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amphibolite A medium-grained, dark-coloured, regional metamorphic rock composed of hornblende and plagioclase with minor epidote, sphene, biotite, and quartz. The rock may show a well developed planar or linear alignment of elongate hornblende crystals as a result of suffering deformation at the same time as regional metamorphism. These alignments define fabrics within the rock known as schistosity and lineation respectively. Amphibolites are formed by medium-grade metamorphism of basic igneous rocks such as basalts, both extrusive and intrusive types.