With a focus on a large set of natural birnessites collected from terrestrial, freshwater systems, we applied and compared the capabilities of Xâ'ray diffraction (XRD), extended Xâ'ray absorption fine structure (EXAFS), Fourier-transform infrared spectroscopy (FTIR), and Xâ'ray photoelectron spectroscopy (XPS) to characterize crystal structure and chemistry. Using XRD, we successfully identified 3 of the 11 natural birnessite samples as hexagonal rancieíte-like phases, but the remaining samples yielded less interpretable "3-line"diffraction patterns with broad, asymmetrical peaks at d-spacings of ~7.2, ~2.4, and ~1.4 Å. EXAFS analysis suggested that many of these samples had characteristics of both triclinic and hexagonal birnessite. However, application of EXAFS to the rancieíte-like phases yielded unreasonably high concentrations of triclinic birnessite as an intergrowth, calling into question the use of synthetic hexagonal H-birnessite as an appropriate standard in the linear combination fitting of EXAFS data for natural birnessites. FTIR spectroscopy of the "3-line"birnessite samples successfully distinguished triclinic and hexagonal constituents, and analyses of peak positions suggested that natural birnessites occur as a full spectrum of triclinic and hexagonal intergrowths. XPS analysis of these samples revealed that higher Mn3+ concentrations relative to Mn2+ and Mn4+ are correlated to increased proportions of triclinic birnessite.
Bibliographical noteFunding Information:
Funding for this work was provided by NSF Grant EAR-1147728 and EAR-1552211 and the Committee on Institutional Cooperation (CIC) and Smithsonian Institution Fellowship. This research also utilized samples from the Smithsonian Mineral Research Collection at the Museum of Natural History. The FTIR laboratory at the Smithsonian Institution was established with generous support from Stephen Turner. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. ESI is supported by the PNNL managed Geosciences Research Program of the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The research was performed in part using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO-1830.
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- Manganese oxide