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.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology