MAGNETITE: A NATURAL HISTORY
English word “magnet” in the 1400s, and of the related words “magnetic” and “magnetism” in the early 1600s.
The phenomenon of magnetism is mentioned in Chinese texts as early as 1800 BCE. By 200 BCE, these texts frequently described in great detail the ironlodestone attraction. Although lodestone was not yet being used for navigation, Chinese geomancers did fashion it into “south-pointing spoons” for divination purposes. Balanced on polished bronze plates, these lodestone spoons aligned themselves along a northsouth axis to aid the feng shui practice of arranging living spaces in harmony with the spiritual forces present in the environment.
Archaeological evidence suggests that some ancient Mesoamerican cultures also knew of magnetism. About 600 BCE, the Monte Alto culture in what is now Guatemala sculpted large pot bellied figures and human heads from magnetite-rich basalt. Archaeologists have detected strong magnetism in these sculptures—always at the navel and face sections. Researchers suggest that Monte Alto stoneworkers used lodestone to orient the sculptures for magnetism-related ceremonial or spiritual purposes.
Another magnetism-related artifact is an Olmec lodestone bar dating to 200 BCE that may have aided in the orientation of temples, dwellings, and graves.
Chinese mariners were using lodestone compasses for navigation by 700 CE. And by 850 CE, after learning to magnetize steel by rubbing it on lodestone, they had invented “wet” compasses with magnetized steel needles that floated on water. Europeans began making wet, steel-needle compasses around 1190.
By 1300, Europeans were utilizing “dry” compasses with magnetized steel needles balanced on pivots. By the dawn of Age of Exploration around 1400, dry magnetic compasses were routinely used for navigation and cartography. By then, mariners also understood the deviation between magnetic and astronomical north, and German miners were using compasses for underground surveying.
DEBATING LODESTONE MAGNETISM
Meanwhile, alchemists continued to debate the cause of lodestone’s magnetism. According to one theory, lodestone emitted an invisible effluent that created a void into which bits of steel would rush. Another theory proposed that garlic and diamonds could negate lodestone’s magnetism. Many alchemists also believed that lodestone possessed a soul, intelligence, and healing properties that cured everything from gout to distemper. Others simply condemned magnetism as a machination of the devil.
Rare and mysterious, lodestone was costly and much in demand by those who could afford it. Among the notable figures of the 15th and 16th centuries who possessed lodestone specimens was the English scientist Sir Isaac Newton, whose gold ring was set with a lodestone fragment that could pick up many times its own weight in steel.
Especially popular among the elite was “armed” lodestone—pieces of lodestone fixed with iron caps on the pole ends that concentrated the magnetic force. The six-ounce, armed lodestone of Italian astronomer and physicist Galileo Galilei could lift 15 pounds of steel. Galileo bestowed armed-lodestone gifts upon many prestigious individuals, including Holy Roman Emperor Ferdinand II and the wealthy Duke of Tuscany and patron of the arts Ferdinand de’ Medici.
England’s Queen Anne (reigned 1702-1714) used a lodestone set into an engraved silver case in her “royal-touch” healing ceremonies that supposedly cured the “king’s evil” (scrofula). Anne refused to touch her afflicted subjects directly, “treating” them instead from a distance with the magnetic force of her armed lodestone.
Lodestone also inspired groundbreaking scientific thought. In 1600, English physician and physicist William Gilbert published De Magnete, a landmark work about magnetism in which he correctly deduced that, like lodestone, the Earth itself was a magnet with north-south magnetic poles. In the late 1600s, the Royal Society of London displayed a six-inch-diameter lodestone sphere to demonstrate the Earth’s magnetic field.
Lodestone remained the sole source of magnetism until 1819, when Danish physicist Hans Christian Øersted discovered that wire conducting electrical current also generated magnetic fields.
For centuries, magnetic iron oxide had been known by such names as “magnet ore,” “black iron ore,” “Magnetstein” (magnetic stone), and “Magneteisenstein” (magnetic iron stone). In 1854, Austrian chemist Wilhelm Karl von Haidinger formally assigned it the name “magnetite.”
Magnetite was among the first minerals analyzed by X-ray diffraction in 1915. Knowledge of its atomic structure and ferro-ferric composition, along with a basic understanding of the emerging field of particle physics, finally combined to explain magnetite’s magnetism.
Magnetite and hematite (ferric oxide, Fe2O3) are the primary ores of iron. The 2.6 billion metric tons of ore now mined worldwide each year include 800 million metric tons of magnetite ore.
MAGNETIC MINNESOTA
Among the Earth’s oldest ores are northeastern Minnesota’s huge banded-iron deposits that consist of both hematite and magnetite. Banded-iron ores are partially altered sedimentary rock that was laid down during the late Precambrian Era. At that time, the Earth’s atmosphere was deficient in oxygen, and what is now northeastern Minnesota was covered by a warm, highly acidic sea rich in dissolved iron and silica.
Some 2.5 billion years ago, the Earth’s atmosphere began changing with the onset of the Great Oxygenation Event. This major biochemical transition lasted one billion years and was triggered by the rapid growth of cyanobacteria, simple, algae-like life forms that obtain energy from photosynthesis and release large volumes of oxygen.
As seawater slowly became oxygenated, it lost its acidity and precipitated iron and silicon. The iron, in the form of magnetite, and silicon, in the form of chert (microcrystalline quartz), accumulated in alternating layers on the sea bottoms. The heat and pressure of deep burial eventually lithified these sediments into hard, durable rock. Meteoric water and hydrothermal fluids later oxidized much of this magnetite into hematite.
The magnetite portion of the Minnesota bandediron ores proved both a blessing and a curse. Geologists could map the Minnesota deposits using only compasses to detect local variations in magnetic declination caused by magnetite concentrations. Magnetite was the preferred ore because it was richer in iron than the hematite ore and could be inexpensively concentrated by magnetic separation.
But on the downside, the large bulk cargoes of concentrated magnetite ore turned Great Lakes ore
ships into giant magnets, each surrounded by its own intense magnetic field that made compass navigation impossible. Until the introduction of global-positioning-satellite navigation systems, shipping magnetite ores on the Great Lakes was a hazardous undertaking.
MAGNETITE IN MODERN USES
In the early 20th century, finely powdered magnetite made possible the development of quality voice recorders. The first voice recorders in the 1890s 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 voicerecording tape revolutionized radio broadcasting by enabling programs to be rebroadcast and archived.
In the 1940s, powdered magnetite was put to work in industrial dense-media-separation processes to “clean” pyrite-rich, high-sulfur coal. Crushed coal floated on density-controlled, powdered magnetite water slurries, while the troublesome pyrite and other gangue materials sank as waste.
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 1960s, magnetite particles present in igneous rocks became the basis of the study of paleomagnetism, a subscience 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. Some “black-sand” ocean beaches consist largely of magnetite and have been mined for iron in the past. Two particularly interesting magnetite sites are in south-central Colorado’s Great Sand Dunes National Park and southern Utah’s Iron Springs Mining District.
At Great Sand Dunes, streaks of dark magnetite appear below the crest of each dune where wind has gravitationally separated the magnetite from the lighter-colored quartz sand. The sandy bottom of the park’s broad, shallow Medano Creek is also heavily streaked with flow patterns of dark magnetite sand.
Mormon immigrants first mined the Iron Springs Mining District west of Cedar City, Utah, for iron in 1852. In many of the district drainages, black magnetite crystals contrast with the lighter-colored gravels and are readily identified by their high density and reflective, triangular crystal faces. Some Iron Springs magnetite specimens are lodestone that exhibit distinct polarity and attract steel objects like paper clips.
The specimens of massive magnetite available at rock shops and online sites are often labeled “lodestone”—but few are natural lodestone. Most have had their magnetic intensity artificially enhanced by contact with strong magnets.
Well-developed octahedrons and natural magnetism have long made magnetite a favorite among mineral collectors. And even placer miners might agree that knowing a little about magnetite’s remarkable history makes those riffle-clogging black sands a little easier to take.