Magnetic Properties of Rocks

ContentsPageTable 201. Magnetic susceptibility of magnetite2952. Susceptibility of magnetic minerals2963. Range of magnetic susceptibility in major rock types2964. Susceptibility and remanent magnetization of rocks297 Magnetic susceptibility of magnetite and of ferromagnetic minerals.—Magnetite, by reason of its relatively high magnetic susceptibility and wide distribution, is easily the most important ferromagnetic mineral. Indeed, the value of the magnetic susceptibility of a rock is generally determined solely by its contained magnetite. The other ferromagnetic minerals are few in number and of rarer occurrence. The most important ones are pyrrhotite, ilmenite, specularite, franklinite, and cubanite. In Table 20–1 are listed values of the magnetic susceptibility of magnetite. These values spread over a wide range and are dependent not only upon the specimen itself but also upon the intensity of the magnetizing field. In natural, large-scale occurrences of magnetite, such as ore bodies, the extremely high values of susceptibility found by Weiss [ 25] for single crystals and by Steinmetz [ 26] are rarely, if ever, observed. A value of about .3 is usually appropriate, but exceptions must be expected, since the susceptibility of magnetite in common with that of other ferromagnetic substances is sensitive to small changes in composition or structure. In Table 20–2 are listed measured susceptibilities for other ferromagnetic minerals. It will be noted that the susceptibility of magnetite is at least ten times that of other minerals. Magnetic susceptibility of rocks.—Published measurements of the magnetic susceptibility of rocks are . . .