PYROPE GARNET—PIEZOMETER OR THERMOMETER?
Use of the pyrope content of garnet as a piezometer should be restricted to an estimate of the minimum pressure of formation. Reactions leading to the growth of garnet inevitably increase the pyrope/almandine ratio, but do so with temperature increase in some cases and pressure increase in others. The volume behavior of Fe-Mg exchange reactions between garnet and other common silicates indicates that, for given bulk compositions, the pyrope/almandine ratio will decrease with pressure.