W0339

The Analysis of Bonding Changes in Pyroxenes at Pressure and Temperature using Procrystal Electron Densities. Robert T. Downs, Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.

Procrystal electron density at a given point in a crystal can be computed by placing spherically averaged wave functions for appropriate atoms at their observed positions and summing each of their contributions. It is fast and easy to compute. A comparison of ab initio and procrystal electron density distributions for ~250 bonds in various minerals demonstrates that the procrystal model can mimic the topological features of the ab initio distribution quite well. In particular, every bond critical point found in the ab initio model is reproduced in the procrystal model within 0.02 Å, while the value of the electron density at the bond critical point is reproduced to within 0.2 e/ų for all bonds except BO and CO. If one accepts the Bader model for analysis of topology, then this implies that the coordination of an atom in a crystal can be quick and simple to determine by using a procrystal model.

With this model, the bonding systematic for more than 100 different silicate pyroxene compositions, some at P and T, are analyzed to reveal a rich collection of environments around the M2 cation. Coordinations numbers are found to range from 4 to 8, and all symmetry changes are accompanied by coordination changes. The analysis reveals a pyroxene structure that has localized ductile behavior within a brittle frame. Topological analysis of this sort has also revealed bonding changes in KTP that accompany the loss of intensity of the generated second harmonic light, and bonding changes in feldspars that accompany subtle compressional variations.