Abstract:
In situ atomic force microscopy (AFM) experiments have provided information about the effect of Ba2+ on crystal growth of calcite {1014} surfaces. The microtopographic features observed have been interpreted by considering both the structural control that the calcite surfaces exert on the incorporation of divalent cations and the supersaturation state of the solution used. Pinning of the calcite growth steps occurs at low Ba concentrations, suggesting specific sites for Ba incorporation. When the Ba content of the solution is increased the advancement of monomolecular steps is observed. Although [441]+ and [481]+ steps advance showing characteristic jagged edges, the parallel steps (i.e., [441]- and [481]-) remain practically immobile. This fact can be explained by considering the nonsymmetrically related distribution of large and small sites along the calcite steps and the easier incorporation of barium on the former. The measured increase in the height of the newly grown steps is also consistent with such preferential incorporation of Ba in certain positions. A further increase in the Ba concentration of the solutions leads to the formation of bidimensional nuclei on the calcite {1014} surfaces. The nature of these nuclei is discussed taking into account the supersaturation of the solution with respect to two possible structures that can accommodate Ba: the calcite-type structure and the aragonite-type structure.
