Activity Measurements and Excess Entropy-Volume Relationships for Pyrope-Grossular Garnets

Abstract

Garnets on the pyrope-grossular join have been synthesized, their unit-cell edges measured, and the activity of component in determined experimentally at 1000°C. There is no significant departure from ideal volume mixing in the composition range 0 to 21 mole% component. When the cell edge data as a whole are considered, however, a small asymmetric positive deviation is obtained. The compositions of garnet solid solution coexisting with anorthite, kyanite, and quartz were reversed at 15 kb/1000°C and 16 kb/1000°C in the presence of lead-silicate flux. The end-member reaction: 3 Anorthite = Grossular + 2 Kyanite + Quartz was determined to lie between 23.0 and 23.3 kb at 1050°C and . From these data an activity coefficient (on a 1 site basis) of between 1.7 and 2.05 is obtained for . This translates to an apparent symmetric excess free energy parameter, . Comparison of the experimental results with enthalpy data indicates an apparent symmetric excess entropy parameter, Ws (at ), of gm.atom, above the value of 0.0 but less than the value of 6.3 J/K.gm.atom based on heat capacity measurements on a synthetic garnet of composition . The data strongly suggest that excess entropies and volumes of pyrope-grossular garnets are asymmetric, with near-zero values at the pyrope end and substantial positive values at the grossular end of the series. Although the exact magnitude of any asymmetry is uncertain, it has been found that a good correlation exists between excess entropy and excess volume in the sanidine-analbite series and similar relationships are anticipated in garnets and other solid solutions.