Abstract

The short- and long-range structural order of various hydrous oxides were investigated using extended X-ray absorption fine structure (EXAFS), X-ray and electron diffraction (XRD and SAED) and in situ chemical analysis (EDS). Materials examined include synthetic sodium birnessite, natural magnesium birnessite, natural nickel copper birnessite, synthetic vernadite (δ-MnO2), a series of natural iron vernadite samples, natural manganese goethite, two natural cobalt nickel asbolane samples, and natural cobalt asbolane. The structure of birnessite is similar to that of chalcophanite, but there are no corner-sharing Mn4+-Mn4+ octahedra. Sodium and magnesium birnessite differ primarily in the stacking mode of their Mn octahedral sheets and their interlayer structure. Similarities and differences between these two minerals have been considered in terms of anion close-packed models. In contrast to birnessite, synthetic vernadite (δ-MnO2) has edge- and corner-sharing Mn4+ octahedra and a three-dimensional anionic framework; δ-MnO2 does not appear to be a c-disordered birnessite. In iron vernadite and manganese goethite, Fe3+ and Mn4+ ions are segregated in coherent scattering domains. In both minerals, Mn atoms form phyllomanganate-like domains; in iron vernadite, Fe domains have a feroxyhite-like local structure. Asbolane has been found to have a mixed-layer structure. Cobalt nickel asbolane has layers of MnO2, N(OH)2, and possibly CoOOH or Co(OH)3 alternating regularly along the c axis. In cobalt asbolane, Mn2+ tetrahedral layers are presumably regularly interstratified with layers of Mno2 and (CoOOH). This study provides new examples of hybrid structures among low-temperature materials and confirms their heterogeneous nature on a very fine scale.

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