Plastic deformation of quartz at room temperature by SEM in situ micropillar compression

Rock material deformation behavior has been studied for many decades using experimental deformation apparatus, usually at high confining pressure and temperature, in order to simulate geological deformation conditions and to reach ductile deformation behavior. Such experiments are usually time consuming, the sample preparation is delicate and only the final deformed product can be analyzed. Additionally, only centimeter to millimeter sized samples can be analyzed. It has been recently discovered that the mechanical behavior of a crystalline material can change as the size of the system in question approaches any characteristic length scale associated with the dislocation processes of the material (Michler et al. 2007). Several proprieties are affected by a material’s internal and in some cases external length scales, such as the yield stress, the hardness, the brittle-to-ductile transition temperature and the fracture toughness. Length-scale dependent values of the mechanical proprieties of rocks materials and minerals have never been tested so far. They are however fundamental for the understanding of the rheology of rocks, as they may constrain for instance the deformation behavior of mylonite or ultramylonite where the grain size reaches micro- to nano- values. In order to address this issue, SEM in situ micropillar compression of natural quartz has been performed. Two sets of samples have been tested with the compression axis perpendicular to respectively the c- and the z-planes. The pillar have been fabricated by a focus ion beam from a bulk single crystal and deformed by a SEM in situ microindenter equipped with a flat diamond punch. The results show that ductile deformation occurs at room temperature with pillars of 1 micron diameter for both orientations, with yield stress of about 8.3 and 6.9 GPa for the pillars oriented parallel to respectively the c- and z-axis. Pillars oriented parallel to the c-axis show rhomb {r} <a+c> slip planes and the ones parallel to the z-axis show basal {c} <a> slip. SEM insitu micropillar compression offers the great advantage to have a very well defined state of stress. In addition it allows real-time in-situ microstructure study during the deformation. It is therefore a very promising method to study plastic deformation behavior and the dislocation mechanisms of rock minerals. Ref.: Michler, J., K. Wasmer, S. Meier, F. Ostlund and K. Leifer (2007): Plastic deformation of gallium arsenide micropillars under uniaxial compression at room temperature. Applied Physics Letters 90, 043123