4.2 Magnetic properties of rocks and minerals

Magnetic properties of rocks and minerals are central to understanding Earth's magnetic field, its history, and the dynamics of the planet's interior. These properties originate from the behavior of electrons in atoms, which produce magnetic responses ranging from negligibly weak to strong enough to preserve a record of ancient field directions.

Iron-bearing minerals, especially magnetite, are the main carriers of magnetism in rocks. Because these minerals can retain magnetization over geologic time, they serve as archives of Earth's past magnetic field. This makes them indispensable for studying geomagnetic reversals, tectonic plate movements, and core dynamics.

Magnetic Material Types

Diamagnetic, Paramagnetic, and Ferromagnetic Materials

All materials respond to an applied magnetic field, but the strength and character of that response varies enormously depending on electronic structure.

• Diamagnetic materials have a weak, negative magnetic susceptibility and are slightly repelled by a magnetic field. All electron magnetic moments cancel out, so there are no unpaired electrons contributing a net moment. They do not retain any magnetization once the field is removed. Common examples: quartz, calcite, water.
• Paramagnetic materials have a small, positive magnetic susceptibility and are weakly attracted to a magnetic field. They contain unpaired electrons whose moments align parallel to an applied field, but the alignment is easily disrupted by thermal energy. They also lose their magnetization when the field is removed. Common examples: biotite, pyrite, siderite.
• Ferromagnetic (sensu lato) materials have a large, positive magnetic susceptibility and are strongly attracted to a magnetic field. Neighboring magnetic moments interact strongly and align spontaneously within regions called magnetic domains. Crucially, these materials can retain magnetization after the external field is removed. This retained magnetization is called remanent magnetization. Common examples: magnetite, hematite, pyrrhotite.

A note on terminology: in geophysics, "ferromagnetic" is often used loosely to include ferrimagnetic materials (like magnetite, where opposing sublattice moments don't fully cancel) and antiferromagnetic materials with a parasitic moment (like hematite). True ferromagnetism, where all moments align in parallel, is seen in pure iron but is rare in natural minerals.

Factors Determining Magnetic Behavior

The magnetic behavior of a material comes down to the presence and alignment of magnetic moments from unpaired electrons:

• In diamagnetic materials, electrons are all paired, so their orbital and spin moments cancel. The only response is a tiny induced moment opposing the applied field.
• In paramagnetic materials, unpaired electrons create net atomic moments, but thermal agitation keeps them randomly oriented unless an external field is applied. The resulting alignment is weak and temporary.
• In ferromagnetic materials, strong exchange interactions between neighboring atoms cause spontaneous alignment of moments within domains. This cooperative behavior produces a much larger magnetization that can persist without an external field.

Magnetic Properties of Materials

Magnetic Susceptibility and Remanent Magnetization

Magnetic susceptibility ($\chi$) measures how readily a material becomes magnetized in an external field. It is defined as the ratio of induced magnetization ($M$) to the applied field strength ($H$):

$\chi = \frac{M}{H}$

In SI units, $\chi$ is dimensionless (though volume and mass susceptibility have different unit conventions). Susceptibility values span many orders of magnitude: diamagnetic minerals like quartz have $\chi$ on the order of $-10^{-5}$, paramagnetic minerals like biotite around $10^{-4}$ to $10^{-3}$, and magnetite can reach values above $1$.

Remanent magnetization (or remanence) is the permanent magnetization that remains in a material after the external field is removed. Only ferromagnetic (sensu lato) minerals carry remanence, because their domain structure can lock in a preferred alignment of moments. This remanence is what makes paleomagnetism possible.

Magnetic Anisotropy

Magnetic anisotropy refers to the directional dependence of a material's magnetic properties. A rock's susceptibility may differ depending on the direction in which the field is applied. Several factors cause this:

• Shape anisotropy: elongated magnetic grains are easier to magnetize along their long axis.
• Crystalline anisotropy: the crystal lattice of a mineral has preferred "easy" magnetization directions.
• Stress/strain anisotropy: tectonic deformation can reorient grains or alter domain structures.

The anisotropy of magnetic susceptibility (AMS) is a widely used technique that measures susceptibility in multiple directions to define an ellipsoid. This ellipsoid reveals the preferred orientation and alignment of magnetic minerals, providing information about rock fabric and deformation history. For example, AMS can identify sedimentary bedding orientations, metamorphic foliation, and flow directions in igneous rocks.

Magnetic Properties of Rocks

Contribution of Rock-Forming Minerals

The magnetic character of a rock is dominated by its iron-bearing minerals:

• Magnetite ($Fe_3O_4$): The most important magnetic mineral in geophysics. It is ferrimagnetic with a very high susceptibility and can carry extremely stable remanent magnetization. Even small concentrations of magnetite dominate a rock's magnetic signature.
• Hematite ($\alpha$-$Fe_2O_3$): Antiferromagnetic with a weak parasitic ferromagnetic moment due to spin canting. Its susceptibility is much lower than magnetite's, but it can carry very stable remanence over billions of years, making it valuable for paleomagnetic studies of old rocks.
• Pyrrhotite ($Fe_{1-x}S$): Ferrimagnetic with strong susceptibility, but its magnetic properties depend on composition (the value of $x$) and crystal structure. It is an important carrier in some sulfide-rich rocks.
• Goethite ($\alpha$-FeOOH): A common weathering product with weak magnetic properties, sometimes relevant in laterites and soils.

Factors Influencing Rock Magnetic Properties

Several factors control a rock's overall magnetic behavior:

• Mineral concentration: More ferromagnetic mineral content means a stronger magnetic signature. Mafic igneous rocks like basalts and gabbros are typically strongly magnetic because they are rich in magnetite.
• Grain size: This profoundly affects magnetic stability. Very fine grains (below ~0.1 μm for magnetite) are single-domain (SD), meaning the entire grain is one magnetic domain. SD grains carry the most stable remanence. Larger grains are multi-domain (MD) and have lower coercivity and less stable remanence. An intermediate range called pseudo-single-domain (PSD) shows properties between the two.
• Mineral distribution: Whether magnetic grains are evenly dispersed or clustered affects the bulk magnetic response and how grains interact magnetically.
• Secondary processes: Alteration, weathering, and metamorphism can create new magnetic minerals, destroy existing ones, or change grain sizes. These processes modify both susceptibility and remanence, sometimes overprinting the original magnetic record.

Rocks dominated by diamagnetic or paramagnetic minerals, such as limestones and quartzites, have very weak magnetic signatures.

Magnetic Remanence in Rocks

Acquisition Processes

Rocks acquire remanent magnetization through several distinct mechanisms, each tied to a specific geological process:

1. Thermoremanent magnetization (TRM): Acquired when ferromagnetic minerals cool through their Curie temperature ($T_C$) in the presence of an ambient magnetic field. Above $T_C$, thermal energy overcomes exchange interactions and the material is paramagnetic. As the rock cools below $T_C$, magnetic moments align with the field and become locked in. TRM is the primary remanence in igneous rocks and high-grade metamorphic rocks. For magnetite, $T_C \approx 580°C$; for hematite, $T_C \approx 675°C$.

2. Chemical remanent magnetization (CRM): Acquired when new ferromagnetic minerals nucleate and grow through a critical grain size in the presence of a magnetic field. As a grain grows past the superparamagnetic threshold, its moment becomes locked in the ambient field direction. CRM forms during diagenesis, hydrothermal alteration, or oxidation (e.g., magnetite altering to hematite).

3. Detrital remanent magnetization (DRM): Acquired when magnetic mineral grains physically rotate to align with Earth's field during deposition in water. As sediment settles and compacts, the alignment is preserved. This is the primary remanence mechanism in clastic sedimentary rocks like sandstones and mudstones.

4. Post-depositional remanent magnetization (pDRM): Acquired after deposition but before full lithification. In water-saturated, unconsolidated sediment, magnetic grains can still rotate within pore spaces and align with the ambient field. pDRM often provides a more accurate record of the field at the time of early burial than DRM, because it smooths out mechanical misalignment during settling.

Preservation and Significance

The usefulness of remanent magnetization depends on how well it is preserved. Preservation requires:

• Stable magnetic minerals with high coercivity (resistance to demagnetization), such as single-domain magnetite or fine-grained hematite.
• Minimal thermal overprinting: If a rock is reheated above the Curie temperature of its magnetic minerals, the original remanence is reset.
• Minimal chemical alteration: Growth of new magnetic phases can overprint or obscure the primary signal.
• No exposure to strong external fields: Lightning strikes, for example, can locally remagnetize surface rocks.

Rocks that retain their original remanence are the foundation of paleomagnetism. By measuring the direction and intensity of remanence in dated rock samples, researchers can:

• Reconstruct apparent polar wander paths, which track the movement of tectonic plates relative to the magnetic pole over time.
• Build the geomagnetic polarity timescale, documenting the history of field reversals recorded in ocean-floor basalts and sedimentary sequences.
• Constrain paleogeographic reconstructions, determining where continents were located in the geological past.
• Study secular variation and the long-term evolution of Earth's core dynamics.