SHOCK-WAVE EQUATION OF STATE OF RHYOLITE

We have obtained new shock-wave equation of state (EOS) and release ad iabat data for rhyolite. These data are combined with those of Swegle (1989, 1990) to give an experimental Hugoniot which is described by U- s = 2.53(+/- 0.08) + 3.393(+/- 0.37)u(p) for u(p) < 0.48 km s(-1), U-s = 3.85(+/- 0.05) + 0.65(+/- 0.03)u(p) for 0.48 less than or equal to u(p) < 2.29 km s(-1), U-s = 1.52(+/- 0.08) + 1.67(+/- 0.02)u(p) for 2. 29 less than or equal to u(p) < 4.37 km s(-1), and U-s = 3.40(+/- 034) + 1.24(+/- 0.06)u(p) for u(p) greater than or equal to 4.37 km s(-1), with rho(0) = 2.357 +/- 0.052 Mg m(-3). We suggest that the Hugoniot data give evidence of three distinct phases-both low-and high-pressure solid phases and, possibly, a dense molten phase. EOS parameters for these phases are rho(0) = 2.494 +/- 0.002 Mg m(-3), K-S0 = 37 +/- 2 GP a, K' = 6.27 +/- 0.25, and gamma = 1.0(V/V-0) for the low-pressure sol id phase; rho(0) = 3.834 +/- 0.080 Mg m(-3), K-S0 = 128 +/- 20 GPa, K' = 3.7 +/- 1.4, and gamma = 1.5 +/- 0.5 for the solid high-pressure ph ase; and rho(0) = 3.71 +/- 0.10 Mg m(-3), K-S0 = 127 +/- 25 GPa, K' = 2.1 +/- 1.0, and gamma = 1.5 +/- 1.0 for the dense liquid. Transition regions of the Hugoniot cover the ranges of 9-34 GPa for the low-press ure-high-pressure solid transition and 90-120 GPa for the high-pressur e solid-liquid transition. Release paths from high-pressure states, ca lculated from the EOS parameters, suggest that the material remains in the high-pressure solid phase upon release. Release paths frost both the high-pressure solid and liquid fall above the Hugoniot until the H ugoniot enters the low-pressure-high-pressure mixed phase region, when the release paths then cross the Hugoniot and fall below it, ending a t significantly higher zero-pressure densities than that of the low-pr essure phase. The low-pressure release paths fall very close to the Hu goniot. Estimates of residual heat deposition, based on shock-release path hysteresis, range from 20 to 60 per cent of the shock Hugoniot en ergy.