Graphite
A soft, black form of carbon found as nodules in some iron meteorites and as seams in ureilites.
Graphite is pure carbon arranged in flat sheets that slide easily over each other, which is why it feels greasy and marks paper; it is the "lead" in pencils. In meteorites, graphite is most common in the IAB irons and in the carbon-rich achondrites called ureilites. It also occurs in some chondrites and, as tiny grains, among presolar materials that formed around other stars.
Graphite records conditions of carbon in early planetary bodies. In iron meteorites, it forms rounded nodules often intergrown with troilite, and sometimes cube-shaped graphite crystals called cliftonite, thought to form from carbon dissolved in metal. In ureilites, graphite fills spaces between olivine and pyroxene grains, and shock has transformed some of it into diamond. Canyon Diablo, the iron that formed Meteor Crater in Arizona, is well known for its graphite-troilite nodules.
On a cut iron meteorite, graphite appears as dull, soft, black patches that do not take a polish well and may crumble. In ureilites it makes dark rims around silicate grains. Graphite is not magnetic and conducts electricity. It can be confused with dark sulfides, so confirmation needs microscopy or X-ray methods.
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