Shock wave compression of iron-silicate garnet

Shock wave Hugoniot data have been obtained for almandine-garnet of composition (Fe_(0.79), Mg_(0.14), Ca_(0.04), Mn_(0.03)) Al_2Si_3O_(12) to pressures of >650 kb. The Hugoniot data indicate the onset of a high-pressure phase at 195 ± 20 kb. Equation-of-state systematics and crystal chemical data (stemming largely from analog compounds) suggest that the high-pressure phase occurs in an ‘ilmenitelike’ structure with an initial density of 4.44 ± 0.04 g/cm^3. This value represents an increase of about 6% over the initial garnet density of 4.180 ± 0.005 g/cm^3. The adiabatic bulk modulus K_0^s and its first pressure derivative (∂K^s/∂P)_T were calculated for the high-pressure phase and found to be 3.19 ± 0.39 Mb and 2.6 ± 0.7, respectively. The major source of probable error in these values results from the indicated uncertainty in the initial density of the high-pressure phase. These results strongly suggest that upper mantle minerals are likely to occur in the ilmenite structure over a substantial part of the lower mantle