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Ab initio, molecular orbital calculations for two different Hartree-Fock basis levels were performed on clusters in the system Al-O-H, and tested by comparing derived vibrational frequencies to the measured values for aluminum oxides and aluminum oxyhydroxide minerals. Models were chosen to reflect surface groups that may be present on aluminous minerals such as [Alpha]-[Al.sub.2][O.sub.3] (corundum) and Al[(OH).sub.3] (gibbsite). Protonation and deprotonation reactions on bridging and terminal oxygen or hydroxyl sites were modeled to investigate the effects of solution pH on the structure of surface groups. Relative deprotonation energies, including solvation effects, were calculated for each site using the self-consistent isodensity polarized continuum model of Keith and Frisch (1994), and then used to predict the order in which each particular surface group protonates.

Molecular orbital calculations on various aluminosilicate [Q.sup.3] T-OH and bridging O species were performed to model atomic structural changes on mineral surfaces that occur as a function of pH. Calculated vibrational frequencies are reported for the terminal T-O, T-OH, and O-H bonds of the central T cation as a test of our models, and the predicted frequencies compare well with experimental vibrational spectra of aluminosilicates. Optimized bond lengths and T-O-T angles to the central [Q.sup.3][Si.sup.4+] and [Al.sup.3+] cations in these molecules change significantly as the T-OH bond is protonated and deprotonated. Protonation of terminal bonds tends to shorten and strengthen the remaining three T-[O.sub.br] bonds that would attach the central cation to the bulk mineral. This result is in contrast to the T-[O.sub.br] weakening that has been suggested previously as a mechanism for proton-assisted dissolution (e.g., Furrer and Stumm 1986; Wieland et al. 1988). Proton affinities (PA) of the [Q.sup.3] T-OH, T-O[H.sub.2], and T-OH-T species are predicted to help delineate the process of proton-assisted dissolution in quartz and feldspars. Theoretical results predict that the PAs of Al-O[H.sub.2] and Al-OH-Si species are comparable; thus, as Al-O[H.sub.2] surface species become stable, protons are also energetically favored to attach to bridging O atoms. In addition, we found that substitution of [Al.sup.3+] for [Si.sup.4+] in the second-nearest-neighbor site of a [Q.sup.3] Si-OH can increase the calculated PA, although the effect is diminished by the presence of a charge-balancing [Na.sup.+] cation. This result has implications for models that attempt to describe the behavior of aluminosilicate surfaces in terms of the component oxides. Addition of [Na.sup.+] to charge balance molecules strongly affects calculated structures, proton affinities, and vibrational spectra. The role of charge-balancing cations was often omitted in previous theoretical studies of aluminosilicates, but the magnitude of the charge-balancing effect could alter the results and conclusions of earlier calculations in this area.