Crystal growth and dissolution of calcite in the presence of fluoride ions: An atomic force microscopy study

Abstract

Growth and dissolution of calcite {1014} surfaces in aqueous solutions in the presence of fluoride ions have been studied by in situ atomic force microscopy (AFM). Supersaturated and undersaturated solutions with respect to calcite were prepared for the growth and dissolution experiments, respectively. The concentration range of solutions containing fluoride ions varied between 0.025 and 50 mM. The crystal growth rates were measured from the step growth of closing of etch pits along the [010] direction as well as the spreading of two-dimensional growth nuclei in the same direction. Dissolution rates were measured from the time-dependent development of the rhombohedral etch pits along both [010] and [421] directions. Low fluoride concentrations (=0.33 mM) in the supersaturated solutions did not significantly affect the crystal growth rates of calcite. At higher concentrations (up to 5 mM), the growth rate decreased substantially, reaching 50% of the respective rate in the absence of fluoride for the same supersaturation. It is suggested that the fluoride ions acted as inhibitors of calcite crystal growth, possibly through adsorption at the active growth sites. For dissolution experiments, the presence of both low and high fluoride concentrations (=1.1 mM and up to 50 mM) accelerated the rate of dissolution along the [421] direction up to 70% of the respective rate in pure water. The presence of fluoride changed the morphology of the dissolving rhombohedral etch pits to hexagonal. The morphology patterns of the dissolving calcite surface recorded by AFM imaging in the presence of fluoride ions suggested also that the fluoride ions were adsorbed onto the calcite surface. It seems likely that calcite growth and dissolution from aqueous solutions containing fluoride ions are governed by a complex interaction between the surface characteristics of the calcite crystal as well as changes in solvent structure, surface hydration, and ion solvation induced by the presence of fluoride. © 2009 American Chemical Society.