SHEAR AND COMPRESSION WAVES IN SHOCKED CALCIUM-CARBONATE

Transverse and longitudinal particle velocity histories were measured in Carara marble subjected to compression shear loading in plate impact experiments. Peak compressive stresses ranged between 0.5 GPa and 6.3 GPa (0.8% and 13.8% density compression). These measurements permitted a direct determination of both shear and longitudinal wave speeds in a material undergoing a shock-induced phase transformation. Such data provide an improved characterization of the shocked state and permit a direct comparison of the density variations of the elastic moduli under shock and hydrostatic loading without invoking assumptions regarding material strength. The present measurements have led to a revised assessment of the shock response of calcite rocks. In agreement with ultrasonic data, the measured shear wave speeds under shock loading undergo a rapid decrease associated with the transformation to CaCO3(II). Subsequently, the shear modulus increases continually up the highest compressions measured, contrary to earlier conjectures. Qualitative evaluation of shear wave amplitudes demonstrates that calcite rock retains strength after yielding and undergoing multiple phase transformations. Comparison of bulk modulus values determined from the present shock measurements with those from ultrasonic measurements indicate that the former are frozen-phase-composition values when the sample is a mixed-phase aggregate. Because of the transformations, mean stresses cannot be determined directly from the bulk modulus values. Under shock loading, polycrystalline calcite samples transform to CaCO3(II) at approximately 2.3 GPa longitudinal stress. The bulk modulus in the shocked samples indicate that by 4.7 GPa peak longitudinal stress (11% compression) another phase is produced. Previous, higher-stress Hugoniot data for calcite and aragonite samples suggest that this phase is aragonite. CaCO3(III) may not occur at all, in which case the bulk modulus values indicate that aragonite may be produced at a peak longitudinal stress of only 3.8 GPa. In contrast to the direct comparisons afforded by the bulk modulus values, comparison of the Hugoniot with the hydrostat in this stress range leads to incorrect conclusions concerning the shock response of calcite rocks. Also, speculation is offered on the role of phase transformation-enhanced plasticity in the dynamic response of calcite rocks.