This project represents a continuation of research previously supported by NSF grant EAR7920095, involving fundamental studies of the structures and crystal chemistry of rock-forming minerals. The experimental aspects of this program employ single-crystal diffraction methods, transmission electron microscopy, and spectroscopic methods to characterize the structures of geologically important low-symmetry complex structures. The major thrust of the current research will be to employ an essentially ionic model to simulate complex structures and calculate their energies. These procedures permit the evaluation of the stabilities of particular structural configuations relative toalternatives, the examination of the response of a particular structure to changes of temperature, pressure, or composition, and the examination of the mechanisms of phase transformations. The success of any particular modeling strategy depends on the veracity of the interionic pair potentials. Depending on specific objectives, these may be derived empirically by appropriate fitting to known structure data, or may be calculated ab initio from fundamental electron density-energy functionals through the modified-electron-gas formalism. Particular mineral systems of interest include pyroxenes, for which present MEG-derived potentials are inadequate; feldspars, particularly the P1 to I1 anorthite transformation; and cordierite.//
