Measures of Basicity in Silicate Minerals Revealed by Alkali Exchange Energetics

TL;DR

This study uses density functional calculations to analyze alkali exchange energetics in silicate minerals, comparing various models and experiments to understand bonding and basicity differences.

Contribution

It provides a comprehensive comparison of first-principles, empirical, and thermodynamic models for alkali exchange in silicates, highlighting their strengths and limitations.

Findings

First-principles calculations align well with experimental data.

Shell model potentials capture some bonding features but miss key basicity differences.

Empirical correlations rank exchange energies but can't distinguish large basicity variations.

Abstract

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.