Manganese geochemistry in carbonate nodules. A review

Mn-bearing carbonate nodules may content carbonate of two structural types: calcite (Mn-calcite, rhodochrosite, siderite, oligonite) and dolomite ones (Mn-dolomite and Mn-ankerite). An idea is diccussed, that each structural Mn-carbonate type is determined by conditions of diagenesis. The latter, in turn, would be determined by sedimentation conditions, such as organic matter abundance and sedimentation rate. As a rule, dolomite-type Mn-carbonates have general structural formulae Са 0.50(Mg, Fe, Mn) 0.50[CO 3], i. e. Mn occupies only Mg(Fe)-positions but not the Ca-positions. A deviation from such empirical rule is rare and may be not always reliable. Mn-module values (MM = Mn/Fe) were near average for the Earth crust [Li, 2000] at Fe-carbonates formation but rose sharply at Ca-carbonate formation.