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METEORITE OR METEORWRONG?
metal, iron, & nickel
About 95% of all known meteorites contain iron-nickel (FeNi) metal. "Iron-nickel" means that the metal is mostly iron but it contains 4-30% nickel as well as a few tenths of a percent cobalt. Iron-nickel metal in meteorites also has high concentrations (by terrestrial standards) of rare metals like gold, platinum, and iridium. It's usually easiest and cheapest to test for nickel, however, because it's more abundant than the rare metals.
Most metal-bearing meteorites are stony meteorites known
as ordinary chondrites; the
rest are irons and stony irons. Among ordinary chondrites, the most
common type, H-group chondrites (41% of all meteorites), have the
most metal, 15-20% by mass. L-group chondrites (37% of all meteorites)
have some metal, 7-11%. LL-group chondrites (13% of all meteorites)
have the least metal among ordinary chondrites, 3-5%. Because chondrites
are rich in metal and the metal is rich in nickel, all chondrites
have a bulk (whole rock) concentrations of Ni (nickel) of 1.0-1.8%
(i.e., 10000-18000 ppm). That's 100-1000
time greater than practically any terrestrial (Earth) rock.
An Earth rock with as much as 1.0-1.8% Ni would be a nickel ore. |
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Metal
grains reflecting light in a sawn slice of the Taffassasset (CR-anom
or achondrite). Click on image for enlargement. Note in
the enlargement on the right the saw marks in the metal grains.
This is a good
way to distinguish metal from shiny sulfide minerals like
pyrite - the sulfides grains won't look so severely scraped.
Photo by Randy Korotev. |
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 Sawn and polished slab of the Campo del Cielo (IAB) iron meteorite. Notice the rusty spots. Click on image for enlargement. Photo by Randy Korotev. |
 Sawn and polished slab of the Gibeon (IVA) iron meteorite. This meteorite has a distinct Widmanstätten pattern. Click on image for enlargement. Photo by Randy Korotev. |
Try ThisSometimes it's hard to tell the difference between metal and shiny nonmetals like some sulfide and oxide minerals. One easy test for grains or slabs that are at least a few millimeters in size is simply to measure the electrical resistance with an ohmmeter. You can buy handheld multimeters in any good hardware store for $30, and they're great for checking the voltage on partially used batteries. In resistance mode (ohms), putting the leads some distance apart on any of these iron meteorites would give a low resistance - <100 or probably <10 ohms. (This won't work on an ordinary chondrite because the iron grains aren't connected.) A shiny hematite or pyrite aggregate will have very high electrical resistance because they do not conduct electricity. |
 Sawn and polished slab of the metal portion of the Glorieta Mountain (ung) pallasite. Photo by Randy Korotev (natural light). |
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 One fragment of the Campo del Cielo (IAB) iron meteorite. Photo by Randy Korotev |
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A sawn face of the he Conception Junction
pallasite |
Iron meteorites, of course, are 100% metal. Pallasites, a rare type of stony-iron meteorite, consist of olivine grains embedded in an iron-nickel metal matrix. Because of they contain much iron-nickel metal, all metal-bearing meteorites are attracted to a magnet.
With a few rare and well known exceptions, naturally occurring terrestrial rock do not contain iron metal or iron-nickel metal. There are two reasons. First, early in Earth's history the iron-nickel it contained metal sank to form the Earth's core. Second, any metal that did not sink has oxidized (rusted) over Earth's long history. The Earth's environment is far more oxidizing (oxygen atmosphere and water) than space, where meteorites originate. Earth rocks do contain iron and nickel, but only in oxidized (non-metallic) form. Therefore, if you find a rock that contains iron-nickel metal, it's probably a meteorite. That sounds simple, but there are two problems.
First, many people find slags and other by-products of metal manufacturing. Some of the samples that have been brought to us may have been from forges or blacksmith shops that are more than 100 years old (see meteorwrongs 026, 027, 061, 065, 070, 075, 093, and 122). Others appear to fall from the sky for unknown reasons (see Getafe). Metal in slags and industrial by-products is mostly iron. Such materials will probably contain little nickel (much less than 1%). So, if you can determine that the sample has little or no nickel, then the sample is not a meteorite. Also, the metal in meteorites have very low concentrations of chromium and manganese, <0.02%. These two elements are common in man-made metals however. If the metal contains more than 0.02% chromium or manganese, it's not a meteorite.
The second problem is that some minerals in terrestrial rocks look like metal but are not. All that glitters is not metal. Many rocks contain small grains of sulfide minerals like pyrite ("fool's gold") or micas that are finely disseminated and shiny. I've had many people tell me, "But, it contains metal!" when there really isn't any. Clue: If there are shiny bits in it but it's not magnetic, then it's not a meteorite (Meteorite Realities).
Look at the photos of how metal in distributed in these photos of ordinary chondrites. The metal does not occur in big round globules. Globs are typical of slags. Notice that the metal is sufficiently soft that saw marks and smearing can be seen on the sawn faces. Sulfide minerals don't do that. Note that the meteorites do not contain vesicles. Vesicles (gas bubbles) are also typical of slags.
Finally, some rare meteorites do not contain any appreciable metal and consequently have low concentrations of Ni. Most of the meteorites known as achondrites are poor in metal and nickel. In other words, many of the rarest types of meteorites contain little or no metal and have low nickel concentrations, just like Earth rocks.
Bottom Line:If you have a chunk of metal or a rock
that contains metal and the metal contains >4% nickel
(Ni), then it is probably a meteorite. If the metal contains >0.02%
chromium (Cr) or manganese (Mn), then it is not a meteorite,
however. |
The DMG Test for NickelI have had some success using a nickel allergy test kit to determine whether metal contains nickel. Such kits are available at well-stocked pharmacies and can be ordered over the Internet. All such tests rely on DMG (dimethylglyoxime), which forms a complex that has a distinct pinkish color with ionic nickel. Some people have allergies to nickel and metal alloys that contain nickel. The kit I tested was designed to determine whether "metallic objects" contain nickel. It consisted of 2 dropper bottles. "Solution A" was DMG in alcohol. "Solution B" was a weak solution of ammonium hydroxide in water. The directions read "Place one drop of solution A and one drop of solution B on a cotton-tipped applicator (use equal amounts of both solutions). Rub wet applicator firmly against the test object for 15 seconds. If applicator turns red, the object contains nickel." Following these directions, I was unable to get a positive result on the iron meteorite pictured above, which contains 6% nickel (the low end of the range among metal in meteorites). The applicator did not turn red, but it did turn a rusty brown color. The problem as I see it, is that the test requires ionic (oxidized) nickel, and ammonium hydroxide does not liberate much ionic nickel from metal.
I tried the test also on a sawn face of an ordinary (H group) chondrite and also obtained a positive result. So, what do you do? Hydrochloric acid is available to consumers is building supply stores as "muriatic acid." Its used to clean mortar off masonry, among other things. It's extremely nasty stuff, and may not be available in quantities less than a gallon, which is enough to ruin a significant portion of your car. Dilute it 50-to-1. The test won't work if the solution is too acidic. Dilute battery acid (sulfuric acid) would probably also work. Some liquid toilet bowl cleaners contain acids strong enough to dissolve metal. They're usually already colored, however. I'm going to try simple vinegar or lemon juice, which are weak acids. Some people have contacted me saying that they obtained a positive result (pink color) when they applied this test to rocks that do not contain metal. I don't understand this. The test is designed for metal and the test is sensitive, but very few terrestrial rocks contain enough nickel to give a pink color. Remember, you're looking for strawberry pink, not rusty pink. Note added later: I recently used this test on an iron
meteorwrong that someone brought to us. If I use the nickel allergy
test kit as is, the results are negative - no pink = no nickel.
When I apply a bit of hydrochloric acid first, I do get a positive
result - a pink cotton swab. Later, we did a chemical analysis
for Ni and obtained 600 ppm. This is a lot of nickel, but is still
10
times
too low
for
a meteorite. (Concentrations of cobalt, gold, and iridium were
also much too low for a meteorite.) |
A sawn, polished, etched slab of the Canyon
Diablo iron (IAB) meteorite showing the Widmanstätten
pattern and large sulfide (troilite) inclusions. The meteorite
specimen is the property of the Collection
of the Arizona State University Center for Meteorite Studies.
Photo by Randy Korotev.
Above: Gujba, a CB chondrite, fell in Nigeria in 1984. Only about 13 CB chondrites are known. Gujba and some other CB chondrites have rounded metal grains. Notice that this specimen has not been polished thoroughly so the saw marks are still very evident in the metal.
Below: Usually, however,
rounded metal blebs means that the "rock"
is a piece of slag. In slags, the metal will be dispersed less evenly
than in a meteorite and there are usually vesicles (gas
bubbles) in the matrix because the matrix was molten. Click on images
for enlargement.
Photos by Randy Korotev. Thanks to Karl for loan of the Gujba specimen
and Jeff for the slag.
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