Catalytic Effect of Magnesium Ions on Polymorphic Crystallization of Calcium Carbonate

The effect of Mg^ ions on the polymorphic crystallization of calcium carbonate has been studied by precipitating carbonate minerals from Ca(HCO_3)_2 (Ca^, 400 mg/l) solutions containing various amount of Mg^ ions (0 to 2000 mg/l) at the temperature of 2 to 80℃. The concentration of Mg^ ions and temperature in a parent solution affect remarkably the aragonite formation. The precipitating fields of calcite and/or aragonite are systematically represented on log[Mg^] vs. l/T diagram. Energetic effect of Mg^ ions on aragonite formation is estimated from an Ahrenius type relationship between required concentration of Mg^ ions and absolute temperature to form aragonite; [Mg^] = A・exp (-E/RT), where E = 13 Kcal/mol, which will be needed to form aragonite nucleous from solution. The presence of calcite seeds favors the calcite formation in the presence of Mg^ ions lower than 400 mg/l