The formation of pyrite crystals depends mainly on the iron content of the sediment. The process of pyrite formation in sediments results from the action of bacteria, which reduce sulphate ions (dissolved in the pore water) to sulphide. If there is iron present, iron sulphide crystals begin to grow.

What is the mechanism of pyrite formation?

Below 300°C, pyrite and marcasite form via an FeS precursor. The precursor phase is crystalline, nearly stoichiometric FeS with a solubility product of 102.9±0.2 ( K = a Fe 2 +a H 2S (a H + ) 2 ). It reacts progressively to mackinawite, hexagonal pyrrhotite, and/orgreigite before forming pyrite or marcasite.

What is the reaction of pyrite formation?

The reaction to produce pyrite is fastest when oxygen is excluded and elemental sulphur is produced from the oxidation of H2S by ferric iron. A reaction between FeS and elemental sulphur will yield pyrite at a much slower rate, although the same basic reaction is involved.Pyrite formation from FeS and H2S is mediated through microbial redox activity

Joana Thiel, James M. Byrne, Andreas Kappler, +1,and Michael Pester

https://orcid.org/0000-0001-6296-4145 Michael.Pester@dsmz.deAuthors

Info & Affiliations

Edited by Donald E. Canfield, University of Southern Denmark, Odense M., Denmark, and approved February 28, 2019 (received for review August 22, 2018)March 18, 2019116 (14) 6897-6902

https://doi.org/10.1073/pnas.1814412116

Significance

Pyrite is the most abundant iron−sulfur mineral in sediments. Over geological times, its burial controlled oxygen levels in the atmosphere and sulfate concentrations in seawater. However, the mechanism of pyrite formation in sediments is still being debated. We show that lithotrophic microorganisms can mediate the transformation of FeS and H2S to FeS2 at ambient temperature if metabolically coupled to methane-producing archaea. Our results provide insights into a metabolic relationship that could sustain part of the deep biosphere and lend support to the iron−sulfur-world theory that postulated FeS transformation to FeS2 as a key energy-delivering reaction for life to emerge.

Abstract

The exergonic reaction of FeS with H2S to form FeS2 (pyrite) and H2 was postulated to have operated as an early form of energy metabolism on primordial Earth. Since the Archean, sedimentary pyrite formation has played a major role in the global iron and sulfur cycles, with direct impact on the redox chemistry of the atmosphere. However, the mechanism of sedimentary pyrite formation is still being debated. We present microbial enrichment cultures which grew with FeS, H2S, and CO2 as their sole substrates to produce FeS2 and CH4. Cultures grew over periods of 3 to 8 mo to cell densities of up to 2 to 9 × 106 cells per mL−1. Transformation of FeS with H2S to FeS2 was followed by 57Fe Mössbauer spectroscopy and showed a clear biological temperature profile with maximum activity at 28 °C and decreasing activities toward 4 °C and 60 °C. CH4 was formed concomitantly with FeS2 and exhibited the same temperature dependence. Addition of either penicillin or 2-bromoethanesulfonate inhibited both FeS2 and CH4 production, indicating a coupling of overall pyrite formation to methanogenesis. This hypothesis was supported by a 16S rRNA gene-based phylogenetic analysis, which identified at least one archaeal and five bacterial species. The archaeon was closely related to the hydrogenotrophic methanogen Methanospirillum stamsii, while the bacteria were most closely related to sulfate-reducing Deltaproteobacteria, as well as uncultured Firmicutes and Actinobacteria. Our results show that pyrite formation can be mediated at ambient temperature through a microbially catalyzed redox process, which may serve as a model for a postulated primordial iron−sulfur world.