Size distribution in sediments, synthesis, and formation mechanism of framboidal pyrite

Pyrite framboids contained in sediments of modern euxinic basins (Black Sea; Framvaren Fjord, Norway; Pettaquamscutt Estuary, Rhode Island) are on average smaller and less variable in size than those contained in sediments underlying dysoxic (Peru Margin) and oxic water columns (Framvaren Fjord, Norway; Wallops Island, Virginia; Great Salt Marsh, Delaware). Application of the Crystal Size Distribution Theory suggests that framboid size is principally a function of framboid growth time and rate.; Framboid formation was found to be restricted to the oxic-anoxic interface in the water column of the Pettaquamscutt River Estuary (Rhode Island). The size distribution of framboids settling through this water column is similar to that of framboids contained within the top several centimeters of the sediments; thus, in euxinic environments, framboid size is apparently fixed above the sediment-water interface. It is shown that {dollar}deltasp{lcub}34{rcub}{dollar}S of pyritic sulfur in sediments of euxinic basins in part reflects a primary (framboidal pyrite) sulfur component incorporated near the oxic-anoxic interface and a secondary sulfur component incorporated during later stages of diagenesis and burial.; Experiments indicate the surface oxidation state of iron monosulfides is an important factor controlling their reactivity and rate of conversion to pyrite. Framboids were only produced in experiments where air was introduced to a solution containing {dollar}rmSigma Hsb2S{dollar} and suspended iron monosulfides; and in these experiments, pyritization rates were faster than when iron monosulfides were aged in solutions containing dissolved polysulfides and colloidal elemental sulfur.; Measurements of the sulfur isotopic composition of reactants and products in experiments where iron monosulfides ('FeS') reacted to form pyrite (FeS{dollar}sb2{dollar}) indicate that pyrite formation via monosulfide precursors proceeds by reactions in which ferrous iron is lost from, rather than added to, monosulfide precursors.; A model is presented for pyrite framboid formation via the magnetic aggregation of uniformly sized greigite {dollar}rm(Fesb3Ssb4){dollar} particles followed by their conversion to pyrite. In this model, pyrite framboid formation is the result of four consecutive processes: (1) nucleation and growth of initial iron monosulfide microcrystals; (2) conversion of microcrystals to greigite; (3) aggregation of greigite microcrystals, i.e., the growth of framboids; and (4) conversion of greigite framboids to pyrite framboids.