PYROXENES AND OLIVINES: STRUCTURAL IMPLICATIONS OF SHOCK-WAVE DATA FOR HIGH PRESSURE PHASES

A reexamination of Hugoniot equation of state data and three new release adiabat points indicates that results for enstatite-bronzite composition pyroxene are compatible with its transforming to a perovskite phase at high pressure (For En_(90): ρ_0 = 4.20 g/cm^3, K_0 ~ 2.6 ± 0.35 Mbar, K'_0 ~ 3.5 ± .65). The release adiabat data, as well as results from porous samples, imply that the shock-wave data do not define an equilibrium, high-pressure phase Hugoniot below about 1.00 Mbar. These also suggest a further transformation to a phase (or assemblage) with density about 5% (or more) greater than that of or thorhombic perovskite. The data would allow such a transformation to occur at pressures as low as 0.60 Mbar under shock, representing an upper bound for the equilibrium transition pressure. Hugoniot data on magnesian olivines also appear to represent states of thermodynamic disequilibrium or a mixed-phase region below about 0.80-1.00 Mbar. However, Hugoniot points for Mgpyroxene and Mg-olivine coincide at pressures above 0.70 Mbar, suggesting that these minerals transform to high-pressure phases (or phase assemblages) of comparable density. Since MgO (presumably as periclase or in a closely related structure) attains relatively low densities at these pressures, the shock- wave data are in strong disagreement with the disproportionation of Mg_2SiO_4 to a MgSiO_3 (perovskite) + MgO assemblage above 0.80-1.00 Mbar. Conversely, the shock- wave data do not preclude a transformation of the type Mg_2Si_2O_6 → Mg_2SiO_4 ("post- perovskite" phase) + SiO_2 (rutile or fluorite structure). Again, these results would imply polymorphism to very dense "post- perovskite" phases. Based on the arguments made for pyroxene, such a transformation could occur at pressures as low as 0.60 Mbar under equilibrium conditions. We note that the combined results of high-pressure experiments allow Mg-pyroxene compositions to be as likely candidates for the lower mantle as olivine.