A group of dark rock-forming silicate minerals, closely related in crystal form, and composition and having the general formula : ABSi2 O6 (...) Colors range from white to dark green or black. Pyroxenes may crystallize in the orthorhombic or monoclinic systems; they constitute a common constituent of igneous rocks, and are similar in chemical composition to the amphiboles (amphibole) (except that the pyroxenes lack hydroxyls).