Molybdenum Burial Mechanism in Sulfidic Sediments: Iron-Sulfide Pathway

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 <i>K</i>_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, Fe₂Mo₂S₄⁴⁺ 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