More than 90% of all meteorites contain iron-nickel (FeNi) metal. “Iron-nickel” means that the metal is mostly iron but it actually an allow that 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, stony irons, and other types of chondrites (see statistics). Among ordinary chondrites, the most common type, H-group chondrites (45%), have the most metal, 15-20% by mass. L-group chondrites (40%) have some metal, 7-11%. LL-group chondrites (15%) 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 times greater than practically any terrestrial (Earth) rock. An Earth rock with as much as 1.0-1.8% Ni would be a nickel ore.
Sometimes 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 test may not 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.
Iron Meteorites and Pallasites
Iron meteorites, of course, are nearly 100% metal, although many contain the iron sulfide mineral troilite. Pallasites, a rare type of stony-iron meteorite, consist of olivine grains embedded in an iron-nickel metal matrix. Because they contain much iron-nickel metal, all metal-bearing meteorites are attracted to a magnet.
With a few rare and 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 metal that it contained 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, then 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 may have been from forges or blacksmith shops that are more than 100 years old. 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 has 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, then it’s not a meteorite. If you have a chunk of metal that attracts a magnet and want to know if it’s a meteorite, obtain a chemical analysis for the elements iron (Fe), nickel (Ni), chromium (Cr), and manganese (Mn).
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 contains shiny bits but does not attract a cheap ceramic magnet, then it’s not a meteorite.
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 typical of slags.
Finally, some rare meteorites do not contain any appreciable metal and consequently they have low concentrations of Ni. Most of the meteorites known as achondrites are poor in metal. In other words, many of the rarest types of meteorites contain little or no metal and have low nickel concentrations, just like Earth rocks.
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 <4% nickel, then the metal chunk or rock is not a meteorite. If the metal contains >0.02% chromium (Cr) or manganese (Mn), then it is not a meteorite, however.
If you have a rock that contains between 1.0 and 1.8% nickel (whole-rock analysis), whether or not it appears to contain metal, then the rock might be a meteorite.
If you have a rock that does not contain metal and has a low concentration of nickel (<1% = <10000 ppm), it could still be a rare type of meteorite, an achondrite. (About 6.5% of stony meteorites are achondrites. The probability is exceedingly small, however, because nearly all (guesstimate: >99.999%) Earth rocks have the same properties – no iron-nickel metal and low concentrations of nickel (<0.3%).
The DMG Test for Nickel
I 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 and palladium.
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.
As an experiment, I applied a tiny drop of 1% hydrochloric acid (0.3 molar) to the meteorite, waited 15 seconds, and repeated the DMG test by swabbing the acid drop. This time I got a positive result (left). The acid dissolved a small portion of the meteorite, putting nickel ions in solution. The manufacturer of the test kit is not likely to suggest this work-around because hydrochloric acid is very corrosive and is likely to ruin jewelry and other metals if used incorrectly. (I rinsed the meteorite in much water after the test.)
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.” It’s 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 100 times too low for a meteorite. (Concentrations of cobalt, gold, and iridium were also much too low for a meteorite.)
The DMG test is very sensitive to nickel and can lead to a “false positive” with some metal samples. A negative (no pink) result probably means that the metal is not from a meteorite. A positive result means that it might be a meteorite or it might not! A correspondent who has done more research on this than I have claims that if the pink color fades away after 5 minutes, then the metal contains Ni, but not enough to be of meteoritic origin.