Shock temperature in calcite (CaCO3) at 95^160 GPa

The temperatures induced in crystalline calcite (CaCO3) upon planar shock compression (95^160 GPa) are reported from two-stage light gas gun experiments. Temperatures of 3300^5400 K are obtained by fitting six-channel optical pyrometer radiances in the 450^900 nm range to the Planck gray-body radiation law. Thermodynamic calculations demonstrate that these temperatures are some 400^1350 K lower than expected for vibronic excitations of the lattice with a 3R/mole-atom specific heat (R is gas constant). The temperature deficit along the Hugoniot is larger than that expected from only melting. In addition to melting, it appears likely that shock-induced decomposition of calcite occurs behind the shock front. We modeled disproportionation of calcite into CaO (solid) plus CO2 (gas). For temperature calculations, specific heat at constant volume for 1 mole of CO2 is taken to be 6.7R as compared to 9R in the solid state; whereas a mole of calcite and a mole of CaO have their solid state values 15R and 6R, respectively. Calculations suggest that the calcite decomposes to CaO and CO2 at V1108 10 GPa along the Hugoniot. Recent reanalysis of earlier VISAR measurements of particle velocity profiles [1] indicates that calcite shocked to 18 GPa undergoes disproportionation at much lower pressures upon isentropic expansion. = 2002 Elsevier Science B.V. All rights reserved.