Magnetic Properties of Minerals

Hello all,

As many of you know, I specialize in finding and collecting the radioactive rare-earth minerals from the Llano Uplift in central Texas. Since a lot of them tend to look alike, I have tried to develop field tests which would aid in the identification of these "black uglies". One test I've come to depend on involves using a length of dental floss to suspend a small specimen of the mineral under test and then observing the specimen's behavior when a rare-earth "supermagnet" is brought close to it. This test has produced a few unexpected surprises.

Gadolinite, a REE, yttrium, beryllium silicate, is strongly paramagnetic. This is not something one would expect from what is essentially a greenish-black glass. The effect is so pronounced that gadolinite specimens collected from the Petrick Pegmatite can actually be picked up with one of these magnets, helping to distinguish gadolinite from the yttrialite that sometimes occurs with it. Chlorophane fluorite from the same location exhibits a pronounced diamagnetism, (being repelled by the magnet). The suspended fluorite specimen will always orient itself so that its broad side will align itself parallel to the magnetic lines of force. It is equally repelled by either pole of the magnet. Graphite and beryllium are so diamagnetic that you can actually float small pieces of them above the magnet.

This led me to wonder whether or not the magnetic behavior of a mineral should be listed in the table of properties used to identify it. I know that the strongly ferromagnetic minerals such as magnetite or pyrrhotite are normally identified by their strong attraction to a permanent magnet, but when was the last time you saw the magnetic properties of fluorite listed? It is only in the last couple of decades that the rare-earth supermagnets have been available for experimenters to play with. These samarium-cobalt or neodymium-iron-boron magnets are capable of magnetic flux densities considered impossible just as few years ago.

In the dark ages, when alnico magnets were the state of the art, a mineral either was attracted to the magnet or it wasn't. Now, with a small magnet salvaged from a computer hard drive, different minerals can be attracted or repelled to a measurable extent. Other magnetically-influenced phenomenon can also become apparent when a mineral is subjected to the intense magnetic field of a supermagnet. I noticed that small crystals of spessartine garnet from one of my new pegmatites would literally fly up to stick to a supermagnet, almost like crystals of magnetite. I selected some of the more strongly attracted grains and observed them under a stereo microscope, expecting to see inclusions of iron or other ferromagnetic material that might explain their ferromagnetism.

I did see that some of the more strongly magnetic crystals were opaque and could very well have been contaminated my iron, nickel or cobalt. I also noticed that some of the more moderately attracted garnet crystals were perfectly transparent with a slight amber tint. The real surprise occurred when I glued a few of these transparent garnet crystals to a glass slide and observed them under polarized light. As expected, there was very little in the way of extinction or polarization-induced color banding, that is however until I brought the supermagnet in close. With the polarzers crossed, the strong magnetic field caused the crystals to come alive with bands and swirls of iridescence. Furthermore, these color bands tended to follow the magnetic lines of force. Moving the magnet around the microscope stage caused the color lines in the garnet crystals to move as well. I know that there are manmade garnets (YIG) that can induce phase shifts into microwaves passing through them in response to an external magnetic field, but this is the first I've heard of this property affecting visible light.

I've strayed from the subject a bit, but what I mean to ask is this; Given the availability and low cost of incredibly powerful permanent magnets, has any thought been given to cataloguing the effects induced into minerals by those ultra-strong magnets? I can only wonder if thare's a whole unexplored field out there that would have been impossible to study just a couple of decades ago.

Frank

"With the polarizers crossed, the strong magnetic field caused the crystals to come alive with bands and swirls of iridescence."

In physics, this is known as Faraday effect: in paramagnetic substances (and particularly so in ferromagnetic substances) magnetic fields induce a double refraction with the optical axis parallel to the direction of the field. Hence, this effect is best observed in a direction perpendicular to the field.

The problem that I see is that several "magnetic" minerals are so because they bear very small magnetic inclusions. The case of magnetism of native platinum has been solved by Daubrée long ago, who showed that this was due to trace amounts of iron. Numerous studies have been devoted by ore dressers to magnetic properties of chromite, cassiterite, etc., where huge money is at stakes.

The test for magnetic effects I use is similar to yours but in reverse. I have a small (4 mm dia.), powerful magnet suspended from a thread which I bring near the mineral. With this method, I can "scan" the surface of a specimen looking for areas of magnetic interaction. Some of the specimens I have scanned would challenge the strength of dental floss had I suspended the mineral.

Franklin, very cool. I wonder if you could use a small magnetic head as a probe and measure the force of attraction as an indicator of whether the magnetic properties are due to contamination or to the overall properties of the specimen. You might be able to use the head from an old magnetic hard drive.

Not surprised to hear of contamination of Pt by iron--Pt is highly siderophilic, which is of course why most of it is to be found in Earth's core.

A few years a go I downloaded and read "Magnetic Susceptibilities of Minerals", USGS Open-File Report 99-529, by Sam Rosenblum and Isabelle K. Brownfield. Generally it had to do with the magnetic separation of minerals. It also contained extensive tables on a wide range of minerals. What intrigued me was that not only minerals containing iron could be separated, but so could those containing copper or manganese. But not those with nickel or cobalt.

As a result, I hung two neodymium magnets (1x3x6cm) from a length of video tape caught between them. The tape twists easily, but has a small restoring force that tends to return it to its original position. I glued a 1cm square mirror on one of the magnets so a light could be reflected from it to make an optical lever. I have found that almost any mineral containing iron, copper, or manganese will cause a displacement, even small crystals. One refinement, the tape needs to be reinforced where it emerges from between the magnets. Pyrite, chalcocite, and romanchite are not attracted.

Hey Don! I made a magnetometer slightly modified from your design. Rather than using two magnets attracted to each other, though, I mounted them to the mirror in the direction where they repel each other (which was a task that took some doing). Wouldn't this result in a stronger field than if you mount the magnets so they attract each other? -Tom

Hi Tom;

I don't think the field would be enhanced. I believe that one of the properties of REE magnets is that the larger they are, the stronger they are. And if they are joined South to North, that makes a larger magnet.

I mounted my pair so the long axis of the magnets is horizontal. This provides higher torque on the video tape when I bring a specimen close to the face at one end. I can get it swinging even with micro crystals of many/most iron, copper, or manganese minerals. Iron bearing garnets, rhodonite, rhodochrocite, black pyroxentes and amphiboles, cuprites, among others, all cause it to swing. A very small displacement of the magnets causes a large displacement in the reflected spot of light when it is directed at a wall across the room.

If I remember correctly, you also built the S.G. balance. Have you used it yet?
How did it work out for you?

Don

I will think about that a bit more... Yes, I did build a SG Balance, but have been busy with other things this winter and spring. Hope to get back to rocks and minerals in a few weeks.

A little about magnetism. If you think about a dipole magnet, you can view the field as lines of force emerging from the North pole and looping around to the South pole. (Note, you can actually see these lines of force if you place a magnet under a glass with a light dusting of iron filings and vibrate the glass, or you can suspend the iron filings in glycerine--really cool if you're a geek like me.) I the magnet is in isolation, every line that emerges from the north pole must terminate on the south. Now bring in a second magnet. If arranged so the magnets attract, now the magnetic field lines have a more direct path if they terminate on the south pole of the second magnet--and that's more energetically favorable. The lines emerging from the north pole of the second magnet loop around a bit further to terminate on the south pole of the second magent. The closer the two magnets are, the less extra distance the 2nd magnet's lines must stretch, so there is a strong attraction between the poles.
When the two magnets are configured so they repel, since no two field lines can cross, this distorts the lines and forces them to travel extra distance before they terminate on the south pole of the same magnet. The further away the magnets, the less distortion--hence the repulsion.

Bottom line, there will be no difference in magnitude for attractive or repulsive force of 2 magnets at the same distance from each other.

OK, Ray. But, am I correct that the larger a neodymium magnet is, the stronger its field? Thus, two of them together (N to S in contact) would be stronger than one alone?

Don, Yes, the larger the magnet the greater the field strength, though maybe not linearly.

I encountered a magnetic mystery in the Min. Rec. vol 38(5) 2007 concerning Bou Azzer. I am puzzled by the remarks concerning the magnetic properties ascribed to sphaerocobaltite on p. 396. As far as I am aware, cobalt carbonate is ultimately antiferromagnetic with a Néel temperature somewhere down below the liquid nitrogen region. At room temperature, I would expect it to be merely paramagnetic, and I am not at all certain that one could easily detect a difference between the paramagnetism of cobalt carbonate and cobaltoan calcite with just a simple test using a hand-held magnet. Was a more sophisticated susceptibility test implied here? I would think at a minimum one would have to use a detached crystal of sphaerocobaltite mounted on a thin fiber.

William, I am no expert on this, but after studying "Magnetic Susceptibilities of Minerals", USGS Open-File Report 99-529, by Sam Rosenblum and Isabelle K. Brownfield. I also would expect sphaerocobaltite to be paramagnetic and not attracted to even strong magnets.