Pyrite formation under conditions approximating those in anoxic sediments I. Pathway and morphology

Experiments were performed utilizing the silica gel technique to synthesize pyrite crystals in four different chemical systems, at room temperature (ca. 23 °C), for periods of up to 24 months. These systems were: FeS(1 − x) (mackinawite) + S(s) ° + H2S(aq), Fe3S4 (greigite) + H2S(aq). FeOOH(goethite) + H2S(aq) and Fe(aq)2 + + S4(aq)2 −. Results indicate that the rate of pyrite formation strongly depends on the nature of solid reactant and solution chemistry (pH and total sulfide concentration). The formation of pyrite is most rapid in the presence of greigite. The pyritization of metastable iron sulfide minerals follows a dissolution-precipitation pathway. Two mechanisms are proposed for the nucleation behavior of pyrite on metastable iron sulfide minerals. The first mechanism is heterogeneous nucleation. This is a result of epitaxial overgrowth of pyrite nuclei at defects on a precursor mineralʹs surface. A second possible mechanism is that pyrite clusters, smaller than the critical nucleus, are concentrated on the surface of precursor iron sulfide minerals under the influence of crystal molecular field. This forms a transition layer between the precursor sulfide mineral and the bulk solution where nucleation of pyrite is accelerated. SEM observations indicate that the morphology of pyrite crystals formed at room temperature is primarily controlled by the degree of supersaturation in the solution from which pyrite is precipitated. With the increasing supersaturation, pyrite morphology changes in the following order: cube → octahedron → spherulite. The influence of other parameters, such as relative abundance of Fe2 + and dissolved zero-valance sulfur, were not observed to be of major significance for morphology.