Amphiboles have long been key players in metamorphic petrology. The concept of facies was developed for rocks of basaltic composition (Eskola 1920, 1939; Fyfe et al. 1958), and within this concept, amphiboles had a leading role. The first appearance of amphiboles, the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene-bearing assemblages, mark a number of major facies boundaries. The transition from greenschist to amphibolite facies is marked, in part, by hornblende replacing actinolite, and the granulite facies is defined by the breakdown of hornblende to orthopyroxene-bearing assemblages. The...

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