Controlled nucleation and crystal growth of various CaC0₃ phases by the silica gel technique

The effect of the environment on the precipitation and crystal growth of CaCO₃ from aqueous solutions at ordinary temperature and pressure has been investigated using the silica gel technique. Thermodynamics fails to predict many of the observed assemblages, suggesting that kinetic mechanisms are more important for these quasi-metastable occurrences. The effect of concentration of reactants, pH, and impurity ions on the precipitation of CaC0₃ in a silica gel has been determined. Aragonite formation, in the absence of impurity cations, seems to be caused by the entrapment of HCO₃⁻, which is a function of the relative concentration of CO₃²⁻ to HCO₃⁻. Synthesis data for vaterite suggests that this phase may be related to the precursor formation of CaCO₃‧H₂0 and also that it is not a stoichiometric CaCO₃ phase; rather, a Ca-rich material with small incorporations of (OH), HCO₃, or CO₂ (aq.). The morphology of calcite and aragonite, in the absence of impurity cations, is a pronounced function of pH. In the presence of the impurity cations, Sr²⁺, Mg²⁺, and Ni²⁺, aragonite formation is controlled by its epitaxial growth on aragonite-like nuclei; for Sr₂₊ unequivocal evidence indicates that this nucleus is SrCO₃, whereas, for Mg the nucleus seems to be MgCO₃·3H₂0, nesquehonite.

The partition of these impurity cations between solution and the various precipitating solids is a complex function of pH, relative concentration of reactants and impurity ions, and amounts of various CaCO₃ phases precipitating simultaneously. The partition coefficients for Sr in aragonite and calcite were 4.37 and 1.04 respectively. Single crystals of calcite, with 7.5 mole percent Mg, were grown by this technique.