Revised shock‐wave equations of state for high‐pressure phases of rocks and minerals

Shock-wave data for the high-pressure phases of a number of rocks and minerals have been reanalyzed using a revised seismic equation of state to constrain the zero-pressure properties of the high-pressure phases. The anomalously low values of dK/dP resulting from a previous analysis are thereby removed. The inferred zero-pressure densities of the high-pressure phases are reduced by an average of 4%, and the values of the zero-pressure seismic parameter Φ0 are reduced by up to 30%. bringing them into approximate agreement with the hypothesis of the molar additivity of Φ. For most of the materials considered, the derived pressure trajectories of density versus the seismic parameter Φ are consistent wth shock-wave data on such materials as MgO, Al2O3, and SiO2 (stishovite) where no zero-pressure assumptions are required. Iron-rich compounds may require further revision. It is demonstrated that in poorly constrained cases the Birch-Murnaghan equation can produce a singularity in dK/dP at high pressure. Possible crystal structures of the high-pressure phases are considered using the revised zero-pressure densities. It seems likely that olivines with less than about 10-mole % FeO content can transform to a phase significantly denser than the isochemical mixture of oxides, in contrast to olivines with higher iron content. The possibility that electron spin transitions occur in iron-rich compounds is considered, but no strong evidence has been obtained. The derived zero-pressure densities of the high-pressure phases are usually not of sufficient accuracy to distinguish between all alternative structures, but in some cases an alternative structure to that previously chosen is preferred here.