The most common mineral of the amphibole group, a complex hydrous aluminum silicate in which calcium, sodium, iron, and magnesium are present in varying amounts; commonly black, dark-green, or brown and occurring in distinct long, slender crystals or in columnar, fibrous, or granular forms. It is a primary constituent of many felsic and intermediate igneous rocks, and, while less common, in mafic igneous rocks, and it is a common constituent in metamorphosed mafic rocks, such as amphibolite.