MAGNETITE: A NATURAL HISTORY

An Iron Oxide that Changed the World

- STEVE VOYNCIK

Whether they will admit it or not, most placer miners have voiced unkind comments about magnetite. And that’s understandable because magnetite is the primary component of the ubiquitous, heavy, black sands that often clog sluice box riffles. But there’s another side to magnetite that deserves respect, for this iron-oxide mineral has profoundly influenced history, culture, industry, and science.

The best-known variety of magnetite is lodestone. Its natural magnetism has created colorful alchemistic lore; greatly advanced navigation, surveying, and cartography; and inspired landmark breakthroughs in scientific thought. Magnetite made possible the early voice recorders that revolutionized radio broadcasting; today, it is used in many industrial dense-media-separation processes.

And that’s not all. More than a half-billion tons of magnetite ore are currently mined worldwide each year as a source of iron. And geophysicists study magnetite grains in igneous rocks to learn about the ancient Earth’s magnetic fields and tectonic-plate movements.

Magnetite (iron oxide, Fe 3 O4 ) consists by weight of 72.36 percent iron and 27.64 percent oxygen. It crystallizes in the isometric system, usually as octahedrons, occasionally as dodecahedrons, and rarely as cubes. Opaque and with a submetallic-to-metallic luster, magnetite is black to dark-gray in color with an occasional hint of blue iridescence. Brittle and with a subconchoidal-to-uneven fracture, it has a Mohs hardness of 5.5-6.5 and a substantial specific gravity of 5.17.