On the Compression of Stishovite

Abstract

Summary Currently available shock-wave compression data for single-crystal α-quartz and pore-free quartzite have been used to determine values for the isentropic bulk modulus of stishovite and its first pressure derivative. The shock-wave data reduction scheme makes use of the observed linearity in the shock-velocity-particle-velocity field over the pressure range 0.4–2.0 Mbar and the first-order Murnaghan equation of state. In addition, an accurate form of the temperature independent Gruneisen parameter (γ), consistent with the linear Us–up relation, has been used in the reduction of the fundamental shock-wave data to the metastable Hugoniot. The temperature derivative of the isentropic bulk modulus is provided by a least-square fit of the γ relation to available Hugoniot data directly measured on porous and fused-quartz samples in the high-pressure regime. The pertinent results are: KS= 3.35±0.19 Mbar; (∂KS/∂P)T= 5.5±0.6; and (∂KS/∂T)p= (−0.35±0.08)× 10−3 Mbar/°K. These results are consistent with recent static-compression and ultrasonic measurements on stishovite and are related systematically to corresponding data for isostructural GeO2 and TiO2. Comparison of the present results with recent shock-wave analyses by other investigators suggests that the first-order Murnaghan form of the equation of state is more appropriate than the first-order Birch equation in reproducing the compression of stishovite in the zero to 2 Mbar pressure range. An evaluation of the composition of the lower mantle in terms of the fundamental oxides, FeO, MgO, and SiO2, using the present results for the elastic properties of stishovite, support marginally the conclusions of previous investigators regarding enrichment of FeO and SiO2 relative to the upper mantle; however, such an interpretation is predicated on the mixed oxide assumption and should be considered in relation to alternative models.