A mineral is defined as a naturally occurring, inorganic solid with a specific chemical composition and a highly ordered internal crystal structure. While thousands of mineral species exist, only a small fraction exhibits strong magnetic properties. All materials, including minerals, interact with a magnetic field on a fundamental level, determined entirely by the arrangement of electrons within the atomic structure.
The Atomic Source of Mineral Magnetism
Magnetism in any solid material originates with the behavior of electrons within their atoms. An electron possesses a quantum mechanical property called spin, which acts like a tiny magnet, creating a magnetic moment. In most atoms, electrons exist in pairs, and their opposing spins cancel out their individual magnetic moments, resulting in no net magnetic field.
For a material to exhibit attraction to a magnetic field, it must contain atoms with unpaired electrons. These unpaired electrons create a net magnetic moment for the atom, which interacts with an external field. The overall magnetic behavior is determined by how these atomic moments are spatially arranged and interact within the crystal structure.
In strongly magnetic minerals, such as those containing iron, cobalt, or nickel, the atomic moments align over large, microscopic regions known as magnetic domains. The collective, parallel alignment of spins within these domains gives rise to the bulk magnetic property. While the presence of magnetic elements is necessary, the specific crystal structure and chemical bonding dictate whether strong magnetism can manifest.
Categorizing Magnetic Behavior in Minerals
The strength and nature of a mineral’s magnetic response allow scientists to classify it into three primary categories. The strongest form of magnetism is ferromagnetism, which allows a material to be strongly attracted to a magnet and retain its magnetization after the external field is removed. The iron oxide mineral Magnetite (\(\text{Fe}_3\text{O}_4\)) is the most well-known example of this strong attraction in nature.
A second group of minerals exhibits paramagnetism, a weak attraction that only occurs when the mineral is placed within an external magnetic field. Paramagnetic materials contain unpaired electrons, but the atomic magnetic moments are randomly oriented and do not spontaneously align in domains. Iron-bearing silicate minerals, such as Pyroxene, often fall into this category due to the iron atoms dispersed throughout their crystal structure.
Finally, all materials exhibit a very weak repulsion from a magnetic field, a property called diamagnetism. This repulsion is caused by the orbital motion of all electrons temporarily altering in response to the external field. Diamagnetism is the dominant behavior in minerals that have no unpaired electrons, like the common rock-forming mineral Quartz (\(\text{SiO}_2\)), but is typically overshadowed by stronger effects.
Identifying Magnetic Minerals and Practical Uses
The magnetic property of minerals serves as an important characteristic for identification in the field and in industry. Highly magnetic minerals like Magnetite and Pyrrhotite are easily identified because they are visibly attracted to a simple hand magnet. Both minerals owe their magnetic strength to high iron content and specific crystal arrangements, though Pyrrhotite’s magnetism is less intense than Magnetite’s.
This sensitivity to magnetic fields is heavily leveraged in practical applications. In geological surveying, researchers use highly sensitive instruments called magnetometers to measure small variations, or anomalies, in the Earth’s magnetic field. These anomalies can indicate the presence of buried deposits of iron ore, copper, or nickel, which are often associated with magnetic minerals.
Magnetic properties are also used extensively in mineral processing and mining to separate valuable minerals from non-magnetic waste material. The technique, called magnetic separation, uses magnetic drums or belts to divert magnetic particles, such as Magnetite, allowing for the efficient concentration of the desired ore.
Furthermore, certain minerals act as tiny recorders of history. When minerals containing iron oxides cool from magma or settle in sediment, their magnetic domains align with the Earth’s magnetic field at that time. This preserved magnetic signature allows geoscientists to study the planet’s paleomagnetic history, including the ancient reversals of its magnetic poles.