HOT-PRESSING AND CHARACTERIZATION OF POLYCRYSTALS OF BETA-MG2SIO4 FOR ACOUSTIC VELOCITY-MEASUREMENTS

Hot‐pressing techniques have been developed for a 2000‐ton uniaxial split‐sphere apparatus (USSA 2000) to fabricate polycrystalline specimens of high‐pressure phases of mantle minerals at pressures up to 18 GPa and temperatures up to 1400°C. Cylindrical specimens (3 mm in diameter and 2–3 mm long) of the beta‐phase of Mg2SiO4 were synthesized using a NaCl cell and recovered at ambient conditions by simultaneously decompressing and cooling along a computer‐controlled P‐T path designed to preserve the high‐pressure phase and anneal strains in the polycrystalline aggregates. These specimens are single‐phased, fine‐grained (≤ 5 micron), free of microcracks and preferred orientation and have bulk densities greater than 99% of x‐ray density. Ultrasonic interferometry measurements of the P‐wave and S‐wave velocities demonstrate that the polycrystals are isotropic and exhibit velocities within 2% of the Hashin‐Shtrikman averages calculated from the single‐crystal elastic moduli. The successful fabrication of these high‐quality polycrystalline specimens makes feasible experiments to determine the pressure dependence of the acoustic velocities for high‐pressure phases of mantle minerals. Copyright 1990 by the American Geophysical Union.