Fourier-transform infrared spectroscopy (FTIR) analysis of triclinic and hexagonal birnessites.
The characterization of birnessite structures is particularly challenging for poorly crystalline materials of biogenic origin, and a determination of the relative concentrations of triclinic and hexagonal birnessite in a mixed assemblage has typically required synchrotron-based spectroscopy and diffraction approaches. In this study, Fourier-transform infrared spectroscopy (FTIR) is demonstrated to be capable of differentiating synthetic triclinic Na-birnessite and synthetic hexagonal H-birnessite. Furthermore, IR spectral deconvolution of peaks resulting from MnO lattice vibrations between 400 and 750cm-1yield results comparable to those obtained by linear combination fitting of synchrotron X-ray absorption fine structure (EXAFS) data when applied to known mixtures of triclinic and hexagonal birnessites. Density functional theory (DFT) calculations suggest that an infrared absorbance peak at ~1628cm-1may be related to OH vibrations near vacancy sites. The integrated intensity of this peak may show sensitivity to vacancy concentrations in the Mn octahedral sheet for different birnessites.
Department of Geosciences, Penn State University, University Park, PA 16802, USA. Electronic address: ftling@princeton.edu.
Florence T
FT
Ling
Department of Mineral Sciences, NHB 119, Smithsonian Institute, Washington, DC 20013-7012, USA.
Jeffrey E
JE
Post
Department of Geosciences, Penn State University, University Park, PA 16802, USA.
Peter J
PJ
Heaney
Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA.
James D
JD
Kubicki
Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA.
Cara M
CM
Santelli
Birnessite; Density functional theory; EXAFS; FTIR; Manganese oxide