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

Abstract

Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and δ-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 δ-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.