Crystal chemistry of todorokite structure-types. Three types of atomic substitution occur in the tunnel structures: first, substitution of Mn (super 2+) by other divalent cations (e.g., Mg 2 , Ni (super 2+) , Cu (super 2+) , Zn (super 2+) ) in the "walls" formed by chains of edge-shared [MnO 6 ] octahedra, characteristically three octahedra wide; second, replacement of Mn (super 4+) by similar sized cations (e.g., low-spin Co (super 3+) in the "ceilings" or "floors" typically three, but as many as seven or more, [MnO 6 ] octahedra wide; and third, in the tunnel interiors adjacent to Mn (super 4+) vacancies in the "ceilings." Recommendation that the name todorokite be universally adopted for the predominant manganese oxide mineral accommodating divalent cations of nickel and copper in marine manganiferous concretions.--Modified journal abstract.

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