Authigenic pyrite (FeS2) commonly forms in marine sediments during early diagenesis from microbially reduced iron and sulfide, both inorganic (from deepsea hydrothermal vents) and microbially produced. Authigenic pyrite is rare in fresh-water lacustrine sediments due to lack of sulfide.
Instead, in most lake sediments, reduced iron that has been released into the pore water during diagenesis of clay minerals is bound by phosphate, not sulfide, to form vivianite Fe3(PO4)2 8H2O.
However, pyrite is common in one fresh-water diagenetic setting: coal swamps. This is apparently because there is so much organic material that sulfide can reach high enough concentrations to compete with the phosphate for the available iron.
I don't know, but I suppose that the activation energy of pyrite formation is much lower than that of vivianite formation. Berner (1980) and Canfield et al (2005) will reveal more to me tonight.
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