A group of dark rock-forming ferromagnesian silicate minerals, closely related in crystal form and composition and having the general formula A2-3 B5 (Si,Al)8 O22 (OH)2. (...) Colors range from white to black. Most amphiboles crystallize in the monoclinic system, some in the orthorhombic. They constitute an abundant and widely distributed constituent in igneous and metamorphic rocks (some are wholly metamorphic), and they are analogous in chemical composition to the pyroxenes (pyroxene).