Elastic moduli of precompressed pyrophyllite used in ultrahigh-pressure research

Abstract

The propagation of ultrasonic pulses in pyrophyllite specimens was studied to investigate the effect of specimen precompression on the measured elastic moduli. Measurements were made at room pressure and, for the precompressed specimens, to pressures of 3 kbar. We find pyrophyllite to be elastically anisotropic, apparently the result of the fabric present in our material. The room-pressure adiabatic bulk modulus as measured on specimens made of isostatically compacted powdered pyrophyllite was determined to be 96.1 kbar. We found the wave speeds of ultrasonic pulses in pyrophyllite to decrease with increasing specimen precompression. A limiting value of precompression was found, above which no further decrease in wave speed is observed. For the shear-wave speeds this occurs at 10 kbar, while for the longitudinal wave at 25 kbar. In the limit, the shear waves propagate 20% slower than in the unprecompressed samples; for the longitudinal wave the difference is 30%. The change in bulk modulus resulting from increasing pressure shows a behavior which can be approximated by a bilinear curve with decreasing slope, the discontinuity occurring between 1.6 and 1.8 kbar. In the second portion of the curve, the bulk modulus derivative is determined to be 22.1. We believe that this value indicates the presence of voids which are closing under pressure. A regression analysis of the data shows that a P1/2 equation appears to give the best fit. The resulting empirical law for the bulk modulus derivative at pressure P becomes: dB/dP=[CL0(0.251P−0.147)−1/2 −CS0(2.76P−2.56)−1/2]×10−1, where P is in kbar and in the present measurements CL0, the longitudinal modulus, is 204 kbar and CS0, the average shear modulus, is 113 kbar.