Magnetism

Magnetism in minerals is more complex than simple attraction to a magnet. A few common minerals are magnetic in the everyday sense - they show obvious attraction to a hand magnet - but magnetism as a physical property encompasses five distinct types, and all minerals possess one or another of them. ^[1] The type a mineral exhibits depends on how its electrons are arranged and how the magnetic moments of adjacent atoms interact with each other - which means, ultimately, that magnetic behaviour traces back to crystal chemistry and structure. ^[1]

The Five Types

Diamagnetism is the weakest form and is present in all materials to some degree. A diamagnetic mineral is very weakly repelled by a magnetic field. Most common rock-forming silicates are diamagnetic. ^[1]

Paramagnetism involves a weak attraction to a magnetic field. It arises in minerals that have unpaired electrons - typically those containing transition metals such as iron, manganese, or chromium - whose magnetic moments align slightly with an applied field but randomise again when the field is removed. ^[1]

Ferromagnetism produces strong, permanent magnetism. The magnetic moments of adjacent atoms are aligned parallel to each other and remain so even without an applied field. Native iron is the classic example. True ferromagnetism is rare among minerals. ^[1]

Ferrimagnetism is the type responsible for the strong magnetism seen in common minerals like magnetite (Fe₃O₄). In a ferrimagnetic mineral, adjacent magnetic moments are antiparallel but unequal in magnitude, so they do not cancel - a net magnetic moment remains. Magnetite is the most geologically important ferrimagnetic mineral and is the primary carrier of the palaeomagnetic signal in rocks. ^[1]

Antiferromagnetism occurs when adjacent magnetic moments are antiparallel and exactly equal, so they cancel completely. An antiferromagnetic mineral produces no net magnetism. Ilmenite at low temperatures is an example. ^[1]

Practical Use

In hand-sample identification, the relevant distinction is simply whether a mineral is attracted to a magnet or not. ^[1] Only ferromagnetic and ferrimagnetic minerals show obvious attraction. Magnetite is the most common strongly magnetic mineral in rocks, and its presence can be confirmed quickly with a hand magnet in the field. The more subtle magnetic distinctions - diamagnetic vs. paramagnetic vs. antiferromagnetic - require laboratory measurement and are not detectable with a hand magnet. ^[1]

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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