1998/05/01 by C. M. Koretsky, Carla M. Koretsky, D. A. Sverjensky +3 · 8 citations
Environmental Science · Energy · Computer Science · #Minerals Flotation and Separation Techniques #Iron oxide chemistry and applications #Geochemistry and Geologic Mapping
paper · pdf · doi:10.2475/ajs.298.5.349
In this study, a method for estimating site types and ionizable site densities at mineral surfaces by consideration of ideal crystal surfaces has been developed. Periclase, rutile, goethite, hematite, corundum, kaolinite, andalusite, sillimanite, sanidine, albite, anorthite, and quartz surfaces were considered, both because they represent a wide range of crystal structures and because many adsorption and dissolution studies have focused on these minerals. For each mineral, the predominant cleavage or growth faces were studied. To avoid choosing arbitrary slices of the crystal parallel to each cleavage or growth face, the total Brown bond strengths (and the resulting partial charges on coordinatively unsaturated atoms) were calculated for the bonds that must be broken to generate each possible ideal slice on a given plane. The charge-neutral or nearly charge-neutral slices produced with a minimum total strength of bonds severed were examined with the aid of the commercial computer program Crystal-Maker C^ . Once the ideal mineral surface for a given plane was chosen, the number of sites per unit surface area was calculated using five different methods. For the minerals considered, calculated site densities for a given surface ranged from 0 to 40.8 sites/nm 2^ , a considerably larger range than the density of 2.3 to 10 sites/nm 2^ that is often cited. The results from each of the methods were compared to available experimental estimates of surface hydroxyl site densities from temperature desorption experiments, infrared data, and tritium exchange methods and to ionizable or reactive site densities from acid-base titrations and chemical reaction methods. Estimates based on the number of broken bonds gave the best agreement with site densities using the tritium exchange method. However, estimates based on the number of coordinatively unsaturated atoms or on the partial charge of coordinatively unsaturated atoms were also consistent with much of the available tritium exchange data. Predicted site types were compared with the available spectroscopic data. Implications of predicted site types with respect to the interpretation of infrared data and for improving adsorption and dissolution rate models are discussed.