MAGNETISM

Each orbiting electron possesses a magnetic moment equal to 1 Bohr magnetron (µ_B), or 0.927 x 10^-23 Am² (Amps meter²). In isolated ions, the net magnetic moment is equal to the sum of the orbital and the spin contributions. Each filled orbital gives a net contribution of zero to the magnetic moment since the two electrons orbit and spin oppositely. Net magnetic moments are generated only in atoms or ions with incomplete electronic shells. Filled shells contribute zero magnetic moment, since they are the sum of filled orbitals. The most important subshells likely to be incompletely filled are the 3d (first transition row) and the 4f (rare earth elements). The second and third transition rows (4d and 5d electrons) also produce magnetic moments but the elements, and hence the minerals, are rare.

Three d electrons have large spin and relatively low orbital contributions to magnetic moments. In compounds the orbital contribution is affected by, and largely negated by, bonding to other ions. Since the 4s electrons are outside the 3d the 3d electrons are partially shielded and the orbital contribution will not be entirely negated. Three d electrons may also be mobile (in metallic bonding) which further serves to negate their orbital contributions. Thus the spin contribution is largely responsible for the 3d electrons contribution to the magnetic moment and is proportional to the number of unpaired d electrons.

After Zoltai and Stout, 1984, Mineralogy: Concepts and Principles, Table 6.3, p.163.

In 4f electron containing elements, the electrons are well-shielded by outer electrons. The 4f electrons are not involved in bonding and both orbital and spin effects contribute to the total magnetic moment.

Behavior in Minerals

An aggregate of ions or atoms may behave much differently than an individual ion. The concept of magnetic susceptibility becomes important. This is the ratio of induced magnetization to the strength of the external magnetic field causing the induced magnetization. The induced magnetization may be aligned parallel or in some other direction to the external field. Induced magnetization which is not parallel to the external field results from magnetic anisotropy, and may oppose the external field.

Diamagnetic

Minerals possessing ions with totally paired electron spins. No transition elements are present, and the net magnetic moment is zero. In a strong magnetic field diamagnetic materials exhibit a small negative magnetic susceptibility, which means they are weakly repelled from the magnet.

Paramagnetic

Transition metals ions are present but the magnetic moments are randomly distributed. The net field is zero, although an external field will produce some alignment of dipoles, which disappears when the external field is removed. The alignment of the magnetic dipoles produces a small positive magnetic susceptibility and these minerals are attracted to a magnet in a strong magnetic field. Example olivine (Mg, Fe)₂SiO₄.

Ferromagnetic

Adjacent moments are aligned. After an external field is applied the dipoles interact and the field remains locked in. The magnetism is due to unbalanced electron spin in the inner orbits of the elements concerned. The ionic spacing in ferromagnetic crystals is such that very large forces, called exchange forces, cause the alignment of all atoms to give highly magnetic domains. In unmagnetized metal these domains are randomly oriented. After a strong magnetic field is applied the domains align and the material remains a strong magnet after the external field is removed. Upon heating the domains may become randomly aligned once again. This transition to a paramagnetic state is called the Curie temperature, after Pierre Curie who was instrumental in elucidating the behavior of paramagnetic materials. In metallic iron the Curie temperature is 770C. Other examples of ferromagnetic materials are the metals cobalt and nickel, and alloys such as alnicol.

Antiferromagnetic

Alternate atoms have oppositely directed moments. The magnetic susceptibility is low but increases with increasing temperature up to the Néel temperature. Above this temperature the susceptibility falls and the material is paramagnetic. The Néel temperature is named after L.E.F. Néel, who discovered the phenomenon of the transition from antiferromagnetism to paramagnetism in 1930. Examples include Cr metal, and compounds like MnO, MnS, and FeO.

Ferrimagnetic

Adjacent atoms have antiparallel alignment, but the magnitude of the magnetic moments of different ions is different. Cancellation is incomplete and strong magnetism may exist. Alternatively, the number of magnetic moments aligned in one direction may be different than in another direction. Ferrimagnetic materials may have magnetism similar to that of ferromagnetic materials. Some minerals have been incorrectly described in the literature as being ferromagnetic when in fact they are ferrimagnetic. Examples include ilmenite FeTiO₃, magnetite (Fe₃O₄ or Fe²⁺Fe₂³⁺O₄₎ and pyrrhotite (Fe_1-xS, x=0.0 0.2). The Curie temperature for magnetite is 85C, much lower than for metallic iron.

Applications of magnetic properties in mineralogy

Magnetic separation based on the differing magnetic susceptibilities of different minerals is used in processing minerals since many minerals, especially those containing iron, are attracted to or repelled from a magnet in a strong magnetic field. Magnetic separation is used in both laboratory and commercial scales for mineral separation. Another application of magnetism in geology is the aerial remote sensing of local magnetic fields. An airplane flies over an area towing a magnetometer, which measures local perturbations of the earth's magnetic field. These aircraft fly low (100 to 300 meters) and use highly sensitive magnetometers. Many sulfide ore bodies are associated with magnetite and although the magnetite itself may have no economic value the sulfides often are valuable. This method is rapid and relatively cheap, especially in areas of rough terrain. Any positive magnetic anomalies must be verified by subsequent geophysical and geochemical exploration.

© 2005 by David L. Warburton

Questions or comments? mailto:warburto@fau.edu

Last updated: June 29, 2005