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.