a common mineral embracing many varieties varying in color and in composition. it occurs in monoclinic crystals; also massive, generally with fibrous or columnar structure. the color varies from white to gray, green, brown, and black. it is a silicate of magnesium and calcium, with usually aluminium and iron. some common varieties are tremolite, actinolite, asbestus, edenite, hornblende (the last name being also used as a general term for the whole species). amphibole is a constituent of many crystalline rocks, as syenite, diorite, most varieties of trachyte, etc. see hornblende.