Abstract Reaction paths for nucleation and growth of pyrite and marcasite from solution have been investigated experimentally. Conditions were chosen to avoid the precipitation of metastable Fe-S phases which can act as precursors for FeS 2 formation. The experiments indicate that FeS 2 nucleation is extremely slow below 100°C. Instead of FeS 2 nuclei, the reaction of ferrous ions and polysulfide ions produces initially amorphous FeS. Although the nucleation of FeS 2 is inhibited below 100°C, pyrite and marcasite can grow from solutions devoid of polysulfides and undersaturated with respect to possible Fe-S precursor phases. The inability of pyrite to rapidly nucleate explains high supersaturation with respect to pyrite and marcasite in anoxic environments. Although pyrite is the stable Fe-S phase in these environments, it will not control the Fe 2+ and H 2S (or HS −) concentrations until its growth rate exceeds the dissolution rate of far more soluble, metastable FeS precursor phases.