MAGNETITE  , a

mineral forming the natural magnet (see MAGNETISM), and important also as an iron-ore. it is an iron-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 oxide of iron having the formula Fe304, .corresponding with 72.4% of metal, whence its great value as an ore . It may be regarded as a ferroso-ferric oxide, FeO.Fe203, or as iron ferrate, Fe"Fe2"'04 . Titanium is often present, and occasionally the mineral contains magnesium, nickel, &c . It is always strongly magnetic . Magnetite crystallizes in the cubic system, usually in octahedra, less commonly in rhombic dodecahedra, and not infrequently in twins of the" spinel type " (fig . I) . The rhombic faces of the dodecahedron are often striated parallel to the longer diagonal . There is no distinct cleavage, but imperfect parting may be obtained along octahedral planes . Magnetite is a mineral of wide distribution, occurring as grains in many massive and volcanic rocks, like granite, diorite and dolerite .

It appears to have crystallized from the magma at a very

early period of consolidation . Its presence contributes to the dark colour of many basalts and other basic rocks, and may cause them to disturb the compass . Large ore-bodies of granular and compact magnetite occur as beds and lenticular masses in Archean gneiss and crystalline schists, in various parts of Norway, Sweden, Finland and the Urals; as also in the states of New York, New Jersey, Pennsylvania and Michigan, as well as in Canada . In some cases it appears to have segregated from a basic eruptive magma, and in other cases to have resulted from metamorphic action . Certain deposits appear to have been formed, directly or indirectly, by wet processes . Iron rust sometimes contains magnetite . An interesting deposit of oolitic magnetic ore occurs in the Dogger (Inferior Oolite) of Rosedale Abbey, in Yorkshire; and a somewhat similar pisolitic ore, of Jurassic age, is known on the continent as chamoisite, having been named from Chamoison (or Chamoson) in the Valais, Switzerland . Grains of magnetite occur in serpentine, as an alteration-product of the olivine . In emery, magnetite in a granular form is largely associated with the corundum; and in certain kinds of mica magnetite occurs as thin dendritic enclosures . 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 hand magnetite is sometimes altered to haematite .

On weathering, magnetite commonly passes into

limonite, the ferrous oxide having probably been removed by carbonated waters . Closely related to magnetite is the rare volcanic mineral from 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 .