Formation mechanisms of pyrite in Earth's diverse geological systems

Pyrite is ubiquitous across various geological systems in Earth's crust, spanning from sedimentary systems to hydrothermal, metamorphic, and magmatic systems. The widespread occurrences of pyrite make it a powerful tracer of geological processes, but effective applications require a thorough understanding of its formation mechanism. In sedimentary systems, pyrite is formed via reactions between Fe²⁺ and sulfides (H₂S_aq/HS⁻), typically through FeS_am/mc/aq intermediates that transform to pyrite via polysulfide or H₂S pathway. The polysulfide pathway is well-characterized, with the formation rates positively correlated with the concentrations of FeS_am, elemental sulfur, total sulfide, and H⁺, while the kinetics of the H₂S pathway remain controversial. Pyritization of metazoan and plant tissues during early diagenesis is another key mechanism. In hydrothermal systems, pyrite is formed through both precipitation from solutions and replacement/pyritization of Fe/S-bearing minerals such as sulfides, oxides, oxyhydroxides, and carbonates. Replacement is via a coupled dissolution-reprecipitation process, with the rate-limiting step (dissolution or precipitation) and the overall rate of pyrite formation controlled by precursor mineralogy, temperature, solution pH, and total sulfide concentration. In magmatic systems, pyrite is formed via exsolution from Monosulfide Solid Solution (MSS), subsolidus reactions involving Intermediate Solid Solution (ISS), precipitation from magmatic-hydrothermal fluids, replacements of pyrrhotite and/or pentlandite, and vapor deposition at volcanic fumaroles. Despite significant advances, some aspects of pyrite formation remain unclear. These include the evolution of Fe-S-bearing aqueous complex species before and after the precipitation of FeS_am/mc, the role and mechanisms of organic matter in pyrite nucleation, the kinetics and mechanisms of pyrite crystal growth, the rate and the hydrogen yield of the H₂S pathway, the kinetics and mechanisms governing pyrite formation from replacement of some precursor minerals. Future work need to address these gaps using synchrotron-based in-situ experimental setups and analytical techniques capable of providing time-resolved evolution of the mineralogy, phase proportions, and aqueous speciation during phase transformations.