Elasticity of the pyrope (Mg3Al2Si3O12)-majorite (MgSiO3) garnets solid solution

Dense isotropic polycrystalline specimens of ich garnets (Py-100, Py(62)Mj( 38), Py(50)Mj(50), Py(21)Mj(79) and Mj(100)) along the pyrope (Mg3Al2-Si3O1 2 = Py-100)-majorite (MgSiO3 = Mj(100)) join were fabricated in a 2000-ton uniaxial split-sphere anvil apparatus (USSA-2000) at pressures from 10 to 1 8.5 GPa and temperatures from 1200 to 1850 degrees C, within their stabilit y fields in runs of 2-4-h duration, using hot-pressing techniques developed by Gwanmesia et al. (1993). These specimens are single-phased, fine-graine d (less than or equal to 5 mm), free of microcracks, and have bulk densitie s greater than 99% of the corresponding single-crystal X-ray density. Elast ic compressional (P) and shear (S) wave velocities were determined at room pressure and temperature for these polycrystalline garnet specimens by phas e comparison ultrasonic interferometry. For Mj(100), the P and S wave veloc ities are within 1% of the Hashin-Shtrikman averages calculated from the si ngle crystal elastic moduli measured by Brillouin spectroscopy. Both the el astic bulk modulus (K) and the shear modulus (G) decrease continuously with increasing majorite content from pyrope garnet (Py-100) to pure majorite g arnet (Mj(100)) The compositional dependence of K and G are given by K = 17 2.3 (40) - 0.085X, and G = 91.6 (10)- 0.038X, where X = mol% majorite), res pectively, indicating that substitution of Si for Mg and Al decreases both K and G by about 5% along the solid solution series.