Shock temperature measurements in Mg_2SiO_4 and SiO_2 at high pressures

Temperatures in the high pressure shock state have been determined by measurement of optical radiation from pure samples of forsterite (Mg_2SiO_4), α-quartz, and fused silica. Shock waves of known amplitude were produced by tantalum flyer impact using a two-stage light gas gun. Shock pressures in the ranges 150-175 GPa and 70-115 GPa for Mg_2SiO_4 and SiO_2 respectively were achieved, and temperatures in the range 4500-6800 K were measured. The observed temperatures in Mg_2SiO_4 are consistent with the occurrence of a shock-induced phase transition with a transition energy of ∼ 1.5 MJ/kg. Measured Hugoniot temperatures versus pressure in both fused and crystalline SiO_2 shocked to the stishovite regime suggest the occurrence of a previously unknown transition, beginning at pressures of approximately 107 GPa and 70 GPa for α-quartz and fused quartz, respectively. The energies and temperatures appear to be consistent with the onset of melting of stishovite under shock loading.