Ureilites


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 ReductionR1 (lowest)R2R3R4 (highest)
Graphite/metal (vol%)>1010-110
Rim thickness of reduced olivine<15 µm15-50 µm<50 vol%>50 vol% of olivine
Degree of hardness soft medium very hard extreme
Diamondsnonefewirregular distributionabundant

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

Summary of Ureilite Characteristics

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.


Dhofar 979 (19°45.4' N, 54°56.2' W)

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

Dho 979. (left) 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. (right) Cross polarized light image of same sample. Image © T.E. Bunch, 2005


Northwest Africa 771 (26°33.60' N, 11°33.33' W)

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.

NWA 771. Image width 13 mm. Image © T.E. Bunch, 2005


Northwest Africa 1569

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.


Northwest Africa 1834

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.

NWA 1834. Typical ureilite; reduction level is R1; (left) dark grains are graphite. Plane polarized light, base width = 8 mm. (right) Same sample, cross-polarized light. Image © T. E. Bunch, 2005


Northwest Africa 1841

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.


Northwest Africa 1913

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.

NWA 1913. (left) Plane light photomicrograph of a typical ureilite; reduction level, R2; base width is 5 mm. (right) Same sample, cross-polarized light. Image © T. E. Bunch, 2005


Northwest Africa 1926

Morocco
Found 2003
Achondrite (ureilite, polymict)

NWA 1926. Image © T. E. Bunch, 2004

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.

NWA 1926. Poikilitic typical ureilite (small olivine grains included in much larger pigeonite, gray). Cross-polarized light photomicrograph; base width is 2.5 mm. Image © T. E. Bunch, 2004


Northwest Africa 2082

Morocco
Found 2003
Achondrite (ureilite)

NWA 2082. Image © T. E. Bunch, 2004

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.


Northwest Africa 2088

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 2352

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.

NWA 2352. Typical ureilite, medium-grained; R2; cross-polarized light; base width = 7 mm. Image © T. E. Bunch, 2005

NWA 2352. Large (2.4 mm), optically twinned pigeonite pyroxene with included grains of olivine. Cross-polarized light. Image © T. E. Bunch, 2005


Northwest Africa 2374

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.

NWA 2374. (left) Typical ureilite that shows preferred orientation of olivine and pigeonite. R2; Plane polarized light, base width = 7 mm. (right) Same sample, cross-polarized light. Image © T.E. Bunch, 2005


Northwest Africa 2376

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.

NWA 2376. (left) Highly reduced (R4/5) typical ureilite; partial plane polarized and reflected light. Base width = 3 mm. (right) Same sample, cross-polarized light. Image © T. E. Bunch, 200


Northwest Africa 2624

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.

NWA 2624. (left) Sawtooth texture caused by enhanced reduction along fractures and cleavage planes in olivine and pyroxene. Only slight reduction has occurred as shown by the low abundance of blue-gray areas (R1). Small, buff-colored grains are graphite. Partial plane polarized and reflected light. (right) Same, cross-polarized light. Image © T. E. Bunch, 2005


Northwest Africa 2625

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.

NWA 2625. (left) Plane polarized light photomicrograph of typical ureilite that shows elongate olivine and pigeonite with preferred orientation, similar to Dho 979. Graphite distribution cross-trends foliation plane; R1; base width = 4 mm. (right) Same. Cross-polarized light. Image © T. E. Bunch, 2005


Northwest Africa 2634

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.

NWA 2634. (left) Plane light photomicrograph of a highly shocked ureilite, gray-brown grains are recrystallized pigeonite pyroxene, olivine contains planar shock features. Shock level = S3-5; R2. Base width = 5 mm. (right) Same general area, cross-polarized light. Image © T. E. Bunch, 2005


Northwest Africa 2651

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.

NWA 2651. Partial cross-polarized photomicrograph that shows saw-toothed edges of olivine and pigeonite decorated with dark, interstitial carbonaceous material and reduced metal. Width = 7.2 mm. Image © T. E. Bunch, 2005


Northwest Africa 2703

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.


Northwest Africa 2705

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.

NWA 2705. The big olivine grain oriented E-W has included graphite (black); this is called a poikilitic texture (relic igneous); most smaller grains are recrystallized with 120 degree grain boundary junctions, a metamorphic texture. Initially, this assemblage formed from either partial melting - with the fractionation and separation of lower temp phases (notably plagioclase and clinopyroxene - or by crystal settling (cumulate separation). All grains show a preferred orientation, together with poikilitic texture, that is strongly suggestive of cumulate formation. (photo base = 5 mm) Image © T. E. Bunch, 2005

NWA 2705. The textures described in the caption above next underwent metamorphism/recrystallization followed by strong shock. This large olivine grain displays beautifully developed basal deformation bands, some kinked. Image © T. E. Bunch, 2005


Northwest Africa 3108

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.

NWA 3108. (left) Typical ureilite texture that shows extensive reduction alteration (blue-gray). Reduction level = R3; partial plane polarized and reflected light. Base width = 3 mm. (right) Same sample (different area), crossed polarized light, base width = 5 mm. Image © T. E. Bunch, 2005


Northwest Africa 3109

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.

NWA 3109. Typical ureilite texture, R2; plane polarized light; base width = 3 mm. (right) Same sample, crossed polarized light. Image © T. E. Bunch, 2005