magnetite, also called lodestone, or magnetic iron ore, Mineral Information Instituteiron oxide mineral (FeFe2O4, or Fe3O4) that is the chief member of one of the series of the spinel group. Minerals in this series form black to brownish, metallic, moderately hard octahedrons and masses in igneous and metamorphic rocks and in granite pegmatites, stony meteorites, and high-temperature sulfide veins. The magnetite series also contains magnesioferrite (magnesium iron oxide, MgFe2O4), franklinite (zinc iron oxide, ZnFe2O4), jacobsite (manganese iron oxide, MnFe2O4), and trevorite (nickel iron oxide, NiFe2O4). All are magnetic, although franklinite and jacobsite are only weakly so; magnetite, which frequently has distinct north and south poles, has been known for this property since about 500 bc. For detailed physical properties, see oxide mineral (table).