Adiabatic release measurements in α-quartz between 300 and 1200 GPa: Characterization of α-quartz as a shock standard in the multimegabar regime

a-quartz has been used prolifically in recent years as an impedance matching standard in the multimegabar regime (1 Mbar = 100 GPa). This is due to the fact that above similar to 90-100 GPa along the principal Hugoniot alpha-quartz becomes reflective, and thus shock velocities can be measured to high precision using velocity interferometry. This property allows for high-precision measurements, however, the accuracy of impedance matching measurements depends upon the knowledge of both the Hugoniot and the release or reshock response of alpha-quartz. Here, we present the results of several adiabatic release measurements of alpha-quartz from similar to 300-1200 GPa states along the principal Hugoniot using full density polymethylpentene (commonly known as TPX), and both similar to 190 and similar to 110 mg/cc silica aerogel standards. These data were analyzed within the framework of a simple, analytical model that was motivated by a first-principles molecular dynamics investigation into the release response of alpha-quartz. Combined, this theoretical and experimental study provides a method to perform impedance matching calculations without the need to appeal to any tabular equation of state for alpha-quartz. As an analytical model, this method allows for propagation of all uncertainty, including the random measurement uncertainties and the systematic uncertainties of the Hugoniot and release response of alpha-quartz. This work establishes alpha-quartz for use as a high-precision standard for impedance matching in the multimegabar regime. We also note that the experimentally validated model framework should prove to be useful in the development of wide range equations of state for silica, a major constituent in the Earth's crust and mantle. Such models are crucial for accurate simulations of high-velocity giant impacts that are thought to be prevalent in the final stages of terrestrial planet formation.