A shock-induced phase change in iron-silicate garnet

Abstract

Hugoniot measurements on iron-silicate garnet (Fe_(0.79), Mg_(0.14), Ca_(0.04), Mn_(0.03))_3 Al_2Si_3O_(12) initial density 4.18 g/cm^3, demonstrate that above a shock pressure of 205 ± 20 kbar transition to a high-pressure phase(s), having an apparent zero-pressure density and bulk modulus of 4.44 ± 0.06g/cm^3 and 3.3 ± 0.3 Mbar, occurs. Crystal chemical systematics and Debye-Scherrer X-ray patterns of a recovered phase(s), which may be the shock-induced high-pressure form, suggest possible formation of a phase with a density of∼ 4.48/cm^3. Occurrence of such a polymorph of garnet in the mantle would give rise to an increase in density and seismic velocity below ∼ 600km in the earth.

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