Single-crystal elasticity of andradite garnet to 11 GPa

The high-pressure elastic properties of single-crystal andradite garnet Ca₃Fe₂³⁺Si₃O₁₂ were determined by Brillouin scattering to 11 GPa. The pressure dependence of the elastic stiffness constants and aggregate bulk and shear moduli were obtained by inversion of the data to finite Eulerian strain equations. The inversion yields C₁₁ = 286.7±0.6GPa, C₁₂ = 88.6±0.6GPa, C₄₄ = 83.8 ±0.3 GPa, K_0s = 154.5±0.6 GPa, G_0s = 89.7±0.4 GPa, (∂K_0T/∂P)_T =4.71±0.1, and (∂G₀/∂P)_T = 1.25 ±0.05. Both individual and aggregate elastic moduli define nearly linear modulus-pressure trends. The elastic anisotropy of andradite garnets increases weakly in magnitude with compression. Previous studies of the high-pressure elasticity of andradite garnet are highly discrepant, with reported pressure derivatives of the bulk modulus varying by 46% and pressure derivatives of the shear modulus varying by 253%. We are able to provide plausible explanations for these discrepancies. In particular, differences between previous x-ray diffraction data and a static compression curve constructed from our Brillouin data can be attributed to the effects of non-hydrostatic stresses on the x-ray data.