We are studying how the mineral fayalite deforms under stress while it is subject to high pressures and temperatures. Specifically, we are studying X-ray diffraction spectra obtained from experiments with the D-DIA apparatus at Brookhaven National Laboratory, in which the x-ray diffraction pattern is studied as the sample is deformed under high pressure. This yields the spacing between the lattice planes as the sample is deformed. By fitting the diffraction spectra, we can calculate the spacing between lattice planes of fayalite, and observe how the spacing changes with time as the crystal deforms. The deformation can be modeled using the “Elastic Plastic Self-Consistent” model, in which the material is treated as a cluster of independently oriented grains. When stress is applied to the material, deformation occurs because the lattice planes slip by each other. A variety of slip systems are used to model the different ways in which these planes can move. The model allows us to calculate the aggregate properties of the material from the microscopic properties of the individual grains.