complex of the form Fe(SH)+ and (2) solid FeS. When excess polysulfide is present, a complex of form [Fe(SH) Sx]− is present. This complex should readily allow for (1) the reduction of polysulfide by sulfide which produces S22− in unprotonated form, and (2) the change of Fe(II) from high spin to low spin during the formation of pyrite. The reduction of polysulfide by sulfide was proposed by Rickard (1975), but at the pH of the solutions studied herein, S22− in solution should be protonated. The 22− ion is critical in the formation and structure of pyrite (Tossel et al., 1981). The proposed complex allows for a cyclic intermediate which cleaves the reacting polysulfide to form S22− unprotonated. As this process occurs, there is a change in the spin state of the Fe(II) from the high spin t2g4eg2 to the low spin t2g6 state which is an electron configuration exhibiting kinetic inertness. On change of the Fe(II) spin state, the complex irreversibly decomposes to form pyrite. The complex may be a cluster complex containing two or more formula units similar to ferredoxin complexes. Single crystal pyrite morphology is observed for low temperature syntheses. This is the morphology of pyrite commonly found in salt marsh sediments. The morphology found at the higher temperature is also single crystals, but noticeable weak clustering (framboid like) is observed. The reactions studied do not give any additional information on the low temperature synthesis of pyrite framboids because framboids were not observed. Other mechanisms must be operative for framboid formation in natural sediments.