AMBIENT TEMPERATURE P-V EQUATION OF STATE FOR GARNET SAND OF PFEIFFER BEACH

Synchrotron powder X-ray diffraction analysis was performed on a natural beach-sand garnet from Pfeiffer Beach, located in Big Sur, California. The ambient-pressure lattice parameter, a, is 11.529 Å. The lattice parameter and, accordingly, unit-cell volume were determined through isothermal compression to 19 GPa in a symmetric diamond anvil cell, in which helium was used as a nearly-hydrostatic pressure-transmitting medium. The isothermal bulk modulus, K_0T, was found to be 167.8 ± 2.7 GPa, using a second-order (i.e. dK/dP fixed at 4) Birch-Murnaghan equation of state. These results are consistent with previous elasticity measurements on a broad range of garnet compositions, and we will present our measurement in relation to prior work on bulk modulus - volume - cation valence systematics (e.g. Hazen et al., 1994).