MAGNETITE: A NATURAL HISTORY
Rock&Gem Magazine|July 2021
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.

Abundant and widely distributed, magnetite is present in most mineral environments. It occurs in particulate, crystalline, and massive forms and is a common accessory mineral in igneous, metamorphic, and sedimentary rocks. When magnetite weathers free from host rocks, its density enables it to concentrate gravitationally on beaches and in placer deposits.

PHYSICAL PROPERTIES

Magnetite’s most notable physical property is its natural magnetism, which is by far the most intense of any natural material. One of nature’s basic forces, magnetism is produced when moving electrons generate electromagnetic fields characterized by the dipole effect of polarization. A familiar example of polarization is the north-south poles of bar magnets, in which like poles repel and opposite poles attract.

The story of magnetite’s magnetic properties begins with iron. Because of its atomic structure, iron has the greatest magnetic susceptibility—the tendency to become magnetized—of any metal. Cobalt and nickel can also become magnetized, but not nearly to the extent that iron can. The magnetic susceptibility of a metal, mineral, or alloy is determined by the amount of iron, nickel, or cobalt present in it and the degree of atomic alignment that is possible within its structure.

Many minerals exhibit trace magnetism, but few have any significant level of magnetic susceptibility. On the magnetic-susceptibility scale, nonmagnetic minerals are rated at zero. Magnetite, the only mineral with obvious magnetism, is rated at 20. Next is chromite (iron chromium oxide, FeCr 2 O4 ), with a magnetic-susceptibility rating of 1.0.

Because the iron in magnetite is present as both ferrous (Fe+2) and ferric (Fe+3) ions, magnetite can be better described as Ferro-ferric oxide with the complete formula Fe+ 2 Fe+ 32 O4. Its crystal lattice consists of tightly packed cubes with oxygen ions occupying the corners and iron ions at the interstices (openings). The ferrous and ferric ions occupy fixed, precise tetrahedral and octahedral sites. This arrangement permits a continuous, directionally aligned flow of electrons between the ferrous and ferric ions.

The resulting electrical vector generates a magnetic field just as a wire conducting an electrical current also creates a magnetic field. Magnetite’s directional vector and magnetic field keeps its iron ions in directional alignment, with each behaving as a tiny bar magnet to maintain its magnetism.

COMMON BEHAVIORS

Although “normal” magnetite is attracted to magnets, it does not attract bits of steel (or other bits of magnetite). Only lodestone, the relatively rare variety of “automagnetized” magnetite, has sufficient magnetism to attract steel. The word “lodestone” stems from the obsolete word “lode,” which meant “course.” When lodestone appeared in Middle English in the early 1500s, it meant “leading stone” or “course stone,” alluding to its use in compasses.

Not all magnetite can become lodestone. In order to magnetize, it must have a specific composition and structure. Unlike normal magnetite, lodestone contains traces of maghemite (cubic iron oxide, Fe2 O3, a hematite polymorph) and ions of titanium, aluminum, and manganese. These impurities create an inhomogeneous structure that increases lodestone’s magnetic force and makes it a permanent magnet.

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