Most metal-bearing meteorites are stony meteorites known as ordinary chondrites; the rest are irons and stony irons. Among ordinary chondrites, the most common type, H-group chondrites (31% of all meteorites), have the most metal, 15-20% by mass. L-group chondrites (28% of all meteorites) have some metal, 7-11%. LL-group chondrites (5% of all meteorites) 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 of 1.0-1.8% (i.e., 10000-18000 ppm). That's 100-1000 time greater than practically any terrestrial (Earth) rock.

Notice (left) that metal grains are typically less than a millimeter in size.

  

A sawn and polished face of the Brahin pallasite.
Photo by Randy Korotev.

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 with ionic nickel that has a distinct pinkish color.

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 (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.)

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 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 10 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. 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. 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.