to also influence the crystallization of calcite in conditions favorable to the precipitation of vaterite. 57 As the surface of calcite is known to be negatively charged, the positive magnitude of the ζ-potentials indicates the adsorption of additive Ca 2+ ions on the particle surfaces, regardless of pH. In fact, the isoelectric point (IEP) of calcite is commonly reported to be between pH = 8 and 9. The final pH (∼8.6) for the case where initial pH = 9, falls within this range (Figure ). Since IEP is the pH where ζ-potential is zero, the particles could not have acquired a positive value of ∼13 mV (Figure ) without the adsorption of additive Ca 2+ ions.