A method is proposed for estimating the zero-pressure parameters of the high-pressure, high-temperature phases formed by intense shock loading of rocks and minerals. The method involves an empirical relationship between the zero-pressure mean molar volume and the slope at the base of the P-V curve. Equations of state are fitted to shock-wave data for eighteen rocks and minerals. Most of the materials collapsed to a denser phase or assemblage of phases when shocked to sufficiently high pressure. If a phase change occurs, parameters of the high-pressure phase are found for a range of ρ_0 the raw Hugoniot and an estimated metastable Hugoniot. The polymorphic transitions involve a considerable reduction in volume, ranging from 33 to 49% for feldspar and quartz-rich rocks such as albitite, anorthosite, and granite, 20% for such basic rocks as diabase and dunite, and about 12% for some dense already closely packed minerals such as spinel, hematite, and magnetite. The parameter (dK/dP)_0, which is related to the Grüneisen ratio, is found to decrease across phase changes and upon iron substitution
