Studies of the growth and dissolution kinetics of the CaCO3 polymorphs calcite and aragonite .1. Growth and dissolution rates in water

Growth and dissolution rates of calcite and aragonite in water were measured by the pH-stat method as functions of the super- and undersaturation from (S - 1) = -0.6 to 3, at temperatures T = 20-70 degrees C, and for different calcium and carbonate concentrations. For calcite from fits of power laws r = p\S - 1\(n) to the experimental growth and dissolution rates almost linear laws are obtained. From the high activation energy of the prefactor p it follows that for moderate deviations from saturation growth and dissolution are determined by processes at the surface and not by diffusion from the bulk of the solution. Measurements with constant (Ca2+)(HCO3-) ion product and different (Ca2+):(HCO3-) ratios show that the growth and dissolution rates are independent of the individual concentrations but depend solely on the ion product. The results are interpreted by a new two-step growth model. The first, chemical step involves a formation of CaCO3 molecules in the adsorption layer of the crystals possibly via CaHCO(3)( )ion pairs, the second, surface kinetic one an incorporation of the formed CaCO3 molecules into the crystal lattice by surface and step diffusion to kink sites. While the surface kinetics is rate determining at low deviations from equilibrium, the formation and decomposition of CaCO3 dominate growth and dissolution at large super- and undersaturations. The same growth model can be applied to aragonite. The interpretation of the experimental results is, however, complicated because of the occurrence of different crystallographic faces, which allows only a description of the measured rates as overall rates. Results for growth may be interpreted by different power laws for different faces or by two-dimensional nucleation. For dissolution an influence of diffusion in the bulk of the solution is observed.