Mineralogy Faqs

Popular Mineralogy Interesting mineralogy and earth science for the amateur mineralogist and serious collector - #17

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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.

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. It is even possible to break out You may have heard it said that the core of spheres of cristobalite from the encasing obsidian. the Earth is solid iron. The innermost core is ©2008 A. A. Sicree

______solid metal and the outer shell of the core is liquid metal, but the composition of these Josephinite: A Rock Out of the Core? layers is thought to be a mixture of iron and nickel rather than pure iron. In fact, metal in In the streams of Josephine County, southwestern Oregon, nuggets occur which are predominantly the Earth’s core probably is similar to the composed of nickel-iron. Known as josephinite, this iron-nickel metal found in the iron meteorites. material is more properly termed a rock rather than a

Disko Island, off the east coast of mineral. Studies have shown that in addition to being made up of more than one nickel-iron alloy mineral, Greenland, is famous for producing native josephinites can also contain andradite garnets, iron- iron. This iron occurs in the porphyritic cobalt alloy minerals, iron-nickel arsenide and iron- feldspar basalt of Ovifak on Disko Island copper-nickel sulfide minerals. Masses up to 100 (a.k.a. Qeqertarsuaq). Native iron is usually pounds have been found but small nuggets are more common. found as small blebs in the basalt, but larger masses are found as well. One large mass Even more fascinating than the composition of these weighed more than 20 tons. nuggets are the controversial theories for their origins. One theory claims that josephinites resulted from Native iron formed when hot basaltic reductive desulfurization during metamorphism – in magmas erupted through local coal beds. The other words, that they were produced from iron-nickel super-heated coal served as a reducing agent sulfide minerals. Evidence for this theory is based on josephinite mineral textures. Another theory claims that reacting with iron in the basalt to produce the nickel-iron was derived from deep in the Earth’s native iron metal. This is similar to what mantle, perhaps from the core-mantle boundary or even happens in a blast furnace when iron ore the outer core itself. The nickel-iron could have been minerals are mixed with “coke” (essentially carried upward by plastic flow of rock within the cooked coal) at a high temperature. The iron mantle, then obducted upward through the crust. ore is reduced and molten iron is the result. Although some critics have derided this theory, other researchers have discovered that josephinites have In addition to its fame as a locality for excesses of the isotopes helium-3, neon-21, and xenon- native iron, Disko Island is geologically quite 129. This has been interpreted as support for the idea that josephinites came out of the Earth’s mantle. Thus, interesting. Various mineral deposits and they could be the “deepest” rocks ever brought up the fossil beds occur, and more than 2000 hot Earth’s surface and perhaps the only available samples springs are found on the island. ©2008 A. A. Sicree of the materials making up the Earth’s core.

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ACROSS 17 18 1 ability to break a mineral 19 20 21 10 what coprolites are fossils of 14 planet where we find minerals 22 23 24 25 26 27 28 15 opposite of an outee 29 30 31 32 33 16 said when you drop the Hope Diamond 34 35 36 17 means rich 18 calcite displays double ___ 37 38 39 40 19 country road 41 42 43 44 20 the Sun King 45 46 47 48 21 where you find josephinite 22 lightweight metal 49 50 51 52 23 a bachelor 53 54 55 56 25 where diamonds are sold 57 58 59 60 61 62 63 29 useful for finding minerals 31 means the same 64 65 66 33 a girl, not a caribou 67 68 34 period of geologic time 35 tetragonal (ab) 2 44 useful for finding gems 36 Geographic Info System small sudden attack 46 International Harvester 37 __ Dog, __ 3 Intern’l Red Cross 48 produces gold, copper 38 your old lady 4 volcanic ring islands 50 wear down rocks 39 atmospheric pressure unit 5 Islamic B.C. 54 E Pluribus ___ 40 color of malachite (ab) 6 not the truth 56 Planet of the ____ 41 ___ Fawkes Day 7 short for more data 57 what owl says 42 over the engine of a car 8 land, ____ firma 58 affectionate name 44 Chinese guy and a suit 9 cheers 59 Terlingua calcite state 45 had a farm _ _ _ _ O! 10 drunken ___ 60 price label on mineral 47 Org. of African Unity 11 used to remove minerals 61 = Employ Opportunity 48 a carbonate mudstone 12 A.D., ___ Domini 63 Mount Desert Island

49 thorium silicate mineral 13 2000 pounds 66 Japanese blue and green

51 peninsular state 18 corundums LAST MONTH’S SOLUTION: Historic 52 Internet Provider 22 large simian T R I L O B I T E S CONE 53 ruthenium 24 supersonic plane O A S E S L O M A HOHO 55 geological time 26 blocks a stream P R O A C L AMSHELLS OE V I A DY I 56 Attorney General 27 rock (ab.) G W E N H STRATUM 57 stones used on knives 28 studies rocks RA Y S Y A W MINI 62 Zulu military unit 30 long ___ AD O I O W A DOC N 64 long tube 32 orthorhomic PJ A N N S AUK ME H A S K PLUG GAR 65 make a duet 35 nice ___ IO N S SAD GAEA 66 mountains in Peru 36 type of chisel C R I N O I D BR IL L 67 black silica rock 38 Yosemite naturalist M O D EON TI 68 studies gems 39 how synthetic gems come Q U A T E R N A R Y KR I S UG L Y E O NS AGATE DOWN 40 Green River fossil fish AH S O B R ACHIOPOD 1 magnetite type magnet 41 measuring the Earth 43 clumsy guy