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Molybdenum profiles in dated sediment cores provide useful historical information about anoxia in anthropogenically impacted natural waters but would be of greater service if Mo fixation mechanisms were better understood. Here, we explore Mo scavenging by precipitated FeS in a model system consisting of an FeIII-bearing kaolinite (KGa-1B) dispersed in NaHS solutions. Test solutions contain 18 microM thiomolybdates (mainly MoOS3(2-)). Optically measuring dissolved polysulfides monitors the rate of FeS production from FeIII minerals. Even though the exposed clay surface area is large (450 m2/L), the clay itself sorbs little Mo at pH 8.6. As FeS forms, Mo is taken up in initial Mo/Fe mole ratios of 0.04-0.06, irrespective of HS- concentration (4-40 mM range). After about a day, Mo expulsion from the solids begins, accompanied by net polysulfide consumption. These changes reflect recrystallization of amorphous FeS to more ordered products such as greigite. FeS captures some MoO4(2-) but captures thiomolybdates more effectively. Kaolinite accelerates conversion of MoOS3(2-) to MoS4(2-), as predicted previously, and thiomolybdates facilitate reduction of FeIII minerals in the clay compared to Mo-free solutions. FeS is a potentially effective, transient scavenging agent for Mo in sulfidic environments, although FeS2 and organic matter appear to be the ultimate sedimentary hosts.

Relative to continental crust, sediments underlying sulfidic marine waters are molybdenum-rich, a property preserved in the rock record and useful for characterizing paleoenvironments. The enrichment mechanism is not agreed upon but is attributed at least partly to deposition of Fe–Mo–S compounds, which are as yet uncharacterized. Here, we determine the composition and stability of colloidal Fe–Mo–S precipitates formed at mildly basic pH and H2S­(aq) > 10–5 M. The first product consists simply of FeMoS4, with K sp = 10–14.95. Within hours, FeMoS4 irreversibly transforms by internal self-reduction to a Mo­(IV) product of similar composition. The reduced product is insoluble in 1 M HCl but soluble in concentrated HNO3, implying that it would be recovered with pyrite in a common assay of sediments. X-ray absorption fine structure data show that Mo­(IV) in the colloids is coordinated by a split first shell of about five sulfur atoms at average distances of 2.31 and 2.46 Å and in its second shell by an iron atom at about 2.80 Å. These properties resemble those determined for Mo in modern anoxic lake sediments and in Phanerozoic black shales. The atomic environment around Mo suggests that the colloidal products may be inorganic polymers containing cuboid, Fe2Mo2S4 4+ cores. Such materials are so far unreported by mineralogists, although a rare mineral, jordisite, may be a related, but more Mo-rich material. The low solubility of FeMoS4 makes it a feasible precipitate in euxinic waters like those in the modern Black Sea. We propose that colloids similar to those studied here could account for Mo-enrichment in euxinic basin sediments and black shales.