Measures of Basicity in Silicate Minerals Revealed by Alkali Exchange Energetics

The energies of the alkali exchange reactions are computed from density functional calculations for a series of alkali silicate minerals. First-principles calculations are compared against (1) experiment; (2) thermodynamic models; (3) calculations using empirical interatomic potentials (with both polarizable shell model potentials and pairwise potentials with partial charges); and (4) empirical correlations based on optical basicity and Pauling bond strength. The first-principles calculations correlate well with experimental values. The shell model potentials appear to account for Si-O-Si versus Si-O oxygen basicity, but don't recover the differences in basicity between Al-O-Si and Si-O-Si donors. The pair potentials don't even qualitatively reproduce the reactivity trends established in the first-principles calculations. Empirical correlations based on optical basicity and Pauling bond strength successfully rank-order the exchange energies, but fail to describe the large difference in basicity between Si-O-Si and Si-O oxygen atoms. Knowing which models are capable of predicting ion exchange energies in silicate minerals improves understanding of the physics of bonding in ion-exchanged glasses.