Ureilites

Ureilites are calcium-poor achondrites, consisting mainly of olivine, pigeonite (their dominant form of pyroxene) and carbon (2.2%). The carbon is in the form of either graphite, diamond, or lonsdaleite (a rare pure carbon mineral like diamond, but with a different crystal structure). The ureilites are the only achondrites containing significant amounts of free metal (Fe, Ni) (5%). They also contain the highest proportions of heavy rare gases such as argon, krypton and xenon, among all meteorites. The origin of ureilites remains an enigmatic. They appear to result from a complex igneous history, involving perhaps carbonaceous chondrite-like material as a starting point.

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