The uptake of cobalt into ferromanganese nodules, soils, and synthetic manganese (IV) oxides

Physics

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Scientific paper

Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and -MnO 2 . Previously, it was suggested that Co 3+ ions in these minerals replace either Mn 3+ or substitute for Fe 3+ in incipient goethite epitaxially intergrown with -MnO 2 . Neither of these interpretations is now considered to be satisfactory on account of the large discrepancy of ionic radius between octahedrally coordinated low-spin Co 3+ and high-spin Mn 3+ or Fe 3+ in oxide structures. The close agreement between the ionic radii of Co 3+ and Mn 4+ suggests that some cobalt substitutes for Mn 4+ ions in edge-shared [MnO 6 ] octahedra in many manganese(IV) oxide mineral structures. It is proposed that hydrated cations, including Co 2+ ions, are initially adsorbed on to the surfaces of certain Mn(IV) oxides in the vicinity of essential vacancies found in the chains or sheets of edge-shared [MnO 6 ] octahedra. Subsequently, fixation of cobalt takes place as a result of oxidation of adsorbed Co 2+ ions by Mn 4+ and replacement of the displaced manganese by low-spin Co 3+ ions in the [MnO 6 ] octahedra or vacancies.

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