The rate and mechanism of polymorphic transformation of calcium carbonate in water

The transformations of vaterite and aragonite to calcite in water were investigated at temperatures ranging from 25.0 to 80.0°C and ion activities of Ca2+ and CO2-3 ranging from 10-2.5 M to 10-4.1 M and 10-4.1 M to 10-5.7 M, respectively. It was conf irmed that these transformations proceeded by the dissolution of the metastable phases (vaterite and aragonite) and the growth of calcite, and that the rate-determining step was the growth of calcite for the both transformations. The rate constant of transformation was in good agreement with that of calcite growth from supersaturated solutions. The apparent activation energy of the transformation is about 70 kJ mol-1 and suggests that the rate of transformation of metastable phase is controlled by the surface reaction of calcite growth. The transformation rate of vaterite is independent of the activities of Ca2+ and CO2-3 ions under most conditions. The transformation is considerably retarded in solutions with very high pH (>10) and low concentration of total carbonate ion. Li+ strongly retards the transformation, whereas other alkali metal ions and Cl-, Br- and NO-3 scarcely affect the transformation rate. The incorporation of Li+, having a small ionic radius, is thought to cause a lattice strain on the surface of calcite and thus causes the retardation.