Ureilite (ure)

perhaps the freshly fallen meteorite smelled good, or perhaps because it was shaped like a loaf of bread, which some ureilite are. However, not all of the stones were eaten, and Novo Urei became the type specimen of one of the best-represented achondrite groups.

Image source: http://www.saharamet.com/meteorite/gallery/ureilite/index.html.

There are two subgroups of ureilite. Main group ureilites are composed mainly of coarse-grained olivine (Fa5-25), pigeonite (En75Wo15) and orthopyroxene (En80-90), set in a dark carbonaceous matrix of graphite and diamond, Ni-Fe metal, and troilite. Polymict ureilites consist of a mixture of different lithologies. Besides clasts from main group ureilites, they contain magmatic inclusions, dark carbonaceous clasts, chondritic fragments of different origins, and various other inclusions. This suggests a surface or regolith origin for the polymict ureilites, an assumption supported by the values for noble gases that have been implanted into the regolith by the solar wind. However, both the origin and the formation history of the ureilites remain
enigmatic. Their mineral and oxygen isotopic compositions suggest that they are residues of partial melting, and therefore represent primitive achondrites that probably formed on several parent bodies. On the other hand, REE patterns and other chemical characteristics indicate that ureilites are highly fractionated igneous rocks that formed in different regions of the same parent body; probably a moderately differentiated C-class asteroid that was disrupted by an impact event and then rapidly cooled. An impact history would also explain the occurrence of high-pressure minerals such as diamond and londsdaleite that are formed by intense shock metamorphism. Even this theory isn’t without its problems though. Recently, a ureilite from the Libyan Sahara, named DaG 868, was found to contain diamonds, but paradoxically, appears to be nearly unshocked.

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