Most placer miners have voiced their opinions about pesky 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.
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, Fe3O4) 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.
Magnetite’s most notable physical property is its natural magnetism, which is by far the most intense of any natural material. 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, FeCr2O4), with a magnetic-susceptibility rating of 1.0.
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 1500’s, it meant “leading stone” or “course stone,” alluding to its use in compasses.
The Earth’s magnetic field is not strong enough to magnetize magnetite; the leading theory of how lodestone becomes magnetized focuses on lightning strikes. Lightning is an instantaneous and massive discharge of electrons that generates a very brief, but extraordinarily intense, local magnetic field. Upon striking a magnetite deposit, lightning’s magnetic field forces magnetite’s iron ions into more perfect alignment to increase its magnetism. Lodestone’s occurrence at or near the surface and not at depth supports the theory of lightning as the origin of its magnetism.
In the early 20th century, finely powdered magnetite made possible the development of quality voice recorders. The first voice recorders in the 1890’s utilized thin, steel wire as a recording medium but had poor voice reproduction. In 1928, German scientists designed voice recorders with magnetite microparticles loosely impregnated in plastic tape. As this tape moved through a recording “head,” a magnetic field governed by electrical sound signatures aligned the magnetite particles in specific patterns. A playing head then detected the tape’s magnetic field and amplified the electrical signal for quality reproduction of the original voice.
Previously, all radio programs were, by necessity, broadcast “live.” Magnetite-based voice recording tape revolutionized radio broadcasting by enabling programs to be rebroadcast and archived.
Today, magnetite-based, dense-media-separation is used in several mineral-beneficiating processes and, most importantly, in separating mixed, recycled metals. Because of their magnetism, magnetite particles in the slurries can be easily recovered and cleaned for reuse.
In the 1960’s, magnetite particles present in igneous rocks became the basis of the study of paleomagnetism, a sub-science of geophysics that deals with ancient magnetic fields. While originally contained in viscous magma, these magnetite particles aligned with the Earth’s magnetic field like tiny compass needles. When the magma solidified, the magnetite particles “froze” in place, creating a permanent record of the direction and intensity of the Earth’s magnetic field that existed at that time.
Paleomagnetic data enable geophysicists to determine the magnetic-pole proximity of the orientation of tectonic plates over hundreds of millions of years. This data also helps understand the “geodynamo”—the circulating currents of molten core material that generate the Earth’s magnetic field. Finally, paleomagnetic data are a record of the Earth’s magnetic-pole reversals—north-south “flips” that have occurred in the past and will occur again in the future.
As placer miners know, magnetite is abundant and widely distributed, however, we might agree that knowing a little about magnetite’s remarkable history makes those riffle-clogging black sands a little easier to take.