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 by vapor deposition, although it is more likely that they formed from graphite via impact-induced solid-state transformation.
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 | few | 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.
Minerals and compositions:
Texture: The majority of ureilites are known as Typical and are easily recognized by the simple mineralogy (olivine and pyroxene), medium to coarse-grained equigranular textures (with triple junctions), the presence of fine-grained reduced margins on olivine (from the reduction of FeO in olivine), and interstitial ribbon-like graphite and metal. Bimodal ureilites, few in number, show extreme grain size and mineral contents. Polygonal ureilites, also few in number, are fragmental breccias that contain clasts of other ureilites and other meteorites classes. Mosaicized ureilities are typified by very fine-grained (microns), recrystallized silicates from heavy shock loading.
Oman
Found 2004
Achondrite (ureilite)
History: A completely fresh, crusted stone of 1063 g was found by Michael Farmer in the Dhofar region of Oman on January 8, 2004.
Description and classification (T. Bunch and J. Wittke, NAU): medium-grained typical ureilite with very fine-grained, wormy graphite at grain boundaries and included within olivine and pigeonite. Very low reduction level or “smelting” activity (R1); graphite mostly intact. Olivine cores, Fa16.5 (FeO/MnO = 36) and rims, Fa15.8; pigeonite, Fs14.9Wo5.0. Very little metal and terrestrial Fe-oxides present. Shock level, S1 and lightly weathered.
Specimens: type sample, 44.7 g, NAU: main mass, Farmer
Morocco
Found 2000 June
Achondrite (ureilite)
A 313 g stone was found near Nebca, Morocco, by a person prospecting for meteorites. Mineralogy and classification (T. Bunch and J. Wittke, NAU): has typical ureilite texture; contains reversely zoned olivine (Fa12-17; Cr = 0.7 wt. %) and pigeonite (Fs17Wo10); average metal composition, Ni = 3.3 wt. %, Si = 1.6 wt. %, Cr = 0.25 wt. %; troilite contains up to 9 wt. % Cr and 1 wt. % Ni; shock stage, S2; weathering grade, W1. Specimens: 21.2 g plus thin section, NAU; main mass with anonymous finder.
Morocco
Found 2000
Achondrite (ureilite)
A 614 g partially crusted stone was purchased in Erfoud, Morocco in 2001. Classification and mineralogy (T. Bunch and J. Wittke, NAU): typical ureilite texture; olivine grain size up to 1.5 mm with triple junctions, poorly developed lineation, low-Ni metal (mostly oxidized) interstitial to olivine; low-Ca pyroxene is <1 mm and tends to occur in clusters with interstitial carbonaceous matter. Olivine cores are Fa18, dusty rims are reduced to Fs10; (Cr203 up to 0.62 wt. %; CaO up to 0.41 wt.%); 1ow-Ca pyroxene is Fsl8Wo8 to Fs23Wo9; metal, 0.47 to 5.3 wt.% Ni, 0.55 wt. % Cr203, 0.4 wt. % P and 0.29 wt.% Si; sulfides contain up to 1.4 wt. % Cr. Shock level, S2; weathering grade, W1. Specimens: main mass with anonymous buyer; type specimen, 20.5 g, and thin section, NAU.
Morocco
Found 2002
Achondrite (ureilite)
A 20.17 g stone was purchased in Rissani, Morocco in 2003. Classification and mineralogy (J. Wittke and T. Bunch, NAU): typical, medium to fine-grained (<2 mm) ureilite, non-foliated, and with only thin (0.0i5 mm) reduced rims on olivine and pigeonite. Interstitial metal (Ni = 2.92 wt. %; Co, 0.51 %; Si, 2.4 %) is partially oxidized. Millimeter-size graphite occurs at grain boundaries of main phases and contains silica platelets (< 3 µm) and clusters of sulfide and schreibersite. Olivine Mg/(Mg+Fe) = 0.88 with 0.87 wt. % Cr2O3 and 0.57 % CaO; pigeonite Mg/(Mg+Fe) = 0.88 and contains up to 1.0 wt. % Cr2O3. Shock level, S2 and only slightly weathered. Specimens: 4.2 g at NAU; main mass, Oakes.
Morocco
Found 2003
Achondrite (ureilite)
A 39.3 g, complete and fully crusted stone was purchased in Rissani, Morocco June 2003. Description and classification (J. Wittke and T. Bunch, NAU): typical ureilite with prominent development of carbonaceous/reduced material along internal fractures and (001) planes of olivine, in addition to typical interstitial areas. Olivine cores, Fa21.8 (Cr2O3 = 0.78 wt. %, rims, Fa8.2-0.8 (Cr2O3 = 0.80 wt. %); pigeonite, Fs18.3Wo8; olivine metal inclusions, Ni = 0.92 wt. %; interstitial metal, Ni = 4.45 wt. %, Si = 0.57 wt. %. Shock level, S2; low degree of weathering. Specimens: 8.4 g, NAU; main mass, Oakes.
Morocco
Found 2001
Achondrite (ureilite)
A 731 g complete and partially crusted stone was purchased in Erfoud in 2002. Classification and mineralogy (T. Bunch and J. Wittke, NAU): typical ureilite texture with preferred orientation of elongated silicates. Mineral modes (vol. %): olivine, 68 and pigeonite, 32. Olivine cores, Fa19.2-19.6 and reduced rims, Fa1.5-3.0. Pigeonite, mostly Fs17Wo11. Metal along grain boundaries is 9.3 wt. % Ni; and metal together with pyrrhotite as inclusions in pigeonite is 3.6 wt. % Ni. Carbonaceous matter at grain boundaries, no evidence of diamonds. Shock level, S1; weathering grade, W2. Specimens: type specimen, 21.2 g, NAU; main mass, Boswell.
Morocco
Found 2003
Achondrite (ureilite, polymict)
A 36.8 g partial stone was purchased in Rissani June 2003. Classification and mineralogy (J. Wittke and T. Bunch, NAU): fragmental breccia with clasts of a poikilitic ureilite embedded in typical ureilite materials that consists of olivine with homogeneous cores (Fa31) and reduced rims (Fa2-7) and pyroxenes with Fs19.7Wo4.5 in cores and Fs9.5Wo1 at rims. Heavy concentration of carbonaceous matter; unoxidized metal, Ni = 2.95 wt. %. Clasts are composed of large pyroxenes (Fs19.1Wo4.0) that poikilitically enclose homogeneous olivine (Fa21.8). Clast ureilite contains very little carbonaceous matter in contrast to the host ureilite. Shock level, S2; low degree of weathering. Specimens: type specimen, 6.3 g, NAU; main mass, Farmer.
Morocco
Found 2003
Achondrite (ureilite)
A 33 g single, complete stone was purchased in Rissani, Morocco in October 2003. Description and classification (J. Wittke and T. Bunch, NAU): typical ureilite with a large amount (~30 vol. %) of dark, interstitial materials (carbon, fine-grained silicates, and weathering oxides). Olivine cores are Fa17.8 (Cr2O3 = 1.03 wt %); reduced rim olivine, Fa4 (Cr2O3 = 0.73 wt %); pigeonite cores, Fs15.6Wo6.2 (Cr2O3 = 1.21 wt. %), rims, Fs10.1Wo8.6 (Cr2O3 = 0.93 wt. %). Shock level, S2; weathering grade, medium (no metal or sulfides remain). Specimens: type sample, 8.5 g, and one thin section, NAU; main mass, Farmer.
Morocco
Found 2003
Achondrite (ureilite)
A single complete stone of 252 g was purchased in Rissani, Morocco in November 2003. Description and classification (T. Bunch and J. Wittke, NAU): typical ureilite of low metamorphic grade; contains graphite flakes and limited development of silicate rim reduction and interstitial metal. No evidence of preferred grain orientation. Pigeonite shows pronounced {100} lamellar twinning. Olivine cores, Fa10.3, rims Fa7.8; pigeonite, Fs9.1Wo7.3. Shock level, S3; weathering grade, W2. Specimens: 20.7 g, and one thin section, NAU; main mass, Farmer.
Northwest Africa
Found 2003
Achondrite (ureilite)
A complete, dark brown and partially crusted stone of 389 g was purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke, NAU): high amounts of large, laminated graphite grains (up to 1.9 mm in length) occur interstitially to olivine and pigeonite; only small amounts of carbonaceous-metal-silicate masses (CMSM) are present. Very limited olivine rim reduction or smelting, R1 level with less that 0.02 mm of olivine margins affected. Olivine cores, Fa11.9 (FeO/MnO = 21) and rims are Fa10.7 (FeO/MnO = 19); pigeonite. Fs11.4Wo9.4 (FeO/MnO = 13.7). Oxygen isotopes (D. Rumble, CIW): replicate analyses by laser fluorination gave δ17O = 2.86 and 2.89, δ18O = 7.16 and 7.37 and Δ17O = -0.913 and -1.002 per mil. Very lightly weathered in the interior with low-level shock (S2). Specimens: type sample, 21.1 g, NAU: main mass, Regelman.
Northwest Africa
Purchased 2004 August
Achondrite (ureilite)
A single stone of 31.5 g was purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke, NAU): typical ureilite, fine to medium-grained with a high degree of preferred orientation of mafic silicates. High abundance of laminar, interstitial graphite with low abundance of metal (and weathered oxides); graphite to metal ratio = 8:1 (vol. %). Low level of silicate reduction (R2): reduced olivine rims are lightly affected (< 0.02 mm or <10 vol. %), pigeonite only marginally affected. Olivine cores, Fa21.0 (FeO/MnO = 39), Cr2O3 = 0.80 wt. %, rims = Fa12.9. Pigeonite, Fs18.1Wo7.9(FeO/MnO = 31). Low weathering grade and lightly shocked. Specimens: type specimen, 6.7 g, NAU; main mass, Birdsell.
Northwest Africa
Purchased 2004 August
Achondrite (ureilite)
A single stone of 123 g was purchased in Erfoud, Morocco. Description and classification (J. Wittke and T. Bunch, NAU): typical ureilite that mostly consists of medium to coarse-grained olivine and pigeonite. Extensive reduction (R5) has affected more than 60 vol. % of all olivine and ~15 vol. % of pigeonite. Carbonaceous-metal-silicate masses (CMSM) are also extensively developed. Olivine cores, Fa12.3 (FeO/MnO = 19), Cr2O3 = 0.60 wt. %; olivine rims, down to Fa1.2; pigeonite cores, Fs10.9Wo5.2. Also, interstitial Al-rich augite, K-rich glass, Fe3C, and probable diamond. Interstitial metal is mostly weathered to various iron oxides; no observable primary graphite. Low weathering grade and lightly shocked (S2). Specimens: type sample, 21.1 g, NAU; main mass, Birdsell.
Northwest Africa
Purchased 2004 August
Achondrite (ureilite)
A 241 g single, mostly crusted stone, was purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke, NAU): equigranular, fine-grained (<0.8 mm) typical ureilite. Olivine is highly fractured with closely spaced, subparallel fractures normal to [001]. Fractures apparently served as channel ways for reducing (smelting) activity; grain edges have a saw tooth appearance of alternating, greater reduced rims together with deposits of carbonaceous-metal-silicate-masses (CMSM) that permeated fractures and less-reduced rims that are between fractures. Olivine cores, Fa19.6, rims are Fa9.8-13.9; low-Ca pigeonite, Fs16.8Wo2.9-3.8; high silica glass in CMSM, SiO2 = 78.8 wt. % and Al2O3 = 15.4 wt. %. Primary metal inclusions in olivine, Ni = 2.5 wt. % and Cr = 0.60 wt. %; reduction-derived metal, Ni = < 0.45 wt. %. Moderate level of reduction (R3); weathering is very low grade and the shock level is S2. Specimens: type example, 21.6 g, NAU; main mass, G.Hupé.
Northwest Africa
Purchased 2004 August
Achondrite (ureilite)
A 305 g single stone was purchased in Erfoud, Morocco. Description and classification (J. Wittke and T. Bunch, NAU): medium- to coarse-grained (<4 mm), typical ureilite with pronounced preferred orientation of silicates, which are mostly elongated with curvilinear morphology. Pigeonite shows well-defined twinning; graphite and carbonaceous-metal-silicate-masses tend to cross trend the lineation direction with some small masses poikilitically enclosed by large pigeonite. The degree of olivine-pigeonite reduction is low to moderate (R2-3). Olivine cores, Fa20.3 (Cr2O3 = 0.72 wt. %), rims are Fa8-10.8(Cr2O3 = 0.53 wt.%); pigeonite, Fs17.4Wo7.6(Cr2O3 up to 1.56 wt. %). Weathering is low grade and the shock level is S2. Specimens: type specimen, 22.2 g, NAU; main mass, G. Hupé.
Northwest Africa
Purchased 2004
Achondrite (ureilite)
A 600.2 g complete stone with fusion crust was purchased in Erfoud, Morocco. Description and classification (T. Bunch and J. Wittke, NAU): mosaicized-type ureilite; elongated olivine and pigeonite in preferred orientation; highly shocked (S5) with pyroxenes completely converted to mosaic texture, olivine less so; reduction level is R3. Olivine cores, Fa21.4, rims reduced to Fa3.8; pigeonite, Fs18.8Wo9.9; sub-calcic augite, Fs17Wo16.6; vein metal, 0.3-3.7 wt. % Ni. Low weathering grade. Specimens: type specimen, 46 g, NAU; main mass, Farmer.
Northwest Africa
Found 2003
Achondrite (ureilite)
History: An 89 g partial stone was purchased in Erfoud, Morocco in May 2004.
Petrography: (J. Wittke and T. Bunch, NAU): bi-modal ureilite that contains scarce, large pigeonite grains (up to 8 mm in diameter) heterogeneously distributed throughout an equigranular, fine-grained (< 0.9 mm) matrix of olivine and compositionally similar low-Ca pyroxene. Olivine is highly fractured with closely spaced, subparallel fractures normal to [001]. Fractures apparently served as channel ways for reducing (smelting) activity; grain edges have a saw tooth appearance of alternating, greater reduced rims together with deposits of carbonaceous-metal-silicate-masses that permeated fractures and less-reduced rims that are between fractures, similar to NWA 2624.
Geochemistry: Olivine cores, Fa18.6, rims are Fa10.7-14.3; small pigeonite, Fs16.8Wo2.9-3.8; large pigeonite, Fs15.1Wo3.3; reduction-derived metal, Ni = <0.63 wt. %. Weathering is very low grade and the shock level is S2. Classification: achondrite (ureilite) potentially paired with NWA 2624.
Type specimens: A total of 22.6 g and one thin section are on deposit at NAU. Reed holds the main mass holder.
Morocco
Find 2004
Achondrite (ureilite)
History: A complete partially crusted stone weighing 121 g was purchased in Erfoud, Morocco.
Petrography (J. Wittke and T. Bunch, NAU): A fine-grained (≤1 mm) ureilite that has a low level of reduction.
Composition: Olivine (cores, Fa12.3, FeO/MnO = 22), pigeonite (Fs10.6Wo5.1, FeO/MnO = 14), clinopyroxene (Fs6.5Wo37, FeO/MnO = 13). Metal, sulfide, and graphite ribbons also present.
Classification: Achondrite (ureilite); low shock; minimal weathering.
Specimens: A 20.5 g type specimen is on deposit at NAU. G. Hupé holds the main mass.
Morocco
Find 2004
Achondrite (ureilite)
History: A complete 70 g stone was purchased in Erfoud, Morocco.
Petrography: (J. Wittke and T. Bunch, NAU) Specimen shows well-formed, basal deformation lamellae in olivine with small enclaves of slightly rotated domains. The olivines and pyroxenes contain a large amount of fine-grained graphite inclusions. Olivine margins show limited reduction. Modal analyses of silicates (vol%): Olivine = 95, pigeonite = 3, orthopyroxene = 2.
Composition: Olivine (cores, Fa22.3; FeO/MnO = 40-47), pigeonite (small grains, Fs7.8Wo7.7; FeO/MnO = 8), orthopyroxene (Fs3.5Wo2.7).
Classification: Achondrite (ureilite); moderate shock, minimal weathering.
Specimens: A 16.8 g type specimen is on deposit at NAU. G. Hupé holds the main mass.
Morocco
Found 2003
Achondrite (ureilite)
A 51 g single, crusted stone was purchased in Erfoud, Morocco, in 2003. Description and classification (T. Bunch and J. Wittke, NAU): typical fine-grained (<2 mm) ureilite, foliated with large amounts of carbonaceous-reduced metal interstitial masses; olivine and pigeonite have extensive reduced rims. Olivine cores, Fa21.1, rims, Fa2.3-5.0 (Ni = 0.74 wt. %); pigeonite cores, Fs16.9, rims, Fs8.2Wo6.8. Olivine metal inclusions. Ni = 9.9 wt. %, P = 0.42 wt. %; troilite, Ni = 0.74 - 1.15 wt. %. Shock level, S2; low weathering grade. Specimens: 8.6 g, NAU; main mass, Boswell.
Morocco
Found 2003
Achondrite (ureilite)
A 32 g partial stone was purchased in Erfoud, Morocco in 2003. Description and classification (J. Wittke and T. Bunch, NAU): non-foliated ureilite with thin, reduced rims in olivine and sparse, interstitial masses of carbonaceous - reduced metal; prominent interstitial clusters of ribbon-like graphite; Cr2O3 enrichment in silicates. Olivine: cores, Fa23.3 with 0.63 wt. % Cr2O3; rims, Fa7.6 and 0.15 wt. % Cr2O3. Pigeonite, Fs17.8Wo3.3 (Cr2O3 = 1.10 wt. %). Presence of (Fe,Cr)3C in interstitial carbonaceous masses (Fe = 78.8 wt. %, Cr = 13.5 wt. %; Ni = 0.52 wt. %; Co = 0.32 wt. % and C = 6.9 wt. % by difference) and an SiO2 phase. Shock level is S2; lightly weathered. Specimens: 6.5 g, NAU; main mass, Boswell.
