Shock compression and isentropic release of granite
New equation of state data for a weathered granite shocked to about 125 GPa are reported and combined with the Westerly granite data of McQueen, Marsh & Fritz (1967). The shock velocity (Us)-particle velocity (Up) relations can be fitted with two linear regressions: Us = 4.40 + 0.6Up for a range of Up up to about 2 km s−1 and Us = 2.66 + 1.49Up for a range of about 2 to 5 km s−1. The third-order Birch-Murnaghan equation of state parameters are Kos = 51-57 GPa and K′os = 1.4–1.8 for the low-pressure regime and Kos = 251 ± 30 GPa and an assumed K′os = 4 for the high-pressure regime. Compressive waveforms in dry and water-saturated granite were measured at 10–15 GPa using the VISAR technique. The measured wave profiles were successfully modelled using a Maxwellian stress-relaxation material model. Water-saturated granite is characterized by a ˜25 per cent lower yield strength and a ˜75 per cent longer material relaxation time than dry granite.From measurements of partially released states in granite, it is proposed that the high-pressure forms of tectosilicates, including granite, relax isentropically to a metastable, intermediate phase characterized by a dense (about 3.7 g cm−3), highly disordered, six-fold coordinated phase which is subsequently quenched to diaplectic glasses of density −2.3 g cm−3, starting at pressure of −10 GPa. We develop an analytical model to describe the release isentropes in the mixed-phase regime which prescribe release to a glass phase with increasing transformation to the high-pressure phase. Hugoniot and post-shock energies and temperatures derived from the release isentropes are consistent with available data and theoretical expectations for quartz and granite.