METEORITE OR METEORWRONG?
metal, iron, & nickel
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About 95% of all meteorites contain iron-nickel (FeNi) metal.
"Iron-nickel" means that the metal is mostly iron but it contains 4-30%
nickel as well as a few tenths of a percent cobalt. 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 than
the rare metals.
Metal grains reflecting light
in a polished slice of the ordinary chondrite
Faucett (H5). Click on image for enlargement.
Photo by Randy Korotev.
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 (45% of all meteorites), have the
most metal, 15-20% by mass. L-group chondrites (40% of all meteorites)
have some metal, 7-11%. LL-group chondrites (15% 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 (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.
Notice (left) that metal grains are typically less than a millimeter in
reflecting light in a sawn slice of the
Taffassasset (CR-anom or achondrite). Click on image for enlargement.
Note in the enlargement on the right the saw marks in the metal
grains. This is a good way to distinguish metal from shiny sulfide
minerals like pyrite - the sulfides grains won't look so severely
scraped. Photo by Randy Korotev.
Sawn, polished, and etched slab
Campo del Cielo (IAB) iron meteorite. Notice
the rusty spots. Click on image for enlargement. Photo by Randy
Sawn, polished, and etched slab of the
Gibeon (IVA) iron meteorite. This meteorite has a distinct
pattern. Click on image for enlargement. Photo by Randy Korotev.
Sometimes it's hard to tell the difference
between metal and shiny nonmetals like some sulfide and oxide
minerals. 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 won't work on an ordinary chondrite because the iron grains
aren't connected.) A shiny hematite or pyrite aggregate will have
very high electrical resistance because they do not conduct electricity.
Sawn and polished slab of the metal portion of the
Glorieta Mountain (ung) pallasite. Photo by Randy Korotev