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An Internet guide to mineral magnetism
magnetite
Counting all the magnets in my house would be a real challenge: there are magnets in electric motors, magnets in speakers, magnets holding cupboards shut, magnets in toys, magnets on the fridge. We are surrounded by magnets. But that’s nothing new; Earth is littered with magnetic minerals. Some organisms have even evolved to make use of them. In ancient times, lodestones were humanity’s introduction to the mysterious force. On the Internet, retired NASA scientist David P. Stern ponders the course of civilization without those rare stones and describes a colleague’s research that suggests lodestones may be the result of lightning striking the right kind of iron ore.
As a geologist, I have long known the importance of magnetic data in revealing Earth’s secrets, such as the reality of plate tectonics. But the new discoveries, coming new techniques for measuring weak magnetic fields, are equally impressive. A best place to get caught up on the new findings is the August issue of Elements, an international magazine of mineralogy, geochemistry, and petrology. The editors assembled five articles on ground-breaking research in the field, which I will list below. In an introduction, guest editors Richard J. Harrison and Joshua M. Feinberg drive home the point that detectable traces of mineral magnetism are virtually everywhere in the environment. They also give a little technical background on the mineralogy, which is helpful in understanding the articles that follow. Click here to read the intro as a PDF file.
Before looking at the new discoveries made possible by magnetic minerals (some of which get quite technical), here is one basic fact: all magnetism arises from moving electric charges. On the scale of the Earth, its vast magnetic field is maintained by liquid iron circulating in its outer core; as the iron moves through the planet’s magnetic field an electric current is generated, which, in turn, reinforces the magnetic field—the self-sustaining geodynamo. (For an Internet guide to planetary magnetic fields, see my March, 2008 column, Journey to the Core.) On a much smaller scale, magnetic fields are created by single, spinning electrons orbiting inside an atom. As the primer on the classes of magnetic materials at The Institute for Rock Magnetism at the University of Minnesota explains, “This may be a surprise to some, but all matter is magnetic.” There are, however, five different types of magnetic materials, determined by how the atoms are configured and whether or not their “magnetic moments” act in concert or cancel each other. Of all the minerals, by far the strongest (and important to geologists) is magnetite. For a brief description of that mineral, go to Geology.com. And for a fun, experiment-packed primer on all kinds of magnetism, go to Paul Doherty’s lecture at the Exploratorium, 2000 Years of magnetism in 40 minutes.
As I already mentioned, the plate tectonics revolution was launched largely on magnetic mineral data, first on land and then at sea. On the Internet I came across an inaugural article by Edward Irving in the Proceedings of the National Academy of Sciences on his pioneering work in paleomagnetism, entitled The Role of Latitude in Mobilism Debates. (In the early days, “mobilism” was used before “plate tectonics” became the accepted term.) By measuring magnetic signature recorded in ancient rocks, Irving deduced that the continents had once been at different latitudes. Be sure to scroll down to the bottom and click on Irving’s biography by freelance science writer Tinsley H. Davis; it’s worth reading just to see how Irving eventually shamed Cambridge University into giving him his doctor of science degree—a case study in the difficulties of breaking new ground. For a good summary of how the magnetic minerals in the seafloor basalt became a key piece of evidence in the plate tectonics revolution, go to this page at the Moorland School in the United Kingdom.
The first article in Elements magazine reporting on new research is by John A. Tarduno, a geophysicist at the University of Rochester. Entitled {http://www.elementsmagazine.org/archives/e5_4/e5_4_art_tarduno.pd}Geodynamo History Preserved in Single Silicate Crystals: Origins and Long-Term Mantle Control, it reveals how minute magnetic particles, frozen in ancient silicate crystals, are opening a window on the early history of our planet’s inner core and the formation of the geodynamo. Without those “magnetic time capsules” recording Earth’s magnetic field more than three billion years ago, that history would have remained forever in the realm of conjecture.
The second feature article in Elements pushes the magnetic record further back than I could have imagined, back to the formation of the planets in the early solar system. Magnetism of Extraterrestrial Materials, by Pierre Rochette and Jérôme Gattacceca, at Aix-Marseille University in France, and Benjamin P. Weiss at the Massachusetts Institute of Technology, explores the information that can be teased from magnetic minerals in meteorites—rocks from worlds shattered in the early chaos or representing the stuff from which they formed. For a quick look at the magnetic fields of other planets, go to this page by Paul Doherty at the Exploratorium.
The third article in Elements is about two disparate, yet remarkable new sources of magnetic mineral data: the largest deposit of wind-blown dust in the world, which stretches across China in a layer that is, in places, more than a thousand feet thick; and the leaves of trees along busy highways. Barbara A. Maher, a professor of physical geography at the Center for Environmental Magnetism and Paleomagnetism at the University of Lancaster in the United Kingdom, is the author of Rain and Dust: Magnetic Records of Climate and Pollution. In the first case, she reports on how the magnetic minerals deposited by the wind can be used to reconstruct a record of the Asian summer monsoons, spanning the glacial and interglacial periods of the last 2 million years—perhaps the best land-based climate record. In the second case, Maher examines how magnetic particles emitted from vehicles and industry are trapped by tree leaves, offering a new way to monitor the environment.
The fourth article in Elements is somewhat of an anomaly, venturing outside the geosciences and into biology. In Magnetic Nanocrystals in Organisms, Mihály Pósfai, a mineralogist at the University of Pannonia in Hungary, and Rafal E. Dunin-Borkowski, an experimental physicist at the Technical University of Denmark, report on the frontiers of biomagnetism. Many organisms, from bacteria to pigeons to humans harbor tiny crystals of magnetite or two other magnetic minerals. Experiments have show that some can also sense the Earth’s magnetic field for the purposes of navigation. But figuring out where the magnetoreceptors are hidden in an organism is no simple task. In pigeons, magnetite grains arrayed in six points in the birds’ beaks seem to do the trick, but in other animals the search has just begun. New transmission electron microscopy techniques, including “electron holography,” are beginning to reveal how tiny magnetic particles function in biological systems, but mysteries still abound.
For more on “magnetotactic” see professor of physics emeritus Richard B. Frankel’s page at Cal Poly State University at San Luis Obispo in California. For more on the homing pigeon’s newly discovered organs for detecting Earth’s magnetic field, go to science blog Neurophilosophy’s entry Researchers Identify putative magnetoreceptors. Click on the link at the bottom of the entry for details on how the pigeon’s use their magnetoreceptors. For a history of how traces of ancient life have been identified from the magnetic minerals it leaves behind, go to Caltech professor of geobiology Joseph L. Kirschvink’s Magnetofossil Homepage. The last entry cites the debate over whether or not the magnetic minerals found in a Martian meteorite are evidence that life existed on the Red Planet.
In the last article in the “magnetic” issue of Elements, Crustal Magnetism, Lamellar Magnetism and Rocks That Remember, geophysicist Suzanne A. McEnroe at the Geologic Survey of Norway in Trondheim and her colleagues report on a new type of magnetic memory recorded in the rocks called “lamellar magnetism.” It promises to reveal much more about the structure of the deep crust on Earth and on other planets, like Mars. It is also points to a way of making better data storage devices in the future. Magnetic minerals, in the form of lodestones, helped spur the scientific revolution, but they still have much to teach us.