Low-temperature heat capacities of synthetic pyrope, grossular, and pyrope60grossular40

Abstract

The heat capacities of synthetic pyrope (Mg3Al2Si2O12), grossular (Ca3Al2Si3O12) and a solid solution pyrope60grossular40 (Mg1.8Ca1.2Al2Si3O12) have been measured by adiabatic calorimetry in the temperature range 10-350 K. The samples were crystallized from glasses in a conventional piston-cylinder apparatus.The molar thermophysical properties at 298.15 K (J mol-1 K-1) are: The values for the end members are significantly greater than those presented previously for natural samples of pyrope and grossular. The pyrope60grossular40 exhibits an excess heat capacity below 120 K; the maximum occurs at 45-50 K. The anomalously high heat capacity of pyrope at low temperatures and the excess heat capacity of the solid solution are apparently caused by the unusually large, eight-fold coordination of Mg2+ in the garnet structure.