METEORITE OR METEORWRONG?
metal, iron, & nickel
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Iron meteorites, of course, are nealy 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 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
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.
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
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
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.
If the metal contains <4% nickel, then the metal chunk or rock
is not a meteorite.
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%).
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
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
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
(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
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
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.
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