Mineral Magnetism

All mineral magnetism originates in the movement of electrons. At the atomic level, the most significant movement is electron spin: each spinning electron generates a magnetic field with a Bohr magnetic moment (μ_B), but paired electrons in the same orbital spin in opposite directions and cancel each other’s fields. Whether a mineral is magnetic, and what kind of magnetic behaviour it shows, therefore depends entirely on whether the atoms or ions in its structure contain orbitals with unpaired electrons. ^[1]

The five types of magnetic behaviour - diamagnetism, paramagnetism, ferromagnetism, ferrimagnetism, and antiferromagnetism - are all expressions of this one underlying fact. What differs between them is not whether unpaired electrons exist, but how the magnetic moments of those electrons are oriented relative to one another within the crystal structure. ^[1]

The elements most likely to produce magnetic behaviour are the transition metals with partially filled 3d orbitals. Of these, Fe, Mn, Ti, and Cr are the most geologically abundant. Fe³⁺ and Mn²⁺ have the largest individual magnetic moments because each carries five unpaired 3d electrons - the maximum possible for a d-orbital subshell. ^[1]

Diamagnetism

In diamagnetic materials, every orbital in every ion contains paired electrons, so there are no unpaired spins and no net magnetic moment. Quartz (SiO₂) is the textbook example. When Si loses four electrons to become Si⁴⁺, it acquires the electron configuration of neon, with all orbitals filled. Each O gains two electrons to become O²⁻, also reaching the neon configuration. With no unpaired electrons anywhere in the structure, quartz has zero net magnetic moment. This pattern applies broadly: minerals whose ions have acquired the electron configuration of noble gases are diamagnetic, which in practice excludes the transition metals. ^[1]

Uniquely, diamagnetic minerals are slightly repelled by an external magnetic field, not attracted. The external field causes electron motion within the mineral that generates an opposing magnetic field - a counterintuitive but physically real effect.

Paramagnetism

Paramagnetic minerals contain ions with unpaired electrons, but those electrons’ magnetic moments point in random directions with no mutual alignment. Olivine [(Mg,Fe)₂SiO₄] is a clear example: the Fe²⁺ ions it contains each have four unpaired 3d electrons, and all four spin in the same direction because of Hund’s Rule - which requires each orbital within a subshell to acquire one electron before any orbital receives a second. Each Fe²⁺ therefore has a real magnetic moment, but the moments of different Fe²⁺ ions point in random directions, so the bulk mineral has zero net magnetic moment under ordinary conditions. ^[1]

Place a paramagnetic mineral in an external magnetic field and its Fe²⁺ moments will tend to align with the field, producing a weak attraction. As soon as the field is removed, however, thermal motion of the ions immediately randomises the alignment - paramagnetic minerals cannot retain magnetism. The strength of the induced alignment is described by magnetic susceptibility, which varies with composition and structure. Most iron-bearing silicate minerals - pyroxenes, amphiboles, olivines - are paramagnetic.

Differences in magnetic susceptibility make it possible to separate minerals from each other using a Franz isodynamic separator, which applies a calibrated magnetic field to pull apart diamagnetic from paramagnetic minerals, and to separate paramagnetic minerals with different susceptibilities from one another. ^[1]

Ferromagnetism

Ferromagnetic materials can retain a permanent magnetic polarity even after an external field is removed. The reason is exchange coupling - a quantum mechanical effect that locks the magnetic moments of adjacent ions into parallel alignment within microscopic volumes called domains. Within a single domain, all the magnetic moments point the same way. Neighbouring domains need not be aligned, however, so a piece of unmagnetised iron has randomly oriented domains and zero net moment overall. Placing the material in a strong external field causes domains whose orientation matches the field to grow at the expense of unfavourably oriented domains, until the crystal becomes permanently magnetised. The attraction between a ferromagnetic material and a magnet is many orders of magnitude stronger than for a paramagnetic one. ^[1]

Ferrimagnetism

Ferrimagnetism is a variant in which exchange coupling forces the magnetic moments of ions in certain structural sites to align antiparallel to those in adjacent sites - so some moments cancel each other. The key difference from antiferromagnetism is that cancellation is incomplete: additional ions elsewhere in the structure are not paired with an antiparallel partner, leaving a residual net magnetic moment. The mineral therefore still behaves as if it were ferromagnetic - it can be attracted to a magnet and retain a permanent magnetisation. ^[1]

Magnetite (Fe₃O₄) - by far the most geologically important magnetic mineral - is ferrimagnetic. Its structure consists of cubic close-packed oxygen anions with Fe³⁺ in tetrahedral coordination and both Fe³⁺ and Fe²⁺ in octahedral coordination. Exchange coupling forces the tetrahedral and octahedral Fe³⁺ to have antiparallel spins, so their moments cancel. The octahedral Fe²⁺ ions, however, remain without antiparallel partners - their aligned moments give magnetite its net magnetic moment and ferromagnetic behaviour.

Antiferromagnetism

Antiferromagnetism is the limiting case where the antiparallel alignment is perfectly complete - the moments of all ions cancel without remainder, yielding zero net magnetic moment. Pure ilmenite (Fe²⁺Ti⁴⁺O₃) is antiferromagnetic below -183 °C. The Ti⁴⁺ has no magnetic moment; Fe²⁺ in adjacent lattice planes are oriented antiparallel. At -183 °C - the Néel temperature - the antiparallel ordering breaks down and ilmenite becomes paramagnetic. At room temperature, the magnetic behaviour of natural ilmenite is almost always dominated by exsolution blebs and inclusions of magnetite rather than by the ilmenite itself. Hematite (Fe₂O₃) is antiferromagnetic below -10 °C, but just above that temperature the opposing spin orientations of Fe³⁺ are not quite antiparallel, leaving a small residual moment that gives hematite ferrimagnetic properties. ^[1]

The Curie Temperature

If a ferromagnetic or ferrimagnetic mineral is heated, exchange coupling gradually weakens as thermal vibration randomises ionic orientations. At the Curie temperature (T_c), the exchange coupling is completely destroyed and all ferromagnetic properties are lost - above T_c, the material behaves paramagnetically. When cooled back through the Curie temperature, minerals such as magnetite reacquire their ferromagnetic properties. Critically, if an external magnetic field - such as the Earth’s own field - is present as the mineral cools through T_c, the mineral can acquire a permanent magnetisation aligned with that field. The same alignment can be acquired by minerals that crystallise below their Curie temperature in the presence of a magnetic field. ^[1]

Remnant Magnetism and Plate Tectonics

The practical consequences of the Curie temperature extend far beyond laboratory mineralogy. Magnetite in cooling igneous rocks and hematite forming during diagenesis of sediments can both lock in a magnetic alignment that mirrors the Earth’s field at the time of formation - and that alignment can be preserved for millions or billions of years. Detailed study of this remnant magnetism has provided two of the most powerful lines of evidence in Earth science: documentation that continents have moved relative to each other, and proof that the Earth’s magnetic field periodically reverses its polarity. Together, these observations underpin the theory of plate tectonics. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

Master UPSC Geology Optional

Ex-ONGC Geologist & Rank Holder

Learn the exact analytical answer-writing patterns needed for UPSC Optional from an AIR 2 & AIR 25 holder.

1-on-1 Personalized Mentorship

Elite Batch (Strictly 10 Seats)

Targeted Strategy for AIR 1-100

Bilingual Conceptual Lectures

Join Us

Offline in Delhi