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 natura l powdered aragonite to MnxCa(1-x)CO3 (rhodochrosite-calcite) solid-so lutions by the reaction with aqueous (Ca2Mn)-chloride solutions, at 20 degrees C and 1-atm total pressure, for periods up to 37 months. Resu lts indicate that the transformation of aragonite to mixed rhombohedra l carbonates follows a dissolution-precipitation pathway. Reaction vel ocity of the transformation process was significantly accelerated by i ncreasing 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 intermedia te ordered compound CaMn(CO3)(2) (kutnahorite). Phase relations are si milar to those found in marine muds of the Baltic deeps.