Theoretical equilibrium and growth morphology of CaCO3 polymorphs. I. Aragonite

Periodic bond chain (PBC) analysis is performed using a potential function fitted to elastic, structural and vibrational properties of aragonite. The resulting equilibrium form of the crystal, bounded by six forms having F character, is dominated by {0 1 1}, {1 1 0} and {1 0 2}, other forms being {1 1 1}, {0 1 0} and {0 0 1}, the corresponding specific surface energy γ has a mean value of ≈940 erg/cm2. The theoretical growth shape is defined by five F-forms: {1 1 0}, {0 1 0}, {0 1 1}, {0 0 1}, {1 0 2} and a stepped one, {0 2 1}. Its habit depends on what value of the attachment energy of {0 1 1} is considered. The agreement between the theoretical growth morphology and that of natural crystals is not fully satisfactory: as a matter of fact three out of the eight forms bounding the averaged natural morphology (i.e., {1 1 1}, {0 1 2} and {1 2 1}) are missing in the theoretical growth morphology. From the observation of the natural gypsum/aragonite epitaxy, a simple model is proposed to explain the striking morphological difference between the pseudo-symmetrical zones [0 1 0] and [1 1 0] in natural aragonite crystals. The variation on the attachment energy of {0 1 0}, induced by water adsorption, improves the fit between theory and observation around these zones, especially when the distribution of their vicinal faces is considered. The same reasoning on water adsorption is extended to the specific surface energy of the crystal: a theoretical value of 330 erg/cm2 is calculated for γcrystal/water of the face (0 1 0).