Amphibolite

A metamorphic rock composed mainly of amphibole minerals, especially hornblende and actinolite, together with plagioclase feldspar and little or no quartz. It typically forms from the metamorphism of mafic igneous rocks such as basalt, though some amphibolites derive from chemically similar sedimentary rocks, particularly where a nearby granitic intrusion has driven contact metamorphism. The rock is usually dark, dense and only weakly foliated, often showing a salt and pepper texture from alternating light feldspar and dark amphibole grains, and it can carry subordinate minerals such as epidote, zoisite, chlorite, quartz and titanite depending on the chemistry of its parent rock. Geologists distinguish amphibolite formed from basalt, called ortho-amphibolite, from amphibolite formed from a sedimentary parent, called para-amphibolite, based on differences in mineral assemblage.