iron oxide mineral (FeFe₂O₄, or Fe₃O₄) that is the chief member of one of the series of the spinel (q.v.) 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).