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.
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