Magnetic minerals are naturally occurring, inorganic solids that respond to an external magnetic field. The most magnetic examples are a very small fraction of all the minerals found in the Earth’s crust. Historically, minerals like lodestone (a naturally magnetized form of magnetite) were invaluable in developing the earliest compasses for navigation. Today, these minerals are still important because they preserve a fossilized record of the Earth’s ancient magnetic field, aiding in geological and archaeological studies.
The Atomic Cause of Mineral Magnetism
Mineral magnetism begins at the atomic level with the behavior of electrons. Every electron possesses a fundamental property called spin, which makes it act like a tiny, individual bar magnet. In most atoms, electrons exist in pairs, and the spin of one electron is canceled out by the opposite spin of its partner.
Strong magnetism only appears in minerals whose atoms contain unpaired electrons, such as those found in iron-bearing compounds. Since these unpaired electrons have no partner to neutralize their spin, they create a net atomic magnetic moment. The overall magnetic behavior of the mineral then depends on how these individual atomic moments are organized within the crystal structure.
In strongly magnetic minerals, the atomic moments are not randomly oriented but are aligned over a microscopic area known as a magnetic domain. Within a single domain, all the atomic magnets point in the same direction, creating a strong internal magnetic field. The bulk magnetism of the mineral depends on the collective alignment of these domains, which can be temporarily or permanently influenced by an external magnetic field.
The Three Classifications of Mineral Magnetism
Minerals are broadly categorized into three types based on the way they interact with a magnetic field. This classification is determined by the internal arrangement of their atomic magnetic moments. These differences dictate whether a mineral is strongly attracted, weakly attracted, or even repelled by a magnet.
Ferromagnetism
Ferromagnetism is the property that results in a strong, permanent attraction to a magnet. In a ferromagnetic material, the atomic magnetic moments are all aligned parallel to one another, leading to a large net magnetization even without an external field. This property is lost when the material is heated above a specific temperature, known as the Curie temperature. For pure iron, this temperature is 770°C, above which the thermal energy disrupts the parallel alignment, and the material becomes simply paramagnetic.
Paramagnetism
Paramagnetic minerals exhibit a weak attraction that only exists when they are placed within an external magnetic field. These minerals contain unpaired electrons, meaning their atoms have magnetic moments, but these moments are randomly oriented throughout the material. The external field is strong enough to temporarily align the moments, but this alignment is lost the moment the field is removed. Aluminum is a common non-mineral example of a paramagnetic substance.
Diamagnetism
Diamagnetic materials are characterized by a very weak repulsion from an external magnetic field. This behavior is present in all matter but is usually overshadowed by stronger magnetic effects. In these substances, all electrons are paired, meaning the atoms have no net magnetic moment. The external field induces a temporary, opposing magnetic field in the material, which causes the slight repulsion. Quartz and table salt are common minerals that display diamagnetism.
Key Examples of Naturally Magnetic Minerals
The most powerful naturally occurring mineral is Magnetite (\(\text{Fe}_3\text{O}_4\)), which is a member of the iron oxide group. Magnetite is not strictly ferromagnetic but ferrimagnetic, a related property where magnetic moments are aligned in opposite directions, resulting in a strong net magnetic moment. This internal structure allows it to exhibit strong, permanent magnetism.
Another significant example is the iron sulfide mineral Pyrrhotite (\(\text{Fe}_{1-\text{x}}\text{S}\)), which often exhibits ferrimagnetic behavior due to vacancies in its iron sites. The magnetic strength of pyrrhotite is variable, depending on the exact ratio of iron to sulfur in its chemical formula. It is often found in igneous and metamorphic rocks, contributing to the magnetic signature of these formations.
Hematite (\(\text{Fe}_2\text{O}_3\)), another common iron oxide, is usually considered a weakly magnetic mineral. While its moments are primarily aligned in an anti-parallel fashion (antiferromagnetic), a slight canting or tilting of the spins results in a weak, residual ferromagnetism. The magnetic properties of hematite can sometimes be used to identify ancient environmental conditions because its magnetism is often temperature-dependent.