In situ AFM study of the interaction between calcite {1 0 1¯ 4} surfaces and supersaturated Mn2+–CO32− aqueous solutions

Growth of rhodochrosite (MnCO3) on calcite (1 0 1¯ 4) substrates from supersaturated aqueous solutions was observed in situ using an atomic force microscope (AFM). The supersaturation with respect to rhodochrosite (expressed as βrhod=a[Mn2+]a[CO32−]/Ksp, rhod.; where a[Mn2+] and a[CO32−] are the activities of Mn2+ and CO32− in the aqueous solution) ranged from 48.89 to 338.04. After an induction period, nuclei of the new phase are formed on the calcite substrate. These nuclei readily reach a significant height (2.2±0.2 nm), which remained approximately constant during their lateral spread. The characteristics of the growth pattern of islands of the newly formed phase indicate that there exists an epitaxial relationship between the new phase and the calcite substrate. The islands show a highly anisotropic growth, preferentially spreading along [4 2 1¯] on the calcite substrate at a rate up to 15 times faster than along [0 1 0]. As a result, the islands develop needle and sword blade-like morphologies, elongated along [4 2 1¯] and showing different truncated ends. This unusual elongation is interpreted as the result from a kinetic effect, which is controlled by both the structural characteristic of the calcite (1 0 1¯ 4) surface and the structure and elastic properties of the overgrowing phase. The lateral growth of islands leads to their coalescence and the formation of a quite homogeneous nanometric layer. The characteristics of the epitaxial growth are in agreement with a Volmer–Weber growth mechanism controlling the formation of the epitaxy. The results obtained in these experiments are compared with those obtained in several similar systems.