Iron precipitation under controlled oxygen flow: Mineralogical implications for BIF precursors in the Archean ocean; X-ray diffraction data

Published: 8 June 2022| Version 1 | DOI: 10.17632/34rh43mwjw.1
, Steven Chemtob


Banded iron formation (BIF) deposition in the Archean ocean is thought to have been initiated by the oxidation of dissolved iron into ferric primary precursor phases that descended through the water column and were deposited on the seafloor. Effective interpretation of the trace element and isotopic composition of BIFs in the geologic record requires an understanding of the identity and longevity of those precursor phases. This study follows the evolution of iron mineralogy, speciation, and redox state during the precipitation of iron under variable oxygen fluxes and in fluids containing different iron complexing anions (chloride, sulfate, and phosphate). Additionally, suspensions collected after incomplete oxidation were anaerobically aged to simulate changes to precursor mineralogy during descent below the seawater redoxcline. This dataset accompanies the publication of the same name and comprises X-ray diffraction data for all experimental products described therein. The results, interpreted together with results from sequential dissolution, colorimetric determination of redox state, and Fe K-edge X-ray absorption fine structure (not included here), indicate that under all experimental conditions, intermediate precipitates collected before complete iron uptake contained mineralized ferrous iron. Purely ferric mineral assemblages were not observed until iron was removed from solution. In low-oxygen experiments, intermediate precipitates contained phases with stoichiometric Fe2+, including magnetite, vivianite, and green rust (GR). Chloride green rust (GR1) had more ferric iron than sulfate green rust (GR2) and more readily transitioned to magnetite via disproportionation. GR2 was stable over a broader range of iron uptake; magnetite appeared mainly as an additional oxidation product in sulfate solution. In high oxygen experiments, mineralized ferrous iron occurred in amorphous phases or as a non-stoichiometric component in ferric oxides; final ferric assemblages were less crystalline than in low-oxygen equivalents. Low concentrations of phosphate increased the total iron oxidation rate, but also increased the ferrous content of intermediate precipitates. The incorporation of phosphate stabilized GR2 and facilitated GR2 precipitation over a larger range of iron uptake. In higher concentrations of phosphate, vivianite and poorly crystalline ferric hydroxides formed in place of GR. These findings suggest that over a range of possible ocean chemistries, mixed-valence phases may have been short lived but relevant precursors to BIF. Additionally, anionic chemistry and oxidant concentration are shown to influence the crystallinity and chemical resistivity of final ferric assemblages, affecting their reactivity during later anaerobic burial.


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X-ray diffraction patterns were collected using a Bruker D8 Advance powder diffractometer equipped with a Cu X-ray tube operating at 40 kV and 40 mA. Diffraction profiles were fit and analyzed for peak width at half-maximum (PWHM), peak position, and intensity. Samples were analyzed as filtered powders except for anaerobic aging samples, which were prepared as dried suspensions and thus were more subject to preferred orientation. To limit exposure to oxygen, samples were analyzed using a zero-background Si sample holder with an airtight dome.


Temple University


Mineralogy, Precambrian Era, Aqueous Geochemistry, X-Ray Diffraction