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Scheinost, A.C., H. Stanjek, D. G. Schulze, U. Gasser, and D. L. Sparks. 2001. Structural environment and oxidation state of Mn in goethite-groutite solid-solutions. Amer. Min. 86:139-146.

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American Mineralogist, Volume 86, pages 139–146, 2001

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

ANDREAS C. SCHEINOST,¹ * HELGE STANJEK,² DARRELL G. SCHULZE,³ UBALD GASSER,⁴ AND DONALD L. SPARKS ¹

1 Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717-1303, U.S.A.
2 Lehrstuhl für Bodenkunde, Technische Universität, München, 85350 Freising, Germany
3 Agronomy Department, Purdue University, West Lafayette, Indiana 47907, U.S.A.
4 College of Engineering ISW, P.O. Box 335, CH 8820 Wadenswil, Switzerland

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 dis-tances 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 reso-nance 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 Mn ³⁺ reference compounds, and well separated from that of Mn ²⁺ and Mn ⁴⁺ compounds. Therefore, Mn in goethite is dominantly trivalent regardless of whether the samples were derived from Mn ²⁺ or Mn ³⁺ solutions. This may indicate a catalytic oxidation of Mn ²⁺ during goethite crystal growth similar to that found at the surface of Mn oxides.

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