The transformation of aragonite to MnxCa(1-x)CO3 solid-solutions at 20 degrees C: An experimental study

Experiments were performed to investigate the transformation of natural powdered aragonite to MnxCa(1-x)CO3 (rhodochrosite-calcite) solid-solutions by the reaction with aqueous (Ca2Mn)-chloride solutions, at 20 degrees C and 1-atm total pressure, for periods up to 37 months. Results indicate that the transformation of aragonite to mixed rhombohedral carbonates follows a dissolution-precipitation pathway. Reaction velocity of the transformation process was significantly accelerated by increasing initial dissolved Mn2+/Ca2+ ratios. Manganese was generally enriched in the secondary carbonate compared to the aqueous solution, and the mole fractions (X-Mn) of the solid-solutions varied between 0.54 and 0.91. No evidence was found for the formation of the intermediate ordered compound CaMn(CO3)(2) (kutnahorite). Phase relations are similar to those found in marine muds of the Baltic deeps.