Microstructural investigation of the interaction and interdependence of cataclastic and plastic mechanisms in Feldspar crystals deformed in the semi-brittle field

Plagioclase feldspar (An₅₀) single crystals from the Hogarth Range, NSW, Australia, have been deformed in the brittle-to-semi-brittle regime at 500-800°C, 90-300 MPa effective pressure, and strain rate between 10^-5 and 10^-6. This gem quality feldspar is both macroscopically and microscopically devoid of any microstructure other than very rare growth twins, so all microstructures produced by experimental deformation can be easily identified. The specimens were deformed to failure, which occurred at strains of <4%, and exhibit microstructures of interacting albite twins and microcracks. The shear stress at the tip of a propagating shear fracture is relieved through the production of mechanical twins. Similarly, the elastic strain in a twin is replaced by a microcrack. In addition to the very important role of relieving stress at the tips of shear fractures, groups of twins also accommodate lattice misorientations of several degrees, typical of those seen in subgrains that have been produced by dislocation climb. These observations suggest that the deformation progressed by an alternation of twinning and micro-fracturing, each process initiating the other in turn. Tangled and isolated dislocations were also observed in and near partially healed cracks. Since, in other materials, dislocation tangles have been observed to nucleate cracks, our observations suggest that in general, deformation to higher strains in the semi-brittle regime is probably controlled, not by the simple superposition of brittle and plastic processes operating independently, but by their interaction and interdependence.