Abstract

The oxidation state and coordination number of Mn, Fe, and Co in hydrous manganese oxides have been investigated by high resolution XANES spectroscopy. At the MnK edge, spectral sensitivity is high enough to differentiate between Mn ions of different oxidation states and site octupations. Application of this technique to various poorly crystallized hydrous oxides, including samples of birnessite, vernadite, asbolane, and manganese goethite, indicates that Mn atoms are generally tetravalent. If low-valence Mn ions are present, they certainly make up less than 20 at%.In natural Fe-containing vernadite, Fe atoms are sixfold cooriinated, on the basis of a detection limit for [4]Fe3+ ions of about 10 at%. Co has been found to be trivalent in both cobalt and cobalt nickel asbolane. MnK preedge spectra have also been demonstrated to be sensitive to the local structure of tetravalent manganate. Spectral intensity was found to depend on the ratio of edge- to corner-sharing MnO6 octahedra: the larger the tunnel size of the manganate, the lower the intensity of the preedge. Application of this concept to the structure of δ-MnO2 suggests that vernadite contains as many corner-sharing octahedra as todorokite. On the basis of XANES data, δ-MnO2 does not possess a layered structure and should not be considered as long-range disordered birnessite.

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