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Title: Molybdenum Burial Mechanism in Sulfidic Sediments: Iron-Sulfide Pathway

Journal Article · · ACS Earth and Space Chemistry
ORCiD logo [1];  [2];  [3];  [1];  [1];  [1]
  1. Minnesota State Univ., Mankato, MN (United States)
  2. Univ. of Maryland, College Park, MD (United States)
  3. Central Michigan Univ., Mount Pleasant, MI (United States)
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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 Ksp = 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.

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Organization:
USDOE Office of Science (SC)
Grant/Contract Number:
ACS-PRF 54583-DNI2
OSTI ID:
1459974
Journal Information:
ACS Earth and Space Chemistry, Vol. 2, Issue 6; ISSN 2472-3452
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
ENGLISH
Citation Metrics:
Cited by: 47 works
Citation information provided by
Web of Science

Figures / Tables (14)

Table 1(p. 28)table Table 1
Table 2(p. 29)table Table 2
Table 3(p. 29)table Table 3
Table 4(p. 30)table Table 4
Table 5(p. 30)table Table 5
Figure 1(p. 31)figure Figure 1
Figure 2(p. 31)figure Figure 2
Figure 3(p. 32)figure Figure 3
Figure 4(p. 32)figure Figure 4
Figure 5(p. 33)figure Figure 5
Figure 6(p. 34)figure Figure 6
Figure 7(p. 35)figure Figure 7
Figure 8(p. 36)figure Figure 8
Figure 9(p. 37)figure Figure 9

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Iron
Geological materials
Sulfides
Colloidal particles
Colloids
Molybdenum
euxinic basins
iron sulfide
jordisite
colloids