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MAGNETITE
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MAGNETITE , a See also:mineral forming the natural magnet (see See also:MAGNETISM), and important also as an See also:iron-ore. it is an iron-See also:black, opaque mineral, with metallic lustre; hardness about 6, sp. gr. 4.9 to 5.2. When scratched, it yields a black streak. It is an See also:oxide of iron having the See also:formula Fe304, .corresponding with 72.4% of See also:metal, whence its See also:great value as an ore. It may be regarded as a ferroso-ferric oxide, FeO.Fe203, or as iron ferrate, Fe"Fe2"'04. See also:Titanium is often See also:present, and occasionally the mineral contains See also:magnesium, See also:nickel, &c. It is always strongly magnetic. Magnetite crystallizes in the cubic See also:system, usually in octahedra, less commonly in rhombic dodecahedra, and not infrequently in twins of the" See also:spinel type " (fig. I). The rhombic faces of the See also:dodecahedron are often striated parallel to the longer See also:diagonal. There is no distinct cleavage, but imperfect parting may be obtained along octahedral planes. Magnetite is a mineral of wide See also:distribution, occurring as grains in many massive and volcanic rocks, like See also:granite, See also:diorite and See also:dolerite. It appears to have crystallized from the magma at a very See also:early See also:period of consolidation. Its presence contributes to the dark See also:colour of many basalts and other basic rocks, and may cause them to disturb the See also:compass. Large ore-bodies of granular and compact magnetite occur as beds and lenticular masses in Archean See also:gneiss and crystalline See also:schists, in various parts of See also:Norway, See also:Sweden, See also:Finland and the Urals; as also in the states of New See also:York, New See also:Jersey, See also:Pennsylvania and See also:Michigan, as well as in See also:Canada. In some cases it appears to have segregated from a basic eruptive magma, and in other cases to have resulted from metamorphic See also:action. Certain deposits appear to have been formed, directly or indirectly, by wet processes. Iron See also:rust sometimes contains magnetite. An interesting See also:deposit of oolitic magnetic ore occurs in the Dogger (Inferior Oolite) of Rosedale See also:Abbey, in See also:Yorkshire; and a somewhat similar pisolitic ore, of See also:Jurassic See also:age, is known on the See also:continent as chamoisite, having been named from Chamoison (or Chamoson) in the See also:Valais, See also:Switzerland. Grains of magnetite occur in See also:serpentine, as an alteration-product of the See also:olivine. In See also:emery, magnetite in a granular See also:form is largely associated with the See also:corundum; and in certain kinds of See also:mica magnetite occurs as thin dendritic enclosures. See also:Haematite is sometimes magnetic, and A. Liversidge has shown that magnetite is probably present. By deoxidation, haematite may be converted into magnetite, as proved by certain pseudomorphs; but on the other See also:hand magnetite is sometimes altered to haematite. On weathering, magnetite commonly passes into See also:limonite, the ferrous oxide having probably been removed by carbonated See also:waters. Closely related to magnetite is the rare volcanic mineral from See also:Vesuvius, called magnoferrite, or magnesioferrite, with the formula MgFe204; and with this may be mentioned a mineral from Jakobsberg, in Vermland, Sweden, called jakobsite, containing MnFe2O4. (F. W. Additional information and CommentsThere are no comments yet for this article.
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