ACTIVITY MEASUREMENTS AND EXCESS ENTROPY-VOLUME RELATIONSHIPS FOR PYROPE-GROSSULAR GARNETS

Garnets on the pyrope-grossular join have been synthesized, their unit-cell edges measured, and the activity of Ca3Al2Si3O12 component in Pyr85Gr15 determined experimentally at 1000°C. There is no significant departure from ideal volume mixing in the composition range 0 to 21 mole% Ca3Al2Si3O12 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 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 bewteen excess entropy and excess volume in the sanidine-analbite series and similar relationships are anticipated in garnets and other solid solutions. -from Autho