This project will investigate the atomic scale mechanisms that accompany compression for the important upper mantle minerals including pyroxenes, garnets and olivines. The experimental work will involve obtaining the crystal structures and Raman spectra of various pyroxenes and garnets at pressures to 40 GPa, as well as at modest temperatures below 550 C. Using this data, as well as those obtained from the literature, the structures will be examined with current electron density models to understand bonding changes, especially around the bridging Si-O-Si linkages in pyroxenes and the Si-O-M linkages in garnets (M = octahedral cation). A general understanding of the pressure effect of framework modifiers will be developed. Furthermore, O-O interactions will be modeled in order to investigate the response of the closest-packed lattice to pressure. These models will aid our understanding of the nature of crystalline phases at conditions of the upper mantle. They will also provide a general background to understanding the influence of framework modifiers on all sorts of important industrial materials such as zeolites and non-linear optical behavior, (e.g. KTP).