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
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)
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 two problems.
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).
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).
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.
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.
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 (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
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.