UREILITE
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.
(More info. on the
NWA 4231 Meteorite)
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.)