ELASTICITY OF STISHOVITE AT HIGH-PRESSURE

The elastic-wave velocities of stishovite, the rutile-structured polym orph of SiO2, were measured to 3GPa at room temperature in a piston cy linder apparatus using ultrasonic interferometry on polycrystalline sa mples. These polycrystalline samples (2-3mm in length and diameter) we re hot-pressed at 14GPa and 1050 degrees C in a 2000 ton uniaxial spli t-sphere apparatus (USSA-2000) using fused silica rods as starting mat erial. They were characterized as low porosity (less than 1%), single phase, fine grained, free of cracks and preferred orientation, and aco ustically isotropic by using density measurement, X-ray diffraction, s canning electron microscopy, and bench-top velocity measurements. On t he basis of subsequent in situ X-ray diffraction study at high P and T on peak broadening on similar specimens, it is evident that the singl e crystal grains within these polycrystalline aggregates are well equi librated and that these specimens are free of residual strain. P- and S-wave velocities measured at 1 atm are within 1.5% of the Hashin-Shtr ikman bounds calculated from single-crystal elastic moduli. Measured p ressure derivatives of the bulk and shear moduli, K'(0) = 5.3 +/- 0.1 and G'(0) = 1.8 +/- 0.1, are not unusual compared with values measured for other transition zone phases such as silicate spinel and majorite garnet. Isothermal compression curves calculated with the measured va lues of K-0 and K'(0) agree well with experimental P-V data to 16 GPa. The experimental value of dG/dP is in excellent agreement with predic tions based on elasticity systematics, Theoretical models are not yet able to replicate the measured values of K'(0) and G'(0).