Microstrain in pyrope-grossular garnet solid solution at high pressure: a case study of Py₉₀Gr₁₀ and Py₁₀Gr₉₀ up to 15 GPa

Single-phase, well-sintered, translucent polycrystalline garnets with compositions of Py₉₀Gr₁₀ and Py₁₀Gr₉₀ were synthesized at 6 GPa and 1400 °C using a multi-anvil apparatus. X-ray diffraction (XRD) data for these garnet solid solutions were collected with both high-resolution synchrotron X-ray and standard laboratory X-ray sources. Analysis of the FWHM of the XRD peaks using Williamson–Hall plot yields microstrains around 0.12% for Py₉₀Gr₁₀ and 0.09% for Py₁₀Gr₉₀. The FWHM of Py₁₀Gr₉₀ garnet as well as the derived microstrain remains constant up to 11 GPa, followed by a continuous increase to the experimental peak pressure 15 GPa caused by elastic strain in response to deviatoric/anisotropic stresses. The FWHM and microstrain for Py₉₀Gr₁₀ remain constant to the measurement limit at 7 GPa. The microstrain of pyrope-grossular garnets exhibits a nonlinear dependence on composition, showing two peaks near Py₂₀Gr₈₀ and Py₈₀Gr₂₀, which may be associated with local structural heterogeneities arising from Mg and Ca substitution. Using a third-order Birch–Murnaghan equation of state, the bulk modulus is constrained to be Κ₀ = 171.8 ± 2.1 GPa (with Κ₀ ^′ fixed to 5.92) for Py₁₀Gr₉₀ and Κ₀ = 174.3 ± 2.5 GPa (with Κ₀ ^′ fixed to 4.4) for Py₉₀Gr₁₀, both of which are much larger than that for intermediate composition close to Py₅₀Gr₅₀ but comparable to that for their corresponding end members. The relatively larger microstrain and higher bulk moduli for Py₁₀Gr₉₀ and Py₉₀Gr₁₀ garnets could be related to short-range ordering of Mg and Ca cations in garnet structure due to substitution, which results in different local environments for Mg and Ca cations along the pyrope-grossular solid solution.