| dc.description.abstract | Several naturally occurring potassium feldspar single crystals have been shortened in triaxial deformation experiments in the [012] direction to activate both (010)[001] and (001)1/2[1 ̅10] slip systems. Sanidine single crystals have been shortened in the [001] direction to activate the (121 ̅)[101] slip system. Experiments were conducted in a Griggs solid-medium apparatus at temperatures of 650-950oC, strain rates of ε ̇=1.6*10-6s-1 to 2.1*10-4s-1 and at a confining pressure of 1500 MPa. Samples chosen have various Al/Si ordering, crystal symmetry, water content and hydrogen speciation. At lower temperatures (T≤700oC and ε ̇ = 10-6s-1), feldspars oriented for slip exhibit differential yield stresses of 204-613 MPa. Many exhibit peak stresses (718-556 MPa) followed by strain softening. One sample reached a nearly constant stress and another showed strain hardening. At higher temperatures (T≥900oC), samples exhibit yield stresses of 132-227 MPa before approaching constant flow stresses (PF-2, AF-2) or hardening (PF-7, SF-2, BF-2). Differential stresses decrease with increasing temperature and show a positive correlation with strain rate. The strain rate and temperature dependencies of feldspar strength can be described equally well by a thermally activated power law, exponential glide law, and Peierls lattice resistance law with a common temperature dependence given by activation enthalpy (H*) = 281.7 kJ/mol. Best fit values of the power law, exponential law and Peierls law are n=7.70, α=0.0315 MPa-1, and σp=44.4 MPa, respectively. The excellent fit of these creep laws suggest that dislocation slip is predominant but other mechanisms are active as well. All samples show undulatory extinction due to dislocation slip and all reach the critical resolved stress required for mechanical twinning. Sample-scale cleavage fractures and microcracks are also present in deformed samples with some microstructures that suggest simultaneous brittle-plastic deformation. The combined mechanical and microstructural results reflect brittle-plastic deformation of feldspars, similar to the feldspar microstructures found in naturally deformed rocks of the middle to lower continental crust. The mechanical results for these potassium feldspars are remarkably similar given the differences in ordering, symmetry and water content of the samples. | en |
