n.
Any of a subgroup of amphibole minerals that are calcium-iron-magnesium-rich and monoclinic in crystal structure.
Hornblende, whose generalized chemical formula is (Ca,Na) 2 (Mg,Fe,Al) 5 (Al,Si) 8 O 22 (OH) 2 , occurs widely in metamorphic and igneous rocks. Common hornblende is dark green to black in colour and usually found in middle-grade metamorphic rocks (formed under medium conditions of temperature and pressure). Such metamorphic rocks with abundant hornblende are called amphibolite s.