Powder X ray diffraction study of synthetic pyrope-grossular garnets between 20 and 295 K

The unit cell parameters of synthetic pyrope (Mg3Al2Si3O12) - grossular (Ca3Al2Si3O12) and five pyrope-grossular solid solutions have been measured between 20 and 295 K using X ray powder diffraction methods and their linear thermal expansions have been determined. The low-temperature thermal expansion data, together with existing thermal expansion data between 295 K and 1000 K on synthetic end-member pyrope and grossular, were compared to the heat capacities of garnets along this join. The thermal expansion of pyrope is greater than that of grossular over the whole temperature range from 20 to 1000 K, whereas the Cp of pyrope is only greater than that of grossular up to about 200 K. It was observed that the change in the thermal expansion of pyrope with respect to temperature is greater than that of grossular below 295 K. The thermal expansions of the solid solution compositions show a temperature dependence intermediate between the two end-members. The volumes of mixing along the join show positive asymmetric deviations from ideality. A two-parameter Margules fit to the data, incorporating recently published data on this join, give W-Ca-Mg(V) = 0.12 +/- 0.26 cm(3)/mol and W-Mg-Ca(V) = 1.84 +/- 0.28 cm(3)/mol at 295 K. No measurable changes are observed in the mixing behavior below 298 K. These new volume of mixing data are in good agreement with another recent determination and remove much of the uncertainty on the mixing properties of pyrope-grossular solid solutions. They will enable refinements of important petrologic geobarometers, which are widely used in pressure determinations of rocks of the lower crust and upper mantle. The change in molar volume of garnet can be interpreted using a model of rigid tetrahedral rotation. It is proposed that phonons related to rigid SiO4-tetrahedral and X site cation translations play an important role in the low-temperature thermodynamic behavior of garnet.