EFFECTS OF DIVALENT-CATIONS UPON NUCLEATION, GROWTH AND TRANSFORMATION OF CALCIUM-CARBONATE POLYMORPHS UNDER CONDITIONS OF DOUBLE DIFFUSION

The effects of 7 different divalent cations upon nucleation, growth and transformation of CaCO3 polymorphs were investigated using a double diffusion technique. The presence of the cationic impurities, Fe2+, Mg2+, Ni2+, Co2+, Zn2+ and Cu2+, in the reaction medium favored the formation of aragonite, whereas Cd2+ ions showed no significant effect. Induction time and the positions to observe first precipitation were measured so as to analyze the nucleation behaviors. By applying the equivalency rule, the experimental data were examined on the basis of the kinetics and thermodynamics of crystal growth. The aragonite formation was explained in terms of impurity effect through two mechanisms: (1) inhibition of growth of the metastable phase (aragonite) by adsorption of impurity cations; (2) formation of the unstable solid solution of calcite through transformation of aragonite.