An isometric mineral, 8(Fe₂₊Al₂O₄) ; spinel group; forms series with magnesian spinel, with gahnite, and with chromite; massive or fine grained; Mohs hardness, 7.5 to 8; in metamorphosed argillaceous sediments with andalusite, sillimanite, or garnet; also in contact metamorphic deposits in limestone or marble; in some eruptive mafic rocks; may occur with corundum to form emery.