The concept that almandine-rich garnet is an anomalous phase in thermal metamorphic assemblages is based partly upon the rarity of almandine in hornfelses, and partly upon evidence of breakdown of ‘regionally’ formed garnets when their host-rocks have been thermally metamorphosed. Where amphibolite facies regional gneisses have been re-metamorphosed within the pyroxene-hornfels facies conditions of the late Caledonian Lochnagar aureole, garnet of composition Alm80Py11p6 (Gr+And)4 has reacted with biotite, sillimanite, and muscovite to give cordierite-rich pseudomorphs; where armoured from reaction by immersion in quartz or feldspar, no signs of dissolution are seen. In some totally reconstituted hornfelses, almandine garnet of composition AIm80P13Sp4 (Gr+And)3 appears to have coexisted stably with cordierite and orthoclase. It is concluded that the ‘unstable’ garnet was breaking down not because its P/T stability field had been exceeded, but because the alman-dine-bearing rock-composition field in the thermal (pyroxene-hornfels) facies was more restricted than that in the regional (amphibolite) facies. Chemical data from the hornfelses suggest that this restriction was mainly due to the stability of cordierite—the plane spinel-cordierite-quartz restricts garnet to those rocks with effective mol. (FeO+MgO-t-MnO) A12O3>1, while within that range cordierite-biotite tie-lines restrict garnet to rocks of high (FeO+MnO)/MgO ratio (Fig. 4b). Recorded instances of garnet ‘instability’ in thermal aureoles show similar features to the Lochnagar aureole—garnet breaks down by reaction; unequivocal instances of isochemical breakdown are rare. This, combined with the widespread occurrence of almandine in volcanic and plutonic igneous rocks, suggests that almandine is a physically stable phase not only in the hornblende-hornfels facies, but also in the pyroxene-hornfels facies and possibly in portion of the sanidinite facies as well. The rarity of almandine in thermal aureoles is the result of its very narrow rock composition field under the P/T conditions of such environments. Comparison of thermal and granulite facies garnet-cordierite assemblages suggests that P and T modify the almandine-bearing rock composition field mainly by modifying the limiting Mg/Fe ratio of garnet and the limiting Fe/Mg ratio of cordierite (Fig. 10). The wide rock-composition field of almandine in the amphibolite facies may be contingent upon the inhibition of cordierite by hydrous minerals under relatively high partial pressures of water.