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Popular Mineralogy Interesting mineralogy and earth science for the amateur mineralogist and serious collector - #17 _________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ ___________________________________________________________________________________________________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________________________________________________________________________________ Magnetism and Magnetite, Lodestones and Lightning by Andrew A. Sicree ______________________________ The first lodestones A little bit of science According to ancient Greek legend, the A magnet is any object that possesses an first lodestone was found by a shepherd, external magnetic field. In common usage we Magnes, when the iron tip of his shepherd’s use the term “magnetism” to describe staff stuck to the black stone. Unlike many phenomena such as a steel bar magnet such stories, this legend is quite plausible: sticking to the door of a refrigerator, or a being a natural magnet, the lodestone will piece of lodestone which, although too weak attract iron metal or deflect the needle of a to hold its itself to the refrigerator door, compass. Like the ancient Greeks, deflects the needle of a compass. To contemporary youngsters are fascinated by a scientists, these are two different, but related, magnet’s ability to project an invisible force displays of two important types of magnetic and almost every young mineral collector has phenomena called ferromagnetism and a piece of lodestone in her or his collection. ferrimagnetism. A little bit of etymology All magnets are magnetic because of the motion of the electrons surrounding their The town or district of Magnesia is located atoms (moving electric charges generate in Thessaly in central Greece (was the magnetic fields). In a ferromagnetic material location named for the shepherd or the (such as iron metal, nickel, cobalt and most shepherd named for the location?). steels), the atomic magnetic fields align Historically, this district produced black themselves parallel to an externally applied stones that attracted iron. The classical name magnetic field, and produce a strong magnetic lithos Magnetis or “stone of Magnesia” is the field of their own. In a ferrimagnetic source of the modern term magnet from material, the atomic magnetic fields align which we derive the mineral name magnetite. themselves both parallel and “anti-parallel” Lodestone, on the other hand, is derived (parallel but with a polarity opposite to that of from the Anglo-Saxon lād, meaning “way” or the parallel components) to the applied fields. “journey.” Thus, a loadstone or lodestone The parallel components are stronger than the was a stone that “showed the way.” This was anti-parallel and thus the material is magnetic. because an elongated lodestone could be Magnetite or lodestone is ferrimagnetic. suspended from a string and used as a (Note that older mineralogy texts sometimes navigational tool. called magnetite a ferromagnet; in the 1940’s Louis Néel provided the theory to explain that Being polarized, a lodestone would always magnetite was really a ferrimagnet. Yes, I point in the same direction – a helpful know that there is only one letter difference characteristic when one is sailing a ship on between the two, but that one-letter difference the sea beyond the sight of land. In Dutch, represents a real difference in magnetic the navigational use of the lodestone was characteristics.) expressed in the word zeilsteen, from zeilen, “to sail,” and steen, “stone.” Thus a lodestone Thus, we note that ferrimagnetism is a or zeilsteen was a “sailing stone.” property intrinsic to the material itself. But, if ©2008, Andrew A. Sicree, Ph.D., [email protected] ~ 10-08-1 ~ Please do not reproduce or extract without permission this is so, why aren’t all pieces of magnetite Interestingly, you can change a lodestone lodestones? back into ordinary magnetite if you heat it up. In order for a specimen of magnetite to If you heat lodestone above about 575°C display a strong external magnetic field (i.e., (1067°F), the “Curie point” for magnetite, and become a lodestone), the magnetic “domains” then cool it back down, it will become in the specimen must be aligned to give a net ordinary, non-lodestone magnetite. magnetic field. When magnetite first forms, its magnetic domains (you can think of them The magnet test as many, many small bar magnets, each with Magnetite is the most strongly a North and a South end, making up the ferrimagnetic mineral and of all ferrimagnetic magnetite) are more-or-less randomly minerals it will most vigorously respond to a oriented, thus the magnetite does not behave bar or a horseshoe magnet. A magnet thus like a lodestone (the randomly-oriented bar becomes a useful tool for determining the magnets cancel each other out). If a strong presence of magnetite. Any small but strong electromagnetic field is applied to the rock, it magnet hung on a pivot or suspended from a will cause many of the domains to align string will be attracted to rocks containing the themselves in the same direction. This mineral. aligned magnetite will be a lodestone. Some minerals other than magnetite are A stroke of lightning also affected by a magnet. These include pyrrhotite (Fe7S8), greigite (Fe3S4), So, if you want to make magnetite into maghemite (γ-Fe2O3), goethite (α-FeOOH), lodestone, how do you apply a “strong feroxyhyte (δ-FeOOH), and jacobsite electromagnetic field” to the rock? In Nature, (MnFe2O3) – all of which are ferrimagnetic. a bolt of lightning does the trick. Lodestones In these minerals, the effect is considerably are thought to form when lightning strikes weaker than in magnetite and the property is magnetite in the rock. The pulse of lightning thus harder to detect. Native iron (rare, but realigns most of the magnetic domains in the found on Disko Island, Greenland) and iron- magnetite. When the North and South ends of nickel metal found in meteorites are also most of its magnetic domains are aligned with strongly attracted to a magnet. the same orientation (all the bar magnets are arranged in the same direction), a magnetite One good magnet test technique is to crush specimen will produce an external magnetic a suspect mineral into small grains and place field. Thus magnetite is transformed into them on a smooth sheet of paper. If a strong lodestone. Apparently most, if not all, natural magnet is brought close to the grains, some lodestones are produced by lightning strikes. will jump onto it. Grains of weakly magnetic minerals may not cling to the magnet, but it Evidence in support of this theory comes may be possible to detect their weak from the fact that on the ridge-tops and magnetism when they are disturbed and summits of mountains built of rocks moved slightly when the magnet is passed containing small amounts of magnetite, one over them. may typically observe strong deflections of ©2008, Andrew A. Sicree, Ph.D. one’s compass needle. Elsewhere, these same Dr. Andrew A. Sicree is a professional mineralogist and geochemist magnetite-containing rocks will not deflect a residing in Boalsburg, PA. This Popular Mineralogy newsletter supplement may not be copied in part or full without express compass needle. This phenomenon is permission of Andrew Sicree. Popular Mineralogy newsletter attributed to the effects of lightning striking supplements are available on a subscription basis to help mineral clubs produce better newsletters. Write to Andrew A. Sicree, Ph.D., the higher ground and converting mountain- P. O. Box 10664, State College PA 16805, or call (814) 867-6263 or email [email protected] for more info. top magnetite to lodestone. ©2008, Andrew A. Sicree, Ph.D., [email protected] ~ 10-08-2 ~ Please do not reproduce or extract without permission Native Iron from Disko Island Cristobalite: Pseudo Quartz Native iron is a very rare mineral. In spite Quartz is a very plentiful mineral. It shows up in almost every mineral collection. Even beginning rock of the fact that, on the Earth, iron is a plentiful collectors quickly learn that quartz is hexagonal and element, it almost always occurs combined has a simple chemical formula: SiO2 or silicon dioxide. with other elements. It can be found in the But not all silicon dioxide is quartz. A number of form of oxide minerals (hematite, magnetite, polymorphs exist. These polymorphs have the same goethite, etc.), sulfide minerals (pyrite, chemical composition as quartz but possess different chalcopyrite, etc.), silicates (pyroxenes, crystal structures. Polymorphs of quartz include the amphiboles, etc.), or other minerals. Iron is minerals coestite, stishovite, tridymite and opal (which is amorphous). usually in the oxidized or reduced form rather than being present as an uncombined “native” Another polymorph of quartz is the mineral cristobalite (tetragonal SiO2). Cristobalite occurs in element. Even the iron in iron meteorites cavities in volcanic rocks such as obsidian and rhyolite. usually occurs as the iron-nickel minerals White “snowflakes” in “snowflake obsidian” are sprays kamacite (α-Fe,Ni) and taenite (γ-Fe,Ni). of cristobalite crystals that crystallized out of the glassy obsidian matrix.

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