There are experiments on spessartine by Hsu and low T phase equilibria
on grossular by Liou, among others. To my knowledge no one has addressed
the effect of these solid solutions systematically, although it is clear
from field evidence that spessartine and grossular solid solutions
generally extend the stability of garnets to lower T in metapelites and to
lower P in granitoids relative to those for almandine. One needs to
consider the solid solutions and the associated phases, especially chlorite
in the case of metapelites. Holland and Powell (1998) could be used for
phase equilibria and/or P-T estimates.
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