The ureilite class is named after the type example Novo-Urei, Russia that fell in 1886 and is one of the strangest of meteorites. 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.
Their origin is highly controversial. One school of thought is that they were formed in the interior of a parent body with cumulate crystals that formed crystal layers. Evidence of this is shown in some ureilites ( shown below) where grains are aligned in preferred orientation. A counter suggestion is that they represent a residuum of unmelted material after a partial melt liquid was drawn off (like ham hocks after the broth is removed). Other ideas are that they are unprocessed materials and never melted or they are mixtures of carbonaceous chondrite and basaltic rock melts.
Probably more than 80% of ureilites are classified as typical, characterized by olivine and pyroxene grains that are < one mm in size, anhedral with 120° 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.
After classifying several dozen ureilites, we began to see a pattern of increased reduction of olivine and to a lesser extent, pyroxenes, compared with a concomitant loss of graphite. From these observations, we have 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, we assign 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 (see Table). These characteristics are qualitative and we offer no discussion on their origin. Reduction grades are assigned based upon the state of the entire sample; areas within individual ureilites may reflect variable degrees of reduction.
| Degree of Reduction | R1 (lowest) | R2 | R3 | R4 (highest) |
|---|---|---|---|---|
| Graphite/metal (vol. %) | >10 | 10-1 | 1 | 0 |
| Rim thickness of reduced olivine | <15 μm | 15-50 μm | <50 vol. % | >50 vol . % of olivine |
| Degree of hardness | soft | medium | very hard | extreme |
| Diamonds | none | fed | irregular distribution | abundant |
Replacement textures of metal for graphite in the interstices between silicates (“veins”) are commonly observed in R1 and R2. These textures show the replacement of graphite by Fe metal from the reaction:
FeSiO3 (in olivine) + C (graphite) → Fe (metal) + CO + SiO2
This reaction is supported by the observed presence of: Fe-poor olivine with tiny metal inclusions in the reduced rims, SiO2 and additional metal in the “veins”, and the loss of graphite, the amounts being consistent with the degree of reduction.
