Shock wave compression of ironsilicate garnet

Shock wave Hugoniot data have been obtained for almandine-garnet of composition (Fe•.7•, Mgo.14, Ca•.•, Mno.•) Al,iaO• to pressures of >650 kb. The Hugoniot data indicate the onset of a high-pressure phase at 195 ñ 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 ñ 0.04 g/cm 8. This value represents an increase of about 6% over the initial garnet density of 4.180 ñ 0.005 g/cm 8. The adiabatic bulk modulus Ko s and its first pressure derivative (OKs/OP)• were calculated for the highpressure phase and found to be 3.19 ñ 0.39 Mb and 2.6 ñ 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.