Almost 387 meteorites are known that have been classified as ureilites. Ureilites are achondrites (= meteorites lacking rounded structures called chondrules) dominated by the minerals olivine and pyroxene. This mineralogy matches up with peridotite, an ultramafic intrusive igneous rock. The sample shown below is from the NWA 4231 Meteorite, a ureilite found in 2005 in northwestern Africa.
Ureilites are typically dominated by forsterite olivine (usually ~50-75% of the rock). The pyroxene component in ureilites (usually ~14-35%) varies from augite to pigeonite to enstatite. Fe-Ni metal is frequently seen in meteorites, but is present in ureilites only in very small amounts (often along crystal boundaries).
Ureilites are distinctive in having unusual materials in the interstitial areas between the olivine & pyroxene crystals. Interstitial material ("mesostasis") in ureilites is relatively rich in carbon and noble gases. Microdiamonds & carbide have also been identified in ureilite mesostasis. Ureilite diamonds are usually considered to be the result of shock metamorphism of pre-existing graphite (this may not be the case, however).
Ureilites are now recognized as having potentially played a role in the origin of life on Earth. The Almahata Sitta Meteorites are ureilites that fell in northern Sudan in 2008 as debris from Asteroid 2008 TC3. Amino acids, the "building blocks" of life, have been identified in Almahata Sitta ureilite rocks. Amino acids have been previously reported from the Murchison Meteorite, a carbonaceous chondrite.
The origin of ureilites has not been settled in the literature. Meteorite researchers acknowledge that ureilite mineralogies and textures are similar to mantle peridotites on Earth that have been subjected to partial melting ("residual peridotites"). So, it's tempting to suggest that ureilites are upper mantle rocks from a differentiated asteroidal body (one that has been sufficiently heated to result in the formation of a core-mantle-crust). However, several aspects of ureilite mesostasis chemistry and isotope composition continue to puzzle meteoriticists.
This particular ureilite sample has not been dated, but other ureilites have crystallization dates of 4.55 billion years.
Ureilite (olivine-pyroxene achondrite meteorite) (2.9 cm across) - cut & polished surface of the NWA 4231 Meteorite. The yellowish-greenish-brownish crystals are olivine; the very dark crystals are pyroxene.
Mostly synthesized from:
Hutchison (2004) - Meteorites, a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press. 506 pp.
Kita et al. (2004) - Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite. Geochimica et Cosmochimica Acta 68: 4231-4235.
Michael Cottingham (pers. comm.)