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. (Photo by Randy Korotev)
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. Like Canyon Diablo (above), it 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. (Photo
by Randy Korotev)
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. Thanks to Jeff for
the sample. (Photo by Randy Korotev)
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 of 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 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, then it's probably a meteorite. That sounds simple, but there are
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. 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, 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 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 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. 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 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%).
Test for Nickel
have had some success using a nickel allergy test kit to determine
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.
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.
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."
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.
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 (right).
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.)
tried the test also on a sawn face of an ordinary (H group) chondrite
and also obtained a positive result.
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.
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
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.)
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.