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

Abstract

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.

Stabilities of crystal faces of aragonite-type strontianite (SrCO3) and cerussite (PbCO3) compared by AFM observation of facet formation in acid

“…Among the crystal faces intersecting with the c-axis, the {1 1 2} facets were formed during the dissolution of polar (0 0 1) face of aragonite in the acidic condition [6,7]. This face is observed in natural forms A2 of aragonite and S2 of strontianite in Fig.…”

“…One of the authors studied facet formation processes at the polar aragonite faces with AFM during dissolution in dilute aqueous acetic acid [6,7], and found that the polar faces were replaced with the neutral facets. The changes in the relative stabilities of the crystal faces were explained, as is demonstrated in Fig.…”

Stabilities of crystal faces of anhydrite (CaSO4) compared by AFM observation of facet formation processes in aqueous solutions

“…We have compared relative stabilities of aragonite (CaCO 3 ) faces in dilute aqueous acetic acid and have found that the equilibrium form of the crystal in the acidic condition should be totally different from natural forms, probably obtained in alkaline environments [1,2]. Protonation of carbonate ions changes charge distributions at the surfaces.…”

“…Apparently polar crystal faces of aragonite are stabilized by neutralization of positive charges of Ca 2 + on the top layer, by the negative charges of O atoms pointing out of the carbonate layer at lower levels [1,2].…”

Atomic force microscopic studies of stabilization of NaCl(1 1 3), (1 1 2) and (1 1 0) surfaces in ethanol, CdCl2/ethanol and HgCl2/ethanol

“…Previously, one of the authors compared stabilities of crystal faces of the aragonite (CaCO 3 ) by observing the facet formation in etch pits during the dissolution of the crystal faces with AFM [7,8]. In an acidic solution, the (1 0 0) face which does not appear in natural crystal forms was stabilized and became atom-flat.…”