mafic UK /ˈmafɪk/US /ˈmeɪfɪk/adjective 1noun 1forms 3 adjective1 •Describing rocks, such as silicate minerals, magmas, and volcanic and intrusive igneous rocks, which contain relatively high concentrations of magnesium and iron.Much of the southern half of the southeastern body is a mafic syenite, slightly more felsic than the shonkinite. — 1954 noun1 •A rock with such properties. formsmafics · more mafic · most mafic