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 me asured by the pH-stat method as functions of the super- and undersatur ation from (S - 1) = -0.6 to 3, at temperatures T = 20-70 degrees C, a nd for different calcium and carbonate concentrations. For calcite fro m fits of power laws r = p\S - 1\(n) to the experimental growth and di ssolution rates almost linear laws are obtained. From the high activat ion 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. Measur ements with constant (Ca2+)(HCO3-) ion product and different (Ca2+):(H CO3-) ratios show that the growth and dissolution rates are independen t of the individual concentrations but depend solely on the ion produc t. The results are interpreted by a new two-step growth model. The fir st, chemical step involves a formation of CaCO3 molecules in the adsor ption layer of the crystals possibly via CaHCO(3)( )ion pairs, the sec ond, surface kinetic one an incorporation of the formed CaCO3 molecule s into the crystal lattice by surface and step diffusion to kink sites . While the surface kinetics is rate determining at low deviations fro m equilibrium, the formation and decomposition of CaCO3 dominate growt h and dissolution at large super- and undersaturations. The same growt h model can be applied to aragonite. The interpretation of the experim ental results is, however, complicated because of the occurrence of di fferent crystallographic faces, which allows only a description of the measured rates as overall rates. Results for growth may be interprete d by different power laws for different faces or by two-dimensional nu cleation. For dissolution an influence of diffusion in the bulk of the solution is observed.