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Molecular orbital calculations on various aluminosilicate Q 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 3  Si 41  and Al 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 br  bonds that would attach the central cation to the bulk mineral. This result is in contrast to the T-O 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 3  T-OH, T-OH 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-OH 2  and Al-OH-Si species are comparable; thus, as Al-OH 2  surface species become stable, protons are also energetically favored to attach to bridging O atoms. In addition, we found that substitution of Al 3+  for Si 4+  in the second-nearest-neighbor site of a Q 3  Si-OH can increase the calculated PA, although the effect is diminished by the presence of a charge-balancing Na +  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 +  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 charge­balancing effect could alter the results and conclusions of earlier calculations in this area.

Ab initio calculations on aluminosilicate Q 3 species: Implications for atomic structures of mineral surfaces and dissolution mechanisms of feldspars