The mineralogy and crystal chemistry of deep-sea manganese nodules, a polymetallic resource of the twenty-first century

The relatively high concentrations of cobalt, nickel, and copper in deep-sea manganese nodules, such as those occurring on the sea-bed beneath the north equatorial Pacific Ocean, indicate that these marine sediments are potential ore deposits. In order to explain the strong enrichments of Ni, Cu, and Co in the nodules, the crystal chemistries and structures of the host manganese oxide minerals must be understood. Over twenty manganese(IV) oxide minerals are known, but only three predominate in manganese nodules. They are todorokite, birnessite, and delta-MnO2. All MnIV oxides contain edge-shared MnO6 octahedra linked in diverse ways, leading to a hierarchy of structure-types somewhat resembling the classification of silicates. Todorokite is deduced to contain chains of edge-shared MnOe octahedra enclosing huge tunnels, thus resembling hollandite and psilomelane. Birnessite has a layered structure with essential vacancies in the sheets of edge-shared MnO6 octahedra, while δ-MnO2 is a disordered birnessite. The uptake of Co into manganese nodules involves replacement of low-spin Co3+ for Mn4+ ions in the structures, whereas Ni2+ and Cu2+ substitute for Mn2+ ions in octahedra located in the chains or between layers of edge-shared MnO6 octahedra.