The influence of {Ca²⁺}:{CO₃^2-} stoichiometry ratios on CaCO₃ nucleation and growth in aqueous environments

When calcium carbonate (CaCO₃) crystals form in aqueous environments, the building blocks—calcium and carbonate ions—come together in the solid in an ideal 1:1 ratio. In contrast, the natural and industrial solutions in which they form, contain widely diverging ionic ratios. What is the impact of these non-ideal ionic ratios on the crystals that form? Since most nucleation experiments have been conducted in solutions with ideal ionic ratios, it is unknown how, why and how fast crystals nucleate in industrial crystallization processes and in Earth surface environments. We therefore investigated the effect of the ionic ratio, r_aq, at various, initially constant degrees of supersaturation at ambient temperature and pressure using dynamic light scattering (DLS), cross-polarized light in optical light microscopy (XPL) and Molecular Dynamics (MD) simulations. At ideal ionic ratios and moderate supersaturation, DLS shows that ion adsorption onto small nuclei is the dominant mechanism. At high supersaturation, no continuum of particle sizes is observed with time, suggesting that aggregation of prenucleation clusters into larger particles is the more dominant growth mechanism. At non-ideal ionic ratios, small prenucleation particles do not seem to grow or aggregate. XPL showed that the time needed for new CaCO₃ particle formation and growth to at least 20 μm depends strongly and asymmetrically on m depends strongly and asymmetrically on r_aq. MD confirmed that r_aq affects CaCO₃ nanoparticle formation substantially. At ideal ratios and high supersaturation, the largest nanoparticles were observed. Our results imply that, besides supersaturation, r_aq affects particle size and persistence, growth and ripening time towards μm depends strongly and asymmetrically on nm-sized crystals. Our results may help to improve understanding, prediction and tailoring the formation of CaCO₃ in geological, industrial and geo-engineering settings.