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.

Examples of Reduction Textures

Graphite. Photograph of a slice surface of NWA 2353 that illustrates the presence of metallic carbon (graphite) interstitial to olivine and pyroxene grains. Note the lack of metal and silicate darkening as the result of reduction, type example for reduction level of R1. Width = 5 mm. © T.E. Bunch, 2005

Reduction Level 1. Typical ureilite texture of Dho 979 with an R1 reduction level. Note tiny, wormy graphite at grain boundaries and included within olivine. Blue-gray color denotes scarce areas of reduction. Partial plane polarized light combined with partial reflected light that highlights the altered reduced areas of olivine and pigeonite by imparting a blue-gray hue. © T.E. Bunch, 2005

Graphite Inclusions. Scanning electron microscope – back scattered electrons image (SEM-BSE) of reduced areas in Dho 979. Black is graphite and white is terrestrially oxidized metal at grain boundaries and as ting micron-sized blebs that "decorate" the reduced margins. © J. H. Wittke, 2005

Olivine Mantle. SEM-BSE image that shows the reduced mantle of an olivine grain in  Dho 979 outlined by tiny metal blebs; metal (white) has partially replaced graphite (black). © J. H. Wittke, 2005

Reduction Level 4. Cross-polarized photomicrograph of NWA 2376 large areas of reduction (black). Base width = 5 mm. © T.E. Bunch, 2005

Reduction Level 4. Same area taken in partial polarized/reflected light that shows the shows reduced areas (blue-gray) and graphite (some of the black). Base width = 5 mm. © T.E. Bunch, 2005

Ureilite Textures

Typical ureilite texture (NWA 2353). Reduction level R1; cross-polarized light; base width = 8 mm. © T.E. Bunch, 2005

Typical Ureilite Texture. Note the slight preferred orientation of olivine and pyroxene long dimensions parallel to an E-W plane. Reduction level R1; base width is 5 mm. © T.E. Bunch, 2005

Sawtooth Variation of Typical Ureilite Texture. Sample NWA 2624 illustrates greater reduction activity in fractures and cleavages of olivine and pigeonite. Reduction level, R2; cross-polarized light photomicrograph; base width = 3 mm. © T.E. Bunch, 2005

Poikilitic Texture. In NWA 1926, pigeonite pyroxene “poikilitically” encloses or includes smaller olivine (indicates that olivine crystallized before pigeonite). Base width = 3 mm. © T.E. Bunch, 2005

Bimodal Texture. Example of bimodal texture in NWA 2088. Large, twinned pigeonite pyroxene partially surrounds smaller olivine. Base width = 7 mm. © T.E. Bunch, 2005