Importance of surface structure on dissolution of fluorite: Implications for surface dynamics and dissolution rates

Dissolution rates are usually calculated as a function of surface area, which is assumed to remain constant ignoring the changes occurring on the surface during dissolution. Here we present a study of how topography of natural fluorite surfaces with different orientation changes during up to 3200h of dissolution. Results are analyzed in terms of changes in surface area, surface reactivity and dissolution rates. All surfaces studied present fast changes in topography during the initial 200h of dissolution. The controlling factors that cause the development of topography are the stability of the step edges forming the initial surface and its inclination to the closest stable planes, which are specific for each surface orientation. During an initial dissolution regime dissolution rates decrease significantly, even though the total surface area increases. During a second dissolution regime, some surfaces continue to present significant changes in topography, while for others the topography tends to remain approximately constant. The observed variation of dissolution rates are attributed to a decrease of the density of step edges on the surface and the continuous increase in exposure of more stable surfaces. Calculations of dissolution rates, which assume that dissolution rates are directly proportional to surface area, are not valid for the type of surfaces studied. Instead, to develop accurate kinetic dissolution models and more realistic stochastic dissolution simulations the surface reactivity, determined by the relative stability of the planes and type of edges that constitute a surface needs to be considered. Significant differences between dissolution rates calculated based on surface area alone, and based on surface reactivity are expected for materials with the fluorite structure.