Stabilities of crystal faces of aragonite (CaCO3) compared by atomic force microscopic observation of facet formation processes in aqueous acetic acid.

Crystal faces of aragonite in various orientations were dissolved in water or in a dilute aqueous acetic acid. Facet formation processes were observed in situ using atomic force microscopy (AFM). At the polar (001) surface, which gave atom-resolved AFM images in air, less polar {112} facets were developed in water. At two other polar faces (110) and (010), which are frequently seen in natural forms, electrically neutral facets such as (130) and (100) were developed in the acidic solution. No facet formation was observed at the neutral (130) and (100) faces. Atom-resolved AFM images observed in air at the latter nearly corresponded to the arrangement of H atoms binding to the carbonate ions. The relative stabilities of polar and neutral surfaces strongly depend on pH, since protonation of the carbonate ions changes distributions of positive and negative electric charges at the surfaces. The equilibrium crystal form of aragonite in acidic conditions most probably consists of {100}, {130} and {112} faces, which is in strong contrast to the cases of alkaline conditions. The (001) face having carbonate ions fully exposed to the solution will not be stabilized without a specific adsorbate.