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Published November 1969 | Published
Journal Article Open

Equations of State and Crystal Structures of High-Pressure Phases of Shocked Silicates and Oxides


Shock-wave data are now available for a variety of rocks, minerals, and oxides of geophysical interest in the pressure range appropriate for the lower mantle. These data are analyzed to obtain equation-of-state parameters with emphasis on the shock-induced high-pressure phases. Of twenty-four materials for which Hugoniot data are analyzed, all but MgO, Al_2O_3, and MnO_2 undergo at least one shock-induced phase change below 800 kb. Birch-Murnaghan parameters for the raw Hugoniots, metastable Hugoniots, adiabats, and 25°C isotherms are obtained for the high-pressure phases. On correcting the raw Hugoniot data for MgO and Al_2O_3 for strength effects, we find that the calculated adiabatic equations of state are in good agreement with recent ultrasonic data. The zero-pressure densities of high-pressure phases are obtained by constraining the adiabats calculated from the Hugoniot data such that the zero-pressure densities and the zero-pressure slopes of the adiabats satisfy Anderson's seismic equation of state. Probable crystallographic structures of the high-pressure phases are inferred from the classical laws of crystal chemistry and, in some cases, from static high-pressure recovery experiments on analog compounds. Shock data for SiO_2 (stishovite) indicate that transformation to the fluorite-type structure (observed in TiO_2) does not take place under shock, at least to ∼2000 kb. Fe_2O_3 probably transforms to either the perovskite or B rare earth structure with a zero-pressure density of 5.96 g/cm^³. MgAl_2O_4 (spinel) may transform to the CaFe_2O_4 structure with a zero-pressure density of 4.19 g/cm_³. Feldspars transform to the hollandite structure (density, of ∼3.85 g/cm^³). Olivine-rich rocks containing greater than 10% FeO either disproportionate to the ilmenite and rock salt structure or transform to a new post-spinel polymorph having the Sr_2PbO_4 structure. Pyroxenes containing greater than 10% FeO probably transform to the ilmenite structure. High-pressure forms of sillimanite and andalusite have calculated densities of 4.00 and 3.95 g/cm^³, respectively. This probably represents disproportionation reaction products, Al_2O_3 + SiO_2 (stishovite), which would give a density of 4.09 g/cm^³. The Birch-Murnaghan second-order parameter ξ is nearly zero for MgO and Al_2O_3. 0.73 for stishovite, and ∼1 for the high-pressure phases of the olivines and pyroxenes. The values of K′ = dK/dP are calculated along the Hugoniots and adiabats and are found to decrease at a rate of −0.5 to −1.6 cm³/g when the density is increased either by compression or by iron substitution.

Additional Information

© 1969 by the American Geophysical Union. Manuscript Received: 3 Oct 1968. We appreciate receiving copies of papers prior to publication from N. Mao, O. Anderson, E. Schreiber, D. Chung, and N. Soga. This research was supported in part by the Defense Atomic Support Agency under contract DASA 01-06-C-0069.

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