Elasticity of a grossular‐pyrope‐almandine garnet

The single‐crystal elastic properties of a natural, grossular‐rich garnet solid solution have been measured at ambient conditions using Brillouin spectroscopy. This garnet sample is unusual for a grossular in that it contains appreciable pyrope and almandine components, with a composition of Gr48.4Py27.6Alm23.4Sp0.6. It is thus suitable for investigating the variation of elastic properties with composition in Ca‐rich garnet solid solutions that are stable only at high pressure. The values obtained for the bulk modulus, K = 170.4(6), and for the shear modulus, μ = 101.6(3), of the garnet examined in this study, vary by less than 1% from those obtained from a weighted sum of the end‐member moduli. This suggests that any composition in the grossular‐pyrope‐almandine system, which comprise the majority of upper mantle garnets, is accurately estimated with this simple approach. More complex elasticity‐composition variations observed for the grossular‐andradite garnets may be specific to garnet solid solutions where the cation substitutions occur at six‐coordinated crystallographic sites.