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 degrees and 90 degrees C. At very high precipitation rates (mixing of Me(NO3)(2)/Me(Cl)(2) solutions with NH4HCO3 solutions at 20 degrees 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 Ca(2+)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 degrees and 90 degrees C), Mn2+ was generally enriched in the solid compared to the aqueous solution. Corresponding partition coefficients ranged between 1.5 and 13.3 (20 degrees C), 3.0 and 11.9 (60 degrees C), and 5.3 and 12.9 (90 degrees 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 degrees C is essentially controlled by precipitation kinetics with a limiting value of 1 for D. Whereas no evidence was found between 20 degrees and 90 degrees C for the formation of the intermediate ordered compound CaMn(CO3)(2) (kutnahorite), several solid solutions of intermediate composition (so-called 'pseudo-kutnahorite') were precipitated. (C) 1998 Elsevier Science B.V. All rights reserved.