Molecular orbital energy minimizations were performed with the B3LYP/6-31G(d) method on a [((OH)(3)SiO)(3)SiOH-(H3O+)center dot 4(H2O)] cluster to follow the reaction path for hydrolysis of an Si-O-Si linkage via proton catalysis in a partially solvated system. The Q(3) molecule was chosen (rather than Q(2) or Q(1)) to estimate the maximum activation energy for a fully relaxed cluster representing the surface of an Al-depleted acid-etched alkali feldspar. Water molecules were included in the cluster to investigate the influence of explicit solvation on proton-transfer reactions and on the energy associated with hydroxylating the bridging oxygen atom (O-br). Single-point energy calculations were performed with the B3LYP/6-311+G(d,p) method. Proton transfer from the hydronium cation to an Obr requires sufficient energy to suggest that the Si-(OH)-Si species will occur only in trace quantities on a silica surface. Protonation of the Obr lengthens the Si-O-br bond and allows for the formation of a pentacoordinate Si intermediate (Si-[5]). The energy required to form this species is the dominant component of the activation energy barrier to hydrolysis. After formation of the pentacoordinate intermediate, hydrolysis occurs via breaking the Si-[5]-(OH)-Si linkage with a minimal activation energy barrier. A concerted mechanism involving stretching of the Si-[5]-(OH) bond, proton transfer from the Si-(OH2)(+) back to form H3O+, and a reversion of Si-[5] to tetrahedral coordination was predicted. The activation energy for Q(3)Si hydrolysis calculated here was found to be less than that reported for Q(3)Si using a constrained cluster in the literature but significantly greater than the measured activation energies for the hydrolysis of Si-O-br bonds in silicate minerals. These results suggest that the rate-limiting step in silicate dissolution is not the hydrolysis of Q(3)Si-O-br bonds but rather the breakage of Q(2) or Q(1)Si-O-br bonds.