Elastic Moduli of Precompressed Pyrophyllite

Chao and Ruoff [*] have shown previously that by precompressing pyrophyllite prior to its use in high-pressure applications, one obtains a material which has a uniform density and uniform mechanical properties. We have recently studied the propagation of u trasonir nplIse~ in pyrnphyllit.p specimens 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 [*] C. C. Chao and A. L. Ruoff, Rev. Sci. Instr., 41, 720-721 (1970).