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 (e0.33 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 (e1.1 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.

Do organic ligands affect calcite dissolution rates?

“…The observations presented in this study suggest that the presence of organic ligands negligibly affects calcite dissolution rates. This observation appears to contrast with observations of calcite surfaces during their dissolution by atomic force microscopy; several studies have shown that the presence of trace quantities of dissolved organic and other trace species can dramatically change the shape of etch pits forming and step velocities on calcite surfaces (Teng and Dove, 1997;Astilleros et al, 2002;Arvidson et al, 2006;Harstad and Stipp, 2007;Ruiz-Agudo et al, 2010;Vavouraki et al, 2010;Yoshino et al, 2010). There are several possible reasons for these apparent differences.…”

Specific effects of background electrolytes on the kinetics of step propagation during calcite growth

“…Effect of F À , SO 2À 4 and Li + on growth hillock morphology: surface stabilization due to increased hydration Changes in the morphology of growth features on calcite cleavage surfaces in the presence of NaF and Na 2 SO 4 were previously reported by Vavouraki et al (2008Vavouraki et al ( , 2010. They suggested F À and SO 2À 4 incorporation on specific sites of the calcite surface as the most likely cause of the change in growth morphology.…”

An experimental study of magnesite precipitation rates at neutral to alkaline conditions and 100–200°C as a function of pH, aqueous solution composition and chemical affinity

“…Steefel and MacQuarrie, 1996;Knauss et al, 2005;Steefel and Maher, 2009;Zhu, 2009;Goddéris et al, 2010;Zhu et al, 2010;Liu et al, 2011), yet the applicability of such models has not been verified in detail and may be inconsistent with what is known about mineral precipitation kinetics (e.g. Nielsen, 1964Nielsen, , 1984Nancollas and Reddy, 1971;Reddy and Nancollas, 1976;Reddy and Gaillard, 1981;Inskeep and Bloom, 1985;Mucci, 1986;Van Cappellen and Berner, 1991;Piana et al, 2006;Kowacz et al, 2007;Vekilov, 2007;Bénézeth et al, 2008;Kowacz and Putnis, 2008;Perdikouri et al, 2009;Vavouraki et al, 2010;RodriguezBlanco et al, 2011). In an attempt to explore the link between dissolution and precipitation, magnesite precipitation rates have been measured as a function of chemical affinity and aqueous solution composition using the same solids and experimental methods as used by Saldi et al (2010) for magnesite dissolution.…”