Although tensile fracture has been well understood for more than 50 years, shear fracture remains poorly understood, especially in terms of the nucleation process. The details of the self-organization process by which a population of microcracks evolves into a localized fault, and the factors controlling fault orientation remain poorly understood. With the discovery of high-pressure phase-transformation-induced faulting, there are now two mechanisms of shear failure available for study. Although the fundamental physics of the two mechanisms is different on the finest scale, the evidence suggests that on the scale of self-organization and faulting, the mechanisms behave similarly. Comparison of these two mechanisms can lead to new insights and potentially one can be used to resolve unanswered questions involving the other. In particular, the diagnostic appearance of the transformed material in microlenses makes the attempt to decipher the self-organization process much simpler than attempting to distinguish cracks of many generations in material faulted by "brittle" processes. This study will examine the approach to failure in Mg2GeO4 undergoing the olivine« spinel transformation at pressures greater than 2.5 GPa (pressures too high for "brittle" processes to participate) to map out the microstructural evolution from loading-generated microlenses, through the self-organization process, to faulting.
