THE ELASTIC-CONSTANTS OF A PYROPE-GROSSULAR-ALMANDINE GARNET TO 20 GPA

Laser-induced phonon spectroscopy has been applied to the determinatio n of the elastic constants and equation of state at 25 degrees C of a natural garnet of composition Py(51.6)Al(31.7)Gr(15.8)Sp(0.9) to a pre ssure of 20 GPa. Moduli are linear with pressure. The adiabatic bulk m odulus in GPa is given by K = 170.8 + 4.09 P and the shear modulus is mu = 94.7 + 1.76P. The derivative of the shear modulus with respect to temperature at constant density is derived from the temperature and p ressure dependencies of the elastic constants. The constant pressure d erivatives of the mean longitudinal (V-p) and shear (V-s) velocities a re tabulated. At 298K and 1bar (partial derivative 1n V-s/partial deri vative 1n rho)(p) = 1.7, (partial derivative 1n V-s/partial derivative 1n rho)(p) = 1.5 and (partial derivative 1n V-s/partial derivative 1n V-p)(p) = 0.9. The corresponding values calculated at 1000 K and 20 G Pa are 1.6, 1.7 and 1.1.