A theoretical study of the system CaCO3-MnCO3-H2O provides evidence that solid solution is limited to three mole fraction ranges corresponding to Mn-calcite, kutnohorite and calcic-rhodochrosite. All these authigenic phases may control the concentration of manganese in suboxic marine pore waters. As a consequence the solubility of manganese cannot be adequately described by a single thermodynamic equilibrium constant.