Manganese(II) partitioning during experimental precipitation of rhodochrosite–calcite solid solutions from aqueous solutions

The partitioning of manganese(II) during the precipitation of the complete rhodochrosite–calcite (MnCO3–CaCO3) solid-solution series was investigated using batch-type experiments at temperatures between 20° and 90°C. At very high precipitation rates (mixing of Me(NO3)2/Me(Cl)2 solutions with NH4HCO3 solutions at 20°C), no fractionation of the cations between aqueous solution and solid solution was observed (≡partition coefficient (D) of 1). These results were not influenced by the presence of dissolved magnesium. Mg2+, however, coprecipitating with Ca2+ and Mn2+ in a multicomponent carbonate solid solution (MnXCaYMgZCO3), was generally depleted with respect to the liquid. At lower precipitation rates (slow degassing of CO2 from a Me(Cl)2–NaHCO3 solution between 20° and 90°C), Mn2+ was generally enriched in the solid compared to the aqueous solution. Corresponding partition coefficients ranged between 1.5 and 13.3 (20°C), 3.0 and 11.9 (60°C), and 5.3 and 12.9 (90°C). The D-values decreased with increasing Mn2+/Ca2+ ratios of the aqueous or solid solutions, indicating the influence of precipitation kinetics and/or varying activity coefficients of the solid components. A comparison of the present results with previous experiments leads to the suggestion that manganese(II) partitioning during the formation of rhodochrosite–calcite solid solutions below 100°C is essentially controlled by precipitation kinetics with a limiting value of 1 for D. Whereas no evidence was found between 20° and 90°C for the formation of the intermediate ordered compound CaMn(CO3)2 (kutnahorite), several solid solutions of intermediate composition (so-called ‘pseudo-kutnahoriteʹ) were precipitated.