Urelite Meteorite Guidelines

Urelites   Chart
 Ureilites contain olivine and pyroxenes (pigeonite, augite, or orthopyroxene, depending on the sample) with filling of the intergranular spaces by graphite (rarely, tiny diamonds), Fe metal with very low Ni, sulfides, Fe₃C and minor accessory phases. These minerals form ugly, dark, opaque masses that we refer to as carbonaceous-metal-silicate-masses or CMSM. Mineral rims and internal fractures appear to be encrusted with carbon (graphite) and tiny metal grains. Iron in the metal probably originated by reduction of oxidized iron (FeO) in silicates by reaction with graphite at some moderate to high temperature. Diamonds may have formed from graphite via impact-induced solid-state transformation, although it is more likely that they formed by vapor deposition.

Probably more than 80% of ureilites are classified as typical, characterized by olivine and pyroxene grains that are < one mm in size, anhedral with 120º triple junctions, and are devoid of plagioclase. A small number of poikilitic grains may be present (pyroxene grains included in olivine or the reverse association). Mosaicized ureilites are typified by finer grain size, probably as the result of recrystallization from shock. A few ureilites are classified as Bimodal and are extremely heterogeneous with respect to grain size and mineral content. Some pyroxene grains may reach one to nearly two cm in size.

The NAU  has developed an additional classifying element that is very useful in distinguishing various types of ureilites akin to the petrologic grades for ordinary chondrites. For example, in those ureilites that have unaltered graphite, little or no interstitial metal, and very lightly reduced silicates, are assigned a reduction grade of R1, which is the least reduced and R5 for the most reduced where no graphite remains, large amounts of interstitial metal are present, and olivine rims are heavily reduced with over 50 vol % of grain mass affected .
 

                                         Urelite Reduction Table

The following sections present specimen illustrations and descriptions of differentiated achondrites, which includes the HED clan (howardites, eucrites, and diogenites), angrites, aubrites, and ureilites. A summary of the diagnostic mineralogic characteristics of all achondrites is given in the table below (modified after Table 8.1 in Hutchison, 2004, Meteorites: A Petrologic, Chemical and Isotopic Synthesis, Cambridge University Press).

Notes:  min = minor, <5 vol. %; tr = trace, <0.5 vol. %