Structural environment and oxidation state of Mn in goethite-groutite solid-solutions

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-Mn_xFe_1-xOOH, with x_Mn ≤ 0.47. Rietveld refinement of X-ray diffraction (XRD) data was employed to investigate the statistical long-range structure. The results suggest that increasing x_Mn 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 x_Mn ≤ 0.47, while the local structure around Mn is goethite-like for x_Mn ≤ 0.13, but groutite-like for higher x_Mn. 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 x_Mn > 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.