Magnetic mineralogy

Last updated December 04, 2021

Magnetic mineralogy is the study of the magnetic properties of minerals. The contribution of a mineral to the total magnetism of a rock depends strongly on the type of magnetic order or disorder. Magnetically disordered minerals (diamagnets and paramagnets) contribute a weak magnetism and have no remanence. The more important minerals for rock magnetism are the minerals that can be magnetically ordered, at least at some temperatures. These are the ferromagnets, ferrimagnets and certain kinds of antiferromagnets. These minerals have a much stronger response to the field and can have a remanence.

Weakly magnetic minerals

Non-iron-bearing minerals

Most minerals with no iron content are diamagnetic.^[1] Some such minerals may have a significant positive magnetic susceptibility, for example serpentine,^[2] but this is because the minerals have inclusions containing strongly magnetic minerals such as magnetite. The susceptibility of such minerals is negative and small (Table 1).

Table 1. Susceptibilities of non-iron-bearing minerals ^[2]
Mineral	Volume susceptibility at room temperature $\times 10^{-6}$ (SI)
graphite	-80 to -200
calcite	-7.5 to -39
anhydrite	-14 to -60
gypsum	-13 to -29
ice	-9
orthoclase	-13 to -17
magnesite	-15
forsterite	-12
halite	-10 to -16
galena	-33
quartz	-13 to -17
celestine	-16 to -18
sphalerite	-31 to -750

Iron-bearing paramagnetic minerals

Most iron-bearing carbonates and silicates are paramagnetic at all temperatures.^[1] Some sulfides are paramagnetic, but some are strongly magnetic (see below). In addition, many of the strongly magnetic minerals discussed below are paramagnetic above a critical temperature (the Curie temperature or Néel temperature). In Table 2 are given susceptibilities for some iron-bearing minerals. The susceptibilities are positive and an order of magnitude or more larger than diamagnetic susceptibilities.

Table 2. Susceptibilities of some paramagnetic minerals ^[2]
Mineral	Volume susceptibility $\times 10^{-6}$ (SI)
garnet	2,700
illite	410
montmorillonite	330-350
biotite	1,500-2,900
siderite	1,300-11,000
chromite	3,000-120,000
orthopyroxene	1,500-1,800
fayalite	5,500
olivine	1,600
jacobsite	25,000
franklinite	450,000

Strongly magnetic minerals

Iron-titanium oxides

Many of the most important magnetic minerals on Earth are oxides of iron and titanium. Their compositions are conveniently represented on a ternary plot with axes corresponding to the proportions of Ti⁴⁺, Fe²⁺, and Fe³⁺. Important regions on the diagram include the titanomagnetites, which form a line of compositions Fe_3-xTi_xO₄ for $x$ between 0 and 1. At the $x = 0$ end is magnetite, while the $x = 1$ composition is ulvöspinel. The titanomagnetites have an inverse spinel crystal structure and at high temperatures are a solid solution series. Crystals formed from titanomagnetites by cation-deficient oxidation are called titanomaghemites, an important example of which is maghemite. Another series, the titanohematites, have hematite and ilmenite as their end members, and so are also called hemoilmenites.^[1] The crystal structure of hematite is trigonal-hexagonal. It has the same composition as maghemite; to distinguish between them, their chemical formulae are generally given as γFe₂O₃ for hematite and αFe₂O₃ for maghemite.

Iron sulfides

The other important class of strongly magnetic minerals is the iron sulfides, particularly greigite and pyrrhotite.

Iron alloys

Extraterrestrial environments being low in oxygen, minerals tend to have very little Fe³⁺. The primary magnetic phase on the Moon is ferrite, the body-centered cubic (bcc) phase of iron. As the proportion of iron decreases, the crystal structure changes from bcc to face centered cubic (fcc). Nickel iron mixtures tend to exsolve into a mixture of iron-rich kamacite and iron-poor taenite.^[3]^: 27

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Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted by a magnetic field. Diamagnetism is a quantum mechanical effect that occurs in all materials; when it is the only contribution to the magnetism, the material is called diamagnetic. In paramagnetic and ferromagnetic substances, the weak diamagnetic force is overcome by the attractive force of magnetic dipoles in the material. The magnetic permeability of diamagnetic materials is less than the permeability of vacuum, μ₀. In most materials, diamagnetism is a weak effect which can be detected only by sensitive laboratory instruments, but a superconductor acts as a strong diamagnet because it repels a magnetic field entirely from its interior.

Hematite, also spelled as haematite, is a common iron oxide compound with the formula, Fe₂O₃ and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of Fe^₂O^₃. It has the same crystal structure as corundum (Al^₂O^₃) and ilmenite (FeTiO^₃). With this it forms a complete solid solution at temperatures above 950 °C (1,740 °F).

Magnetism is a class of physical attributes that are mediated by magnetic fields. Electric currents and the magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Magnetism is one aspect of the combined phenomenon of electromagnetism. The most familiar effects occur in ferromagnetic materials, which are strongly attracted by magnetic fields and can be magnetized to become permanent magnets, producing magnetic fields themselves. Demagnetizing a magnet is also possible. Only a few substances are ferromagnetic; the most common ones are iron, cobalt and nickel and their alloys. The rare-earth metals neodymium and samarium are less common examples. The prefix ferro- refers to iron, because permanent magnetism was first observed in lodestone, a form of natural iron ore called magnetite, Fe₃O₄.

Paramagnetism Weak, attractive magnetism possessed by most elements and some compounds

Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, diamagnetic materials are repelled by magnetic fields and form induced magnetic fields in the direction opposite to that of the applied magnetic field. Paramagnetic materials include most chemical elements and some compounds; they have a relative magnetic permeability slightly greater than 1 and hence are attracted to magnetic fields. The magnetic moment induced by the applied field is linear in the field strength and rather weak. It typically requires a sensitive analytical balance to detect the effect and modern measurements on paramagnetic materials are often conducted with a SQUID magnetometer.

Iron ore Ore rich in iron or the element Fe

Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the form of magnetite (Fe^₃O^₄, 72.4% Fe), hematite (Fe^₂O^₃, 72.9% Fe), goethite (FeO(OH), 62.9% Fe), limonite (FeO(OH)·n(H₂O), 55% Fe) or siderite (FeCO₃, 48.2% Fe).

In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism.

In physics and materials science, the Curie temperature (T_C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can be replaced by induced magnetism. The Curie temperature is named after Pierre Curie, who showed that magnetism was lost at a critical temperature.

Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe₃O₄. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. It is the most magnetic of all the naturally occurring minerals on Earth. Naturally magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism.

Paleomagnetism Study of Earths magnetic field in past

Paleomagnetism, or palaeomagnetism, is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Magnetic minerals in rocks can lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists.

Maghemite (Fe₂O₃, γ-Fe₂O₃) is a member of the family of iron oxides. It has the same spinel ferrite structure as magnetite and is also ferrimagnetic. It is sometimes spelled as "Maghaemite".

Pyrrhotite Magnetic iron sulfide mineral

Pyrrhotite is an iron sulfide mineral with the formula Fe_(1-x)S. It is a nonstoichiometric variant of FeS, the mineral known as troilite. Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is weakly magnetic. The magnetism decreases as the iron content increases, and troilite is non-magnetic.

Rock magnetism is the study of the magnetic properties of rocks, sediments and soils. The field arose out of the need in paleomagnetism to understand how rocks record the Earth's magnetic field. This remanence is carried by minerals, particularly certain strongly magnetic minerals like magnetite. An understanding of remanence helps paleomagnetists to develop methods for measuring the ancient magnetic field and correct for effects like sediment compaction and metamorphism. Rock magnetic methods are used to get a more detailed picture of the source of the distinctive striped pattern in marine magnetic anomalies that provides important information on plate tectonics. They are also used to interpret terrestrial magnetic anomalies in magnetic surveys as well as the strong crustal magnetism on Mars.

Iron(II,III) oxide is the chemical compound with formula Fe₃O₄. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe₂O₃) which also occurs naturally as the mineral hematite. It contains both Fe²⁺ and Fe³⁺ ions and is sometimes formulated as FeO ∙ Fe₂O₃. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic. Its most extensive use is as a black pigment. For this purpose, it is synthesized rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.

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In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions (or equivalently, oxygen fugacities) at which a rock forms and evolves can be important for interpreting the rock history. Iron, sulfur, and manganese are three of the relatively abundant elements in the Earth's crust that occur in more than one oxidation state. For instance, iron, the fourth most abundant element in the crust, exists as native iron, ferrous iron (Fe²⁺), and ferric iron (Fe³⁺). The redox state of a rock affects the relative proportions of the oxidation states of these elements and hence may determine both the minerals present and their compositions. If a rock contains pure minerals that constitute a redox buffer, then the oxygen fugacity of equilibration is defined by one of the curves in the accompanying fugacity-temperature diagram.

Antitaenite is a meteoritic metal alloy mineral composed of iron (Fe) and 20–40% nickel (Ni), that has a face centered cubic crystal structure.

Natural remanent magnetization (NRM) is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement of the rock over millions of years from its original position. Natural remanent magnetization forms the basis of paleomagnetism and magnetostratigraphy.

Iron oxide nanoparticles are iron oxide particles with diameters between about 1 and 100 nanometers. The two main forms are magnetite and its oxidized form maghemite. They have attracted extensive interest due to their superparamagnetic properties and their potential applications in many fields.

Environmental magnetism is the study of magnetism as it relates to the effects of climate, sediment transport, pollution and other environmental influences on magnetic minerals. It makes use of techniques from rock magnetism and magnetic mineralogy. The magnetic properties of minerals are used as proxies for environmental change in applications such as paleoclimate, paleoceanography, studies of the provenance of sediments, pollution and archeology. The main advantages of using magnetic measurements are that magnetic minerals are almost ubiquitous and magnetic measurements are quick and non-invasive.

Magnetochemistry is concerned with the magnetic properties of chemical compounds. Magnetic properties arise from the spin and orbital angular momentum of the electrons contained in a compound. Compounds are diamagnetic when they contain no unpaired electrons. Molecular compounds that contain one or more unpaired electrons are paramagnetic. The magnitude of the paramagnetism is expressed as an effective magnetic moment, μ_eff. For first-row transition metals the magnitude of μ_eff is, to a first approximation, a simple function of the number of unpaired electrons, the spin-only formula. In general, spin-orbit coupling causes μ_eff to deviate from the spin-only formula. For the heavier transition metals, lanthanides and actinides, spin-orbit coupling cannot be ignored. Exchange interaction can occur in clusters and infinite lattices, resulting in ferromagnetism, antiferromagnetism or ferrimagnetism depending on the relative orientations of the individual spins.

References

1 2 3 Dunlop, David J.; Özdemir, Özden (1997). Rock magnetism: Fundamentals and Frontiers. Cambridge Univ. Press. ISBN 0-521-32514-5.
1 2 3 Hunt, Christopher P.; Moskowitz, Bruce P. (1995), "Magnetic properties of rocks and minerals", in Ahrens, T. J. (ed.), Rock Physics and Phase Relations: A Handbook of Physical Constants, 3, Washington, DC: American Geophysical Union, pp. 189–204
↑ O'Reilly, W. (1984). Rock and Mineral Magnetism. Boston, MA: Springer US. ISBN 9781468484687.

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[Dunlop-1] 1 2 3 Dunlop, David J.; Özdemir, Özden (1997). Rock magnetism: Fundamentals and Frontiers. Cambridge Univ. Press. ISBN 0-521-32514-5.

[Hunt-2] 1 2 3 Hunt, Christopher P.; Moskowitz, Bruce P. (1995), "Magnetic properties of rocks and minerals", in Ahrens, T. J. (ed.), Rock Physics and Phase Relations: A Handbook of Physical Constants, 3, Washington, DC: American Geophysical Union, pp. 189–204

[O'Reilly-3] O'Reilly, W. (1984). Rock and Mineral Magnetism. Boston, MA: Springer US. ISBN 9781468484687.

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