magnetite (loadstone, lodestone) An important rock-forming iron oxide Fe²⁺Fe³⁺₂O₄ (FeO.Fe₂O₃) and member of the spinel group of minerals; belongs to the magnetite series along with magnesioferrite (MgFe³⁺₂O₄), franklinite, jacobsite (MnFe³⁺₂O₄), and trevorite (NiFe³⁺₂O₄); sp. gr. 5.2 (to 4.6); hardness 5.5–6.5; cubic; normally iron-black with a blue tarnish; black streak; sub-metallic lustre; crystals often octahedral and rhombdodecahedral, but it is also massive and granular; cleavage poor, octahedral; strongly magnetic; found extensively in magmatic rocks associated with basic rocks, in pegmatites, in contact metasomatic zones (especially in limestones in regionally metamorphosed rocks due to the dehydration of iron hydroxides), and also in black sands associated with gold gravels.

magnetite Iron oxide mineral (Fe₃O₄). It is a valuable iron ore, found in igneous and metamorphic rocks. It is black, metallic and brittle. Permanently magnetized deposits are called lodestone. Hardness 6; r.d. 5.2.

mag·net·ite / ˈmagnəˌtīt/ • n. a gray-black magnetic mineral that consists of an oxide of iron and is an important form of iron ore.