magnetite(redirected from Ferrous ferric oxide)
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a mineral, a complex oxide of composition FeO.Fe2O3, containing 31 percent FeO and 69 percent Fe2O3 (a total of 72.4 percent Fe), and frequent admixtures of MgO, Cr2O3, Al2O3, MnO, and ZnO. Magnetite is a ferrite and has the inverse spinel structure. It crystallizes in the isometric system a0 = 8.3963 Å, generally forming octahedral, more rarely dodecahedral, crystals and granular aggregates. Magnetite rarely occurs in the form of colloform aggregates.
Synthesized magnetite is widely used in industry. It is usually obtained by synthesis in the solid phase following the combined roasting of pressed FeO and Fe2O3 powders at temperatures of 1000-1400°C
Magnetite is brittle with an uneven fracture; it exhibits no cleavage. It has a hardness of 5.5-6 on Mohs’ scale and a density of 4,800-5,300 kg/m3. A black mineral, it is opaque, and has a semimetallic, sometimes dull, luster. A good conductor of electricity, magnetite is classified as a ferrimagnet owing to its magnetic properties. Its magnetization is determined by the difference in magnetic moments of the mineral’s two magnetic sublattices: (1) composed of Fe2+ and Fe3+ ions, located at the octahedral points, and (2) composed of Fe3+ ions, located at the tetrahedral points. At room temperature, the saturation magnetization of magnetite is Js = 4.8 ×102 tesla (480 gauss). The coercive force Hc of natural magnetite depends on the admixture content, while that of synthesized magnetite is conditioned by the method used to obtain the magnetite. In magnetite powders, Hc increases with decreasing particle size [in fine powders, Hc ~ 12-16 kA/m (150-200 oersteds)]. Magnetodielectrics are manufactured from these powders. At temperatures above 550-600°C (above the Curie point), magnetite becomes paramagnetic. Magnetite has a melting point of 1591-1597°C. Oxidation of magnetite leads to hematite (martite); muschketowite is the pseudomorph of magnetite after crystalline hematite. Varieties of magnetite are distinguished as a result of increased content of isomorphic admixtures: magnesioferrite, manganmagnetite, vanadomagnetite, chromian magnetite, and aluminian magnetite. Magnetite is included in the titanomagnetite category when found in close penetration with ilmenite and other titanium minerals (decomposition structure of solid solutions).
Magnetite occurs in deposits of various origins; however, the main industrial deposits are located in complex magmatic, con-tact metasomatic, or regional metamorphic formations. Magnetite frequently occurs in magmatic, sedimentary, and metamorphic rocks as an accessory mineral.
V. M. GRIGOR’EV