Pyroxene, a group of silicates of magnesium, calcium, iron, and sodium, with or without aluminium, so named from their infusibility. Though almost identical in composition with hornblendes, they differ in not being pleochroic, and seem to have resulted from more rapid cooling. Some of them, such as augite (q.v.) and diallage (q.v.), crystallise in the Oblique system, but in forms unlike those of the hornblendes. Others, such as hypersthene (q.v.), enstatite (q.v.), and bronzite (q.v.), belong to the Prismatic system, and are known collectively as Rhombic pyroxenes. Pyroxenes occur in diabase, basalt, gabbro, and other basic lavas, seldom associated with either quartz or orthoclase.