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The magnetism of rocks is the vector sum of induced magnetization—dependent on an external field—and remanent magnetization—independent of any external field. The magnetization induced by an external field is a function of the magnetic susceptibility of the rock.


DEFINITIONS : The magnetic susceptibility, k, of a substance is the ratio of intensity of magnetization, I, to the magnetizing field, H; k = I/H, defined with respect to unit volume. The specific susceptibility, χ, and specific intensity of magnetization, J, are defined with respect to unit mass.

Most minerals are paramagnetic or diamagnetic with positive or negative susceptibilities of the order of 10~6 c.g.s units. The susceptibility of such minerals is essentially independent of the applied field. For ferromagnetic materials like iron and nickel, the intensity of magnetization reaches a maximum value within a finite strength of applied field; the susceptibility approaches or exceeds unity. There is for each such substance a characteristic temperature, the Curie point, at and above which the high susceptibility vanishes. The ferromagnetic properties of rocks, which are of prime interest from a geophysical standpoint, are due to the presence of a small percentage of ferrimagnetic minerals. Ferrimagnetism s i distinguished from ferromagnetism (all atomic spin moments parallel) and antiferro-magnetism (two equal but opposite sets of spins with zero net moment) in that there are two opposite but unequal sets of spins. For example, in magnetite the spins of 8Fe+3 ions

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