Magnetism derives from a property called the magnetic moment that results from the spinning and orbiting of electrons. The sum of all the magnetic moments of all the atoms in a mineral gives it magnetism. There are different kinds of magnetism. Minerals may be ferromagnetic, diamagnetic, or paramagnetic.
In ferromagnetic minerals, the moments of a mineral’s electrons are aligned in a constructive way and the minerals have properties similar to those of metallic iron. Ferromagnetic minerals may be magnetized. Magnetite (Figure 3.80) and pyrrhotite are examples of ferromagnetic minerals, but magnetite is much more ferromagnetic than pyrrhotite.
Diamagnetic minerals exhibit little magnetic character overall but may be weakly repelled by a strong magnetic field. Pure feldspars, halite, and quartz all exhibit weak diamagnetism. An impure feldspar, however, may contain iron, which results in paramagnetism, which means it is attracted to a strong magnet. Other commonly paramagnetic minerals include garnet, hornblende, and many pyroxenes.
For routine mineral identification, only a few minerals – for practical purposes only magnetite – can be identified because of their magnetism. In the field, rocks that contain magnetite will attract a magnet. This sometimes help distinguish different rock formations. And, in the laboratory, subtle differences in the magnetic properties of minerals are routinely used to separate different minerals in crushed rock samples. Figure 3.81 shows a device used for this purpose. Magnetism, thus, can be an important property of minerals.