Metal, Iron, & Nickel
Iron-Nickel Metal
More than 95% of all meteorites contain iron-nickel (FeNi) metal. “Iron-nickel” means that the metal is mostly iron but it also contains 4-30% nickel. The metal occurs as two different alloys known as kamacite (lower nickel concentration) and taenite (higher nickel concentration). Both alloys strongly attract magnets. Neither alloy occurs naturally in Earth rocks, so a natural rock that contains kamacite or taenite is a meteorite.
The metal in meteorites also contains a few tenths of a percent cobalt; the nickel/cobalt ratio in meteoritic metal is usually in the 10-25 range. 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 and easier to measure than the rare metals.
The metal in meteorites strongly attracts a magnet. If you have a chunk of metal or a rock that contains metal but it does not attract a magnet, then it is not a meteorite. |
Ordinary Chondrites
A sawn face of the Faucett (Missouri) H5 chondrite. All the light-colored specks are grains of iron-nickel metal.
Most metal-bearing meteorites are stony meteorites known as ordinary chondrites; the rest are other types of chondrites, brecciated achondrites, irons, and stony irons (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 bulk (whole-rock) concentrations of 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. In terrestrial rocks the nickel is not contained in metal, however, but in silicate and sulfide minerals.
One cannot determine if a rock contains high concentrations of nickel just “by looking.” FeNi metal looks just like Fe metal. A chemical analysis is required. Metal detectors cannot determine if a rock contains 1-2% Ni or that a chunk of metal contains 4-30% nickel. |
Sometimes it’s hard to tell the difference between metal and shiny nonmetals like some sulfide and oxide minerals. If the “metal” is yellowish, it’s not metal but pyrite (“fool’s gold”).
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 high electrical resistance because they do not conduct electricity.
Metal grains reflecting light in a sawn slice of Tafassasset (primitive achondrite). Note the saw marks in the metal grains. This is a good way to distinguish metal from shiny sulfide minerals like pyrite – sulfide grains won’t look so severely scraped.
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. The concentration of nickel in iron meteorites and the metallic part of pallasites, typically 4-30%, much higher than that in industrial metals except high-nickel steels. The concentration of nickel in industrial iron is usually <1%.
Left: A softball-size piece of the 2-ton Campo del Cielo (IAB) iron meteorite. Right: Sawn, polished, and etched slab of the meteorite. Notice the coarse Widmanstätten pattern. Among collectors, Campo del Cielo is known as a “ruster” because of its tendency to easily rust (right).
Sawn, polished, and etched slab of the Gibeon (IVA) iron meteorite. Gibeon has a finer Widmanstätten pattern than does Campo del Cielo. Widmanstätten patterns only occur on sawn, polished, and etched faces of an iron meteorite. They do not occur in stony meteorites. The linear lamellae are intergrowths of crystals of kamacite and taenite. They are large because they cooled slowly over millions of years in the core of an asteroid. One consequence of this slow cooling is that iron meteorites do not easily bend or break unless they are badly rusted.
Widmanstätten patterns do not occur in stony meteorites. They only seen in iron meteorites that have been cut, polished, and etched. |
A sawn, polished, etched slab of the Canyon Diablo iron (IAB) meteorite showing the Widmanstätten pattern and large, round troilite (iron sulfide) inclusions. The meteorite specimen is the property of the Collection of the Arizona State University Center for Meteorite Studies.
Most collectors would agree that the most attractive iron meteorite is Sikhote-Alin (IIAB). At least 23 tons in mass, the meteorite is the largest to fall in historical times. It fell in the Sikhote-Alin mountains of eastern Russia on February 12th, 1947.
Thousands of pieces of Sikhote-Alin have been recovered. This is a collection of Sikhote-Alin “shrapnel.” Sikhote-Alin is not a “ruster.”
An iron meteorite or pallasite that has been buried in the ground for a long time is unspectacular – until someone hits it with a plow. This is the Conception Junction pallasite found by a landowner in Missouri in 2006.
A sawn slice of Conception Junction. It has experienced considerable weathering and rusting, at least near the exterior.
A slice of the Brahin pallasite. The meteorites was found in 1968 by a school girl in Belarussia.
Gujba, a CB chondrite, fell in Nigeria in 1984. Only about 21 CB chondrites are known. Gujba and some other CB chondrites have rounded metal grains, which are otherwise very rare in meteorites. Like Canyon Diablo (above), it also has rounded blebs of troilite. Notice that this specimen has not been polished thoroughly so the saw marks are still very evident in the metal. Thanks to Karl for loan of the Gujba specimen.
Industrial Slag
Rounded metal blebs usually mean 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. Thanks to Jeff for the sample.
With a few rare and exceptions, naturally occurring terrestrial rock do not contain iron metal or iron-nickel metal. There are two reasons. First, the Earth formed from the same kind of material as the asteroids but 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.
When someone walked into my office with this chunk of metal, I thought “Whoa, this could be an iron meteorite!” The “hole” at the lower left was suspicious, however, and when we sawed into it (inset, upper right), it was full of vesicles. Iron meteorites are cores of asteroids that cooled from liquids over millions of years. They don’t contain vesicles. (The elongated “hole” in Gibeon, above, is not a vesicle, it is a void where a troilite grain was “plucked” during sawing of the meteorite.) Also, when we analyzed it, there was much less that 1% nickel. I don’t know how this thing formed, but it is man-made.
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, however.
Finally, some rare meteorites do not contain any appreciable metal and consequently they have low concentrations of Ni. Unbrecciated 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.
Bottom LineIf 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 unbrecciated achondrite. 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%). |
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.)