Low-temperature heat capacities of synthetic pyrope,

The heat capacities of synthetic pyrope (Mg,Al,Si,O,,), grossular (CasAI,SisO12), and a solid solution pyrope,,grossular,, (Mg,,sCa,,,Al,Si,O,,) have been measured by adiabatic calorimetry in the temperature range l&350 K. The samples were crystallized from glasses in a conventional pistoncylinder apparatus. The molar thermophysical properties at 298.15 K (J mol’ K‘) are: c; So298 % H’h, &P Pyrope 325.3 1 266.27 47852 Grossular 333.17 260.12 47660 Py&% 328.03 268.32 47990 The values for the end members are significantly greater than those presented previously for natural samples of pyrope. and grossular. The pyrope,Ogrossular 4o 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.