SLIP SYSTEMS IN CALCITE SINGLE-CRYSTALS DEFORMED AT 300-800-DEGREES-C

In an attempt to resolve questions recently raised regarding the principal high-temperature slip systems in calcite, optical quality.single crystals have been uniaxially compressed at 300-800-degrees-C and at a constant strain Tate of approximately 3x10(-5) s-1. In addition, a single strain rate cycling test was performed at 650-degrees-C. The tests were all carried out with the compression direction parallel to [2243BAR], which lies at approximately 30-degrees to the c axis and makes angles of 52-degrees and 23-degrees with the poles to two rhombohedral (r) planes. Axial strains of 5-16% were achieved in the tests. The stress-strain curves obtained showed three-stage hardening behavior, while the strain rate cycling data showed flow stresses to be rather insensitive to strain rate with empirical power law fits yielding a conventional stress exponent of 22-30 at strains greater-than-or-equal-to 5%. The active glide systems were identified by slip line analysis. Slip on r[2021BAR] in the so-called negative sense was found to be an important deformation mechanism across the entire range of temperatures investigated, while slip on a single f system in the negative sense became important at 650-degrees-C and above. The active f slip direction was of [1011BAR] type, confirming the existence of a new set of f slip systems, namely, f[1011BAR] as opposed to the generally accepted f[2201BAR] systems. In addition, clear evidence was found for significant slip on the basal (c) plane at temperatures above 600-degrees-C, probably in the [a] direction. By comparison with earlier data on crystals from the same batch, no evidence was found for significant strength asymmetry on the r[2021BAR] and f[1011BAR] slip systems in the positive and negative senses. Notably, the presently observed f and c slip systems have not been taken into account in previous modelling studies of plasticity and texture (i.e., crystallographic preferred orientation) development in calcite rocks, and r slip is usually taken as stronger in the positive than negative sense. Our results therefore imply a need for renewed modelling work on calcite, particularly regarding texture development at relatively high temperatures.