Static lattice energy minimization and lattice dynamics calculations on aluminosilicate minerals

Abstract

Lattice energy minimization and lattice dynamics calculations for the minerals andalusite, sillimanite, kyanite, diopside, cordierite, gehlenite, leucite, orthozoisite, grossular, and pyrope are compared with experimental data and previous calculations. The potential models used in this study included bond-bending interactions, short-range Born-Mayer forces, effective dispersive interactions, long-range Coulomb interactions, and harmonic core-shell interactions for the 0 ions.. Parameters for the potential models were generally taken from the literature, but the core-shell force constant was modified to give better agreement with experimental data for refractive indices. It was necessary to include bondbending interactions for AI-O polyhedra with coordination numbers even larger than four. A method for describing effective potentials with Al-Si disorder and solid-solution is presented. Modified Morse and Buckingham potentials were used to model O-H bonds. Relaxed energy-minimum structures were calculated, allowing cell parameters to change and treating atomic cores and shells as independent entities within the adiabatic approximation. Calculated phonon frequencies for the relaxed structures were used to construct thermodynamic functions. Elastic and dielectric constants were also calculated. Comparisons between calculated structures and other properties with experimental data have shown that the model is genuinely transferable and gives reasonable predictions of crystallographic, physical, and thermodynamic properties. Detailed analysis gives a measure of the reliability of the model.