Structural environment and oxidation state of Mn in goethite-groutite solid-solutions
-
Andreas C. Scheinost
,
Helge Stanjek
Abstract
Both X-ray absorption and diffraction techniques were used to study the structural environment and oxidation state of Mn in goethite-groutite solid solutions, α-MnxFe1-xOOH, with xMn ≤ 0.47. Rietveld refinement of X-ray diffraction (XRD) data was employed to investigate the statistical long-range structure. The results suggest that increasing xMn leads to a gradual elongation of Fe and Mn occupied octahedra which, in turn, causes a gradual increase of the lattice parameter a and a gradual decrease of b and c in line with Vegard’s law. X-ray absorption fine structure (XAFS) spectra at the MnKα and FeKα edges revealed, however, that the local structure around Fe remains goethite-like for xMn ≤ 0.47, while the local structure around Mn is goethite-like for xMn ≤ 0.13, but groutite-like for higher xMn. The spectral observations were confirmed by XAFS-derived metal distances showing smaller changes around Fe and larger changes around Mn as compared with those determined by XRD. Therefore, the XAFS results indicate formation of groutite-like clusters in the goethite host structure for xMn > 0.13, which remain undetected by XRD. The first prominent resonance peak in the X-ray absorption near-edge spectra (XANES) of the Mn goethites was 17.2 to 17.8 eV above the Fermi level of Mn (6539 eV), in line with that of Mn3+ reference compounds, and well separated from that of Mn2+ and Mn4+ compounds. Therefore, Mn in goethite is dominantly trivalent regardless of whether the samples were derived from Mn2+ or Mn3+ solutions. This may indicate a catalytic oxidation of Mn2+ during goethite crystal growth similar to that found at the surface of Mn oxides.
© 2015 by Walter de Gruyter Berlin/Boston
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