Summary: The chemical factors controlling the rare appearance of garnet in Connemara amphibolites are elucidated, including the importance of low oxidation ratio (w), low Mg/(Mg+Fe+Mn) (mg), and high MnO. Pressure is believed to be critical in determining whether garnet is common in amphibolites in any one terrain and it is suggested that the ratio mg × w/MnO (called g) in serial analyses of garnetiferous amphibolites can be used as a measure of the pressure of metamorphism.
89 Connemara amphibolite analyses with plagioclase and hornblende are considered including 31 new analyses, which are reported. Of the 89 analyses, 10 contain garnet, 6 cummingtonite, 16 clinopyroxene including 3 with garnet and clinopyroxene and 2 with garnet and cummingtonite. The coexisting garnet and hornblende in 3 amphibolites have been analysed, including one sample in which brown-green and green hornblende, cummingtonite, plagioclase, garnet, and ilmenite have been analysed. Cummingtonite does not coexist with clinopyroxene or sphene and its presence is controlled by pancity of Ca as well as medium to low pressure in metamorphism.