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

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 Mn reference compounds, and well separated from that of Mn and Mn compounds. Therefore, Mn in goethite is dominantly trivalent regardless of whether the samples were derived from Mn or Mn solutions. This may indicate a catalytic oxidation of Mn during goethite crystal growth similar to that found at the surface of Mn oxides. SCHEINOST ET AL.: Mn IN GOETHITE-GROUTITE SOLID-SOLUTIONS 140 nal oxidation states of these minerals have been confirmed by XANES (Manceau et al. 1992). XANES of moist topsoils detected only Mn and Mn (Schulze et al. 1995). There are several possible reasons for the rarity of α-MnxFe1–x OOH in natural systems. The aqueous Mn species readily disproportionates to Mn and Mn (∆G298 = –109 kJ/mol) (Stumm and Morgan 1970; Burns 1993), and the dominant mobile form, Mn, readily oxidizes at the surface of minerals (Junta and Hochella 1994) or bacteria directly to Mn without an intermediate Mn step (Mandernack et al. 1995). These reactions should limit the source of Mn and Mn, which are necessary precursors for the formation of MnxFe1–xOOH. However, aqueous Mn sorbs onto previously precipitated Mn minerals and oxidizes in a catalytic surface reaction to Mn (Murray et al. 1985). At pH 6, groutite precipitates formed from aqueous Mn at the surface of birnessite (Tu et al. 1994), confirming pH to be a key variable for the preferential formation of groutite over other Mn minerals (Cornell and Giovanoli 1987a; Ebinger and Schulze 1990). The oxidation state of Mn in a particular mineral depends on the oxidation potential of the solution and the structure of the mineral. The latter aspect determines the ligand field affecting Mn, and, in turn, its relative crystal field stabilization energy (CFSE). In octahedral symmetry the CFSE increases from Mn (CFSE = 0) to Mn (CFSE = 3/5·10Dq) to Mn (6/ 5·10Dq) (Burns 1993). This may explain the prevalence of Mn in those minerals, which are able to host Mn in several oxidation states (Manceau et al. 1992; Nesbitt and Banerjee 1998). Due to the (t2g) (eg) configuration, however, Mn minerals are susceptible to the Jahn-Teller distortion, which decreases the symmetry of the octahedra from Oh to D4h. The resulting increase of CFSE may also stabilize Mn in such minerals. Once groutite has formed, it should persist in spite of its metastability relative to the more common manganite, γ-MnOOH, because of the small enthalpy gradient between groutite and manganite (Fritsch et al. 1997). The degeneracy of the Mn electron ground state may be removed by either octahedral compression or elongation. However, most Mn minerals, including groutite, show an elongation (Hoffmann et al. 1997). Ebinger and Schulze (1989) concluded that the increase in a and the decrease in b and c of Mn-substituted goethite were due to this elongation of the Mn octahedra, hence indirectly proving the stabilization of Mn in goethite. Other authors pointed out, however, that the elongation of the Mn(O,OH)6 octahedra would cause structural incompatibility with the shorter Fe(O,OH)6 octahedra, thus limiting the extent to which the MnxFe1–xOOH solid solution would be stable (Manceau et al. 1992). The objective of our work is (1) to confirm the trivalent oxidation state of Mn in goethite and (2) to investigate whether the composition of the two different structures is accompanied by a random distribution of Mn among the Fe sites or by some degree of clustering. We used Rietveld refinements of X-ray diffraction (XRD) patterns to model the structure of MnxFe1– xOOH. Because Mn and Fe scatter very similarly and occupy the same 4c site, only an average (long range) structure can be obtained with this method. Therefore, we used X-ray absorption fine structure (XAFS) spectroscopy at the FeKα and MnKα edge to probe the short-range structure (<8 Å) around Mn and Fe separately and X-ray absorption near edge structure (XANES) spectroscopy to determine the oxidation state of Mn. MATERIALS AND METHODS