Abstract
The ionic model for crystals was devised shortly after the first crystal structures were determined in the early part of this century, but its use was hindered by the difficulty of calculating Coulomb electrostatic sums, and its applicability was diminished by the fact that empirical short-range repulsive potentials could be obtained only for the simplest of structures. Over the past two decades these disadvantages have been overcome, by development of techniques for fitting short-range potentials empirically to data for any structure, and by the important innovation of the modified electron-gas (MEG) formalism by which short-range potentials for closed-shell ions can be determined nonempirically from electron densities. Both techniques have now been employed to examine minimum-energy structures and properties of a variety of mineral systems. In a review of studies of TiO2 polymorphs, MgSiO3 perovskite, forsterite and its high-pressure polymorphs, quartz, diopside, and albite, the modern applicability of the ionic model to examination of the structures and properties of silicates and oxides is explored. The potential of these methods to contribute significantly to quantitative understanding of crystal-chemical and thermodynamic properties of a wide spectrum of minerals is firmly established.
