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Ureilites

Synonyms: olivine-pigeonite achondrites

 

General: The meteorites of this unusual group are named for Novo Urei, a small village in the Mordova Republic, Russia, where several meteorites fell in late 1886. Contemporary reports tell that one stone was soon recovered by local peasants - but not to preserve it for science. On the contrary, the stone was broken apart and eaten! The reports don't reveal the reason for this strange behaviour, maybe the peasants just ate it because the freshly fallen meteorite smelled good; we just don't know. However, not all of the stones were eaten, for good, and Novo Urei became the type specimen of a rather enigmatic group of achondrites.

Description: Ureilites are dark grey or brownish rocks with medium- to coarse-grained textures, and they exhibit a black fusion crust. Due to the fact that most ureilites do contain various ammounts of nano-diamonds, they are very hard to cut, and are good to ruin a few sawblades - a good indication that a new meteorite find actually might be a ureilite.

Mineralogy: The ureilites are subdivided into two groups: the monomict main group and the less common polymict group. Monomict ureilites are primarily composed of coarse-grained olivine and minor pyroxene, mostly in form of Ca-poor pigeonite, set in a dark carbonaceous matrix of graphite and diamond, nickel-iron metal, and troilite. Polymict ureilites consist of a mixture of different lithologies. Besides monomict ureilite clasts, they often contain magmatic inclusions, dark carbonaceous clasts, chondritic fragments, and various other inclusions. This suggests a regolith origin for the polymict ureilites, an assumption that is supported by the values for noble gases that have been implanted into the polymict ureilites by the solar wind.

Formation history: The formation history of the ureilites remains enigmatic. Their mineral and oxygen isotopic compositions suggest that they formed as residues from partial melting, and therefore represent primitive achondrites that formed on several parent bodies. On the other hand, rare-element patterns and chemical characteristics indicate that ureilites are highly fractionated igneous rocks that formed in different regions of one and the same parent body; probably a moderately differentiated C-type asteroid that was disrupted by an impact event and then rapidly cooled. An impact history would also explain the occurrence of several high-pressure minerals, such as diamond and londsdaleite that can be formed by intense shock metamorphism.

Origin: Asteroidal. As to this moment, we don't have any spectral match for the ureilites, and a prospective parent body, i.e., a certain asteroid.

Members: Excluding all probable pairings, the ureilite group comprises about 85 members, and more are certainly to be recovered from the hot deserts of Africa, and Asia.

> Meteorite Classification Index


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