Waco Gem and Mineral Club, Waco, Tx

17.1 Magnetism


17.1: Magnetism

We‘ve all had fun with magnets: those little bars that stick to refrigerators and pick up paperclips. Place one near an iron or steel surface, and it will literally jump out of your hand and stick to the surface. They also attract and repel other magnets, letting you flip and push one around with another without them touching. Magnets perform these fun stunts because they produce a field of force—a magnetic field—caused by movement of electrons.

Some minerals react when placed near a magnetic field. Such minerals are referred to as magnetic minerals. They have one common denominator: iron. A simple test will help you identify a magnetic mineral. Pass it over a compass, and the compass needle will move.

Magnetic minerals come in two basic sorts. Most common are those attracted to a magnet, either strongly or weakly. The most strongly magnetic is magnetite (iron oxide), but there‘s also pyrrhotite (iron sulfide), ilmenite (titanium-iron oxide), hematite (another iron oxide), and franklinite (zinc-iron oxide). Some, such as limonite (hydrated iron oxide) or siderite (iron carbonate), may become weakly magnetic if heated.

A second sort of magnetic mineral is one that is naturally magnetized. That is, it‘s a magnet itself, generating a magnetic field that will attract iron to it. There‘s just one mineral of this sort, a specific variety of magnetite called lodestone.

You can enjoy fun activities with kids seeing how lodestone picks up paperclips or how magnets stick to magnetite and other magnetic minerals. For another fun activity, fill a plastic tub with black sand (often found in association with placer deposits while gold panning) and drop in a few strong magnets. Magnetic minerals in the black sand will clump around the magnets like frizzy hair on a bad hair day.

Magnetism has long fascinated people. For centuries, ships carried lodestones to magnetize compass needles for navigation. Magnetism has helped prospectors distinguish iron ore look-alikes; for instance, magnetite and chromite are outwardly similar, but magnetite is much more strongly magnetic.

The word 'magnet' comes from Magnesia, an area in Greece where lodestone was discovered long ago. It was called magnítis líthos, or 'magnesian stone'. According to another, more interesting legend, a Greek shepherd boy named Magnes discovered lodestone when iron nails in his shoes stuck to a rock. Sounds like a good excuse for being late to school!