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Yilmia

EL6
(ELa6 in Weyrauch et al., 2018)
standby for yilmia photo
Found 1969
31° 11′ 30′ S., 121° 32′ E. In 1969, numerous fragments were found in a shallow depression during nickel exploration in Western Australia. They were not recognized as meteoritic until 1971, when a single large mass with ablation characteristics, along with smaller fragments totaling 24 kg, was found 400 m from the first find. Although originally described as petrologic type 5, all current research concludes that Yilmia is a type 6. The Van Schmus–Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Yilmia has thermometers that indicate a classification of EL6β. A rapid cooling phase was initiated consistent with 30,000°C/day (Kissin, 1989).

Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).

ENSTATITE CHONDRITE SUBGROUPS
Weyrauch et al., 2018
EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite Ni <6.5 wt% Ni >6.5 wt% Ni <6.5 wt% Ni >6.5 wt%

A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that Yilmia is a member of the ELa subgroup.

Planetary-type noble gases have been identified in Yilmia, the carrier of which is thought to be a nanometer-sized phase designated phase ‘Q’ (for ‘quintessence’, including He, Ne, Ar, Kr, and Xe). Noble gases may be adsorbed at low nebular pressures onto this phase, or precursors of this phase, which is thought likely to consist of rare graphite grains, kerogen, or carbon blacks. An alternate scenario proposed by some investigators (e.g., Verchovsky et al., 2002; Matsuda et al., 2010) suggests that an amorphous phase of carbon experienced implantation through ion irradiation of planetary noble gases (the ‘plasma model’), and that this phase now serves as the carrier of the Q-gases. These Q-gases are then released through oxidation processes resulting in a rearrangement of the carbon structure. A subsequent in-depth investigation into the carbonaceous carrier of the Q-phase was conducted by Fisenko et al. (2018) utilizing the L4 chondrite Saratov. They contend that the carrier of the Q-gases is a nongraphitizing carbon phase present as curved, few-layer, graphene-like sheets which were likely formed in the protoplanetary nebula.

The higher incidence of impact shock events for EL chondrites is attested by the higher prevalence of impact-melt breccias among the more metamorphosed members, as well as by the occurrence of abundant silica in the form of tridymite, cristobalite, and sinoite, the latter mineral known to crystallize from an impact melt. Following the impact-shock events, most EL6 chondrites experienced an extended period of annealing to shock stage S1, which was followed by a period of less severe impact-shock events resulting in a shock stage of S2 (Rubin et al., 2009).

While taenite is uncommon in other E chondrites, it is present in Yilmia. The photo above shows a 1.25 g partial slice of this terrestrially weathered, low-metal, enstatite chondrite.


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Pillistfer

EL6
(ELa6 in Weyrauch et al., 2018)
standby for pillistfer photo
Fell August 8, 1868
58° 40′ N., 25° 44′ E. At 12:30 P.M. in Estonian SSR, sonic booms were heard and stones fell at Aukoma, Kurla, Wahhe, and Sawiauk. The weight of these stones was ~14 kg, 7.5 kg, 1.5 kg, and 0.25 kg. These falls are also known by the names of Pilistvere and Pillistvere.

Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troilite and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solution series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamacite were demonstrated to be consistent factors for the resolution of four distinct E chondrite groups: EHa, EHb, ELa, and ELb (see table below).

ENSTATITE CHONDRITE SUBGROUPS
Weyrauch et al., 2018
EHa EHb ELa ELb
Troilite Cr <2 wt% Cr >2 wt% Cr <2 wt% Cr >2 wt%
(Mn,Mg,Fe)S Fe <20 wt% Fe >20 wt% Fe <20 wt% Fe >20 wt%
Daubréelite Abundant Missing Abundant Missing
Kamacite Ni <6.5 wt% Ni >6.5 wt% Ni <6.5 wt% Ni >6.5 wt%

A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphic processes, and some or all of them could represent separate E chondrite asteroids. The revised E chondrite classification scheme of Weyrauch et al. (2018) including selected examples from their 80-sample study can be found here. It was determined that Pillistfer is a member of the ELa subgroup.

The Van Schmus–Wood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (3–7), reflecting peak metamorphic temperature, and a mineralogical type (α–δ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Pillistfer has geothermometers that indicate a classification of EL6β.

Enstatite chondrites were formed in a highly reducing environment. Therefore, they contain virtually no metal in the oxide form—much less by comparison to other chondrites and to the terrestrial planets. Iron in EL6 chondrites is depleted and isotopically fractionated compared to less metamorphosed EL3 and EH chondrites (Wang et al., 2013). A trace element analysis utilizing nonmagnetic micron-scale grains from Pillistfer was conducted by Lavrentjeva and Lyul (2017). They found depletions in siderophile elements and enrichments in lithophile elements, which indicates that nebular metal–silicate fractionation of precursor material occurred, as well as redistribution during parent body metamorphism. The mineral sinoite (silicon oxynitride) has been found to occur in Pillistfer and many other EL chondrites that have a high bulk N content. Sinoite is associated with crystallization from an impact melt, or alternatively, with metamorphic processes. This suggests that Pillistfer experienced a period of high, possibly melt-forming temperatures. A rapid cooling phase was initiated consistent with 0.8°C/day (Kissin, 1989). This was followed by a period of annealing and then a final shock to stage S2.

An isochron age for Pillistfer representing the K–Ar system closure was calculated by Bogard et al. (2010) to be 4,541 (±7) m.y. ago., a similar age to that of several equilibrated E chondrites. A comparison of the younger Ar–Ar ages measured for ordinary chondrites suggests that E chondrites cooled more quickly, possibly reflecting a smaller parent body size, a lower initial heating level, a shallower burial, and/or a collisional disruption prior to K–Ar closure. More recently, employing a broader range of EL chondrite petrologic types (i.e., formation temperatures), Hopp et al. (2013, 2014) determined a lower corrected age range for metamorphic cooling of EL5 and EL6 meteorites of 4.48–4.51 b.y. In a similar manner, the Ar–Ar isochron age for an EL3 chondrite reflected a younger age, possibly representing a late-stage impact ~4.43–4.47 b.y. ago. This better constrained age range would allow for a more extended period of time for parent body cooling and a relaxation of the constraints on the parent body size. However, since the K–Ar closure for the EL parent body occurred 30 m.y. earlier than that of the H-chondrite parent body, the size of the EL parent body was most likely significantly smaller than the H parent body.

Oxygen isotopic studies place the formation of enstatite chondrites on the terrestrial fractionation line, which is taken by some to mean that they formed within the inner Solar System. Based on Mn–Cr isotope systematics and its correlation with heliocentric distance, Shukolyukov and Lugmair (2004) concluded that E chondrites originated ~1.0–1.4 AU from the Sun before being perturbed into their present locations in the asteroid belt. Similarly, Nakashima et al. (2006) calculated a heliocentric distance of >1.1 and 1.3 AU for two EL3 chondrites (ALH 85119 and MAC 88136, respectively) on the basis of their implanted solar noble gas concentrations.

In contrast, the identification of the E-asteroid group, including Hungaria at 1.94 AU, Nysa at 2.42 AU, and Angelina at 2.68 AU, suggests that the actual solar region of formation could lie at a greater heliocentric distance. Cosmochemists are presently trying to construct a suitable theory involving oxygen depletion in this E-asteroid region of the Solar System to explain the conflicting theories. The specimen pictured above is a 5.7 g partial slice showing the abundant free metal that characterizes this group.


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NWA 3132

EL3
standby for northwest africa 3132 photo
purchased
no coordinates recorded A 125.39 g meteorite was found in the Sahara Desert and later purchased by American collector N. Oakes. A sample was submitted to Arizona State University, Center for Meteorite Studies (G. Huss), and NWA 3132 was determined to be a rare EL3 chondrite composed almost entirely of orthopyroxene. Although almost no metal or sulfide is present, vestigial signs of metal can be seen throughout the meteorite. This enstatite chondrite has a shock stage of S2 and a weathering grade of W4. An unequilibrated petrologic type 3 sample is quite rare among EL-group members, and is thus far represented primarily by Antarctic meteorites including ALH 85119, A-881314, A-882067, EET 90299/90992, MAC 88136 pairing group, MAC 02635, MAC 02837/02839, QUE 93351 pairing group, LAP 03930, and PCA 91020, along with EL3 xenoliths identified in the carbonaceous chondrite Kaidun (designated Kaidun IV) and in the ureilite Almahata Sitta.

With the exception of the transitional EH/L chondrite Y-793225, E chondrites have been historically assigned to one of two distinct groups—a high-Fe, high-siderophile (EH) group, and a low-Fe, low-siderophile (EL) group. Surprisingly, it has been demonstrated by Macke et al. (2009) that these two groups do not actually differ in their iron content, and that they are indistinguishable in density, porosity, and magnetic susceptibility as well. However, differences in siderophile, chalcophile, and other mineralogical abundances can be employed to distinguish the two groups. From comparisons of elemental abundance ratios between the EH and EL groups, it has been demonstrated that values for all elements except the refractory siderophiles are consistently lower in the EL group than in the EH group. Certain elemental ratios (e.g. La/Sm, Sb/Ir) easily resolve the two groups. Furthermore, the following mineralogical relationships are diagnostic of their distinct parent bodies:

  • EH group has a higher Si content in kamacite (EH: 1.9–3.8 wt%; EL: 0.3–2.1 wt%)
  • EH group has a lower Mn content in daubreelite (EH: 0.4–1.1%; EL: 1.4–4.0%)
  • EH group has a lower Ni content in schreibersite (EH: <20 wt%; EL: >20 wt%)
  • EH group has a lower Ti content in troilite (EH: <4.8 wt%; EL: >5.5 wt%)
  • EH group has a lower An content in plagioclase (EH: <3 mol%; EL: 13–17 mol%)
  • EH group sulfides are enriched in alkali elements (e.g., Na in caswellsilverite, K in djerfisherite), and chondrule mesostasis is enriched in Na relative to EL group
  • EH group chondrites contain niningerite [(Mg,Fe)S] or keilite [(Fe,Mg)S], the Mg-rich end member of the monosulfide series having the formula [(Mg,Mn,Fe)S]; EL group chondrites contain the Mn-rich end member alabandite [(Mn,Fe)S]
  • EH group chondrites have higher abundances of the siderophile elements Ni, Fe, Au, and Co
  • EH group chondrites contain an average of 15 times the abundance of the volatile element Zn
  • Other good discriminators are Ga, As, Se, and Sb, each of which are found in greater abundances in EH group chondrites

In addition, both Fe- and Zn-isotope compositions are fractionated to different degrees between EL and EH chondrites; EL chondrites are isotopically heavier than EH chondrites, indicating EL chondrites experienced higher volatilization due to its formation closer to the Sun (Mullane et al., 2005), or alternatively, due to elemental fractionation during impact-shock events (Rubin et al., 2009). Studies into the origins of EL chondrites conducted by Goresy et al. (2012) determined that petrologic evidence, including the occurrence within FeNi-metal nodules of repeated sinoite–graphite condensation events associated with oldhamite (CaS) in the sequence CaS ⇒ sinoite ⇒ graphite, was indicative of a nebular condensate origin for these chondrites rather than their formation as an impact-melt breccia of preexisting proto-asteroids. Still, later incidences of impact-shock for EL chondrites are attested by the higher prevalence of impact-melt breccias among the more metamorphosed members, as well as by the occurrence of sinoite crystallized from a melt.

Other comparisons demonstrate that EH group chondrites have smaller average chondrule diameters (220 µm vs. 550 µm; Rubin et al., 2000) and smaller average metal diameters than EL-group members as shown in the table below:


Comparison of EH and EL Chondrule and Metal Diameters
EH EL
Chondrule Diameter 0.045–1.313 mm
(EH3 average 0.278 ±0.229 mm)
0.085–2.125 mm
(EL3 average 0.476 ±0.357 mm)
Metal Diameter 0.008–0.492 mm 0.002–1.107 mm

The disparity in the size and Na content of chondrules within the EH and EL groups can be reconciled by several possible scenarios, including the one proposed by Schneider et al., 2002:

Chondrules from both groups were formed from similar precursor material in the same nebular region. During accretion, the chondrules underwent a size sorting process induced by volatile flows within the regolith, or alternatively, by the abundance of dust in the prospective accretion regions and by the number of chondrule remelting episodes (Rubin, 2010). Photophoresis, utilizing pressure and particle-size dependence, was also a likely size sorting mechanism of chondrules (Hesse et al.,2011). This resulted in the larger EL chondrules becoming more deeply buried than the smaller EH chondrules on their respective parent bodies. It has been reported that EL3 chondrites usually exhibit a preferred orientation of chondrules and other constituents. The deep burial conditions would support such a foliation as the result of continued impact deformation processes. After burial, lithification of the chondrules into bulk rock was quickly achieved. The shallower EH material experienced more rapid cooling, and thus retained more of its volatile component such as Na, while volatiles were lost during a more extended cooling period in the more deeply buried EL material. In an attempt to model the precursor material of Earth, it was calculated that approximately 55% of the precursor component could have been of EL chondrite composition, which is the meteoritic material that provides the best match to the Earth in O-isotope composition, bulk Fe/Al weight ratio, and bulk FeO concentration (T. Burbine and K. O’Brien, 2004). However, it was not possible to model the Earth’s precursor based solely on known chondrites. Alternatively, the Earth could have been formed from chondritic material that subsequently underwent differentiation and loss of a basaltic component, or that had a significant Si component sequestered into the core or lower mantle.

Based on isotopic studies, the meteorites of the EL3 group, as opposed to the EL6 group, are thought to be the best candidates for the building blocks of Earth (Boyet et al., 2018). Further information on the classification and petrogenesis of the E chondrites can be found on the Saint-Sauveur page. The specimen of NWA 3132 shown above is a 0.8 g partial slice. The photo below shows the main mass. standby for northwest africa 3132 photo
Photo courtesy of Nelson Oakes—Meteorites–R–Us


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NWA 2965

EL3/6, impact-melt breccia
‘Fossil’ or ‘Paleo’ Meteorite
(EL6/7 in MetBull 91; aubrite pairing in MetBull 92)
Revised classifications have been submitted to NomCom and MetBull
nwa 2965
click on photo for an enlarged view Purchased August 2005
27° 30′ N., 12° 30′ W. Numerous individual pieces of an extensively weathered ‘relict’* or fossil meteorite, weighing together as much as 3,000 kg, were found in Western Sahara, reportedly near the village of Al Haggounia. Stones of various sizes were found both on top of the surface and buried under the soil in geological strata associated with both quaternary limestones and cretaceous limestones. This is evidence that the fall occurred later than the deposition of these strata (Chennaoui et al., 2007). The 14C age of this meteorite was determined by Chennaoui–Aoudjehane et al. (2009) to be 23 (±2) t.y., in agreement with geological evidence.

A large portion of this meteorite was purchased by a collector at the 2006 Tucson Gem and Mineral Show and in subsequent purchases from a Moroccan source. The vast majority of Northwest Africa 2965 is surface material which is very highly weathered and dark brown in color with a porosity of 40%. It contains an abundance of very dark fractures filled with oxidation products such as goethite. Although primary minerals are present (e.g., enstatite, plagioclase, troilite, daubreelite, alabandite, oldhamite, and others), secondary minerals occur throughout, including those within the mm- to cm-size ubiquitous pores, considered by some to have formed by weathering of metal phases. However, in his study of the Al Haggounia 001 pairing, A. Rubin (2016) attributed the presence of these vesicles (~6.8 vol%) to impact-induced evaporation of sulfides, in a similar manner to those in the aubrite Mayo Belwa. He reasoned that the sparsity of metal observed in some parts of the mass, especially in the less weathered bluish-gray portions, is the result of metal–sulfide melt drainage into nearby regions as represented by Al Haggounia 001 with its large component of limonite (32.6 vol%) replacing FeNi-metal (0.29 vol%) along with sulfide (4.0 vol%).

A very low abundance of radial pyroxene chondrules have been identified (<5 vol%), as well as other fine-grained, rounded enstatite and plagioclase aggregates. The chondrules contain a Na–Al–Si–rich glass phase consistent with an unequilibrated chondrite. Recognizing this scarce population of chondrules and the other unequilibrated features of this meteorite has finally enabled investigators to arrive at a consensus for the classification of this possibly ‘fossil’ meteorite.

Northwest Africa 2965 was initially analyzed at Northern Arizona University (T. Bunch and J. Wittke), and due to its apparent lack of chondrules and its fine-grained igneous-like matrix, it was determined to be a recrystallized EL6/7 chondrite. An alternate classification of this enstatite meteorite as the first enstatite metachondrite, a newly proposed metamorphic category defined by Irving et al. (2005), was also considered. Subsequent to this study, additional material reported to be from the same meteorite, but having a less altered bluish color, was studied at the University of Washington, Seattle (A. Irving and S. Kuehner). This material was classified as an EL3 chondrite (the likely paired NWA 2828 was classified as an aubrite in MetBull 91).

This meteorite contains only trace amounts of FeNi-metal, but some portions near the top strata bear a dark goethite-rich rind consistent with leaching of Fe from the interior of the stone during an extended terrestrial residence in a wetland location. Analyses of the trace FeNi-metal and of rare kamacite inclusions identified within enstatite grains of NWA 2828 revealed a Si content consistent with EL chondrites, but different from aubrites (Irving et al., 2010). The investigation also determined that while major elements are depleted, trace siderophile elements still have abundances typical of E chondrites and were possibly preserved through an electroplating process. According to investigator T. Bunch (pers. comm.), this meteorite contains very tiny prismatic enstatite crystals with µm-sized oblate-to-spherical glass inclusions that might be presolar condensates. Also present are vermicular carbon, well-formed and poorly-formed ‘graphite’, and unresolved carbon grains (<2 µm in size).

Prior to the analyses of NWA 2965, a 171.5 g stone with an identical appearance was analyzed at the Lunar & Planetary Laboratory, University of Arizona (Lowe, Hill, Domanik, and Lauretta), and the Southwest Meteorite Laboratory, Payson, Arizona (Killgore). The details of their analysis and the reasons for their ultimate classification can be found in the abstract NWA 2736: An unusual new graphite-bearing aubrite. They conclude that NWA 2736 has an igneous texture, and is best described as an unusual enstatite achondrite (aubrite) rather than an enstatite metachondrite or enstatite impact melt. Notably, they compare the impact-shock features to those of Happy Canyon, which is typically classified as a highly metamorphosed EL chondrite rather than an aubrite.

Utilizing a larger volume of this meteorite that provided a better representation of its various components, investigators from NAU and UWS have released results of their most exhaustive study to date at the Fall 2006 Meeting of the American Geophysical Union. They suggest that all of the similarities found among NWA 2965, NWA 2736, and NWA 2828 make it likely that these independently classified stones, along with NWA 4232 and several others, are paired. In further support of a pairing, they determined that all of these stones share a common subsurface excavation site in Algeria delimiting a 40 km strewn field. Based on visual evidence, the previously classified EL6 chondrite NWA 002, a meteorite purchased in Morocco in 1999 having an appearance virtually identical to NWA 2965, may also belong to this pairing group.

Succeeding excavation at the strewn field led to the recovery of several large masses which were studied at Université Pierre & Marie Curie, Paris (A. Jambon, O. Boudouma, and D. Badia) under the name Al Haggounia 001. A classification of aubrite was assigned to this material in light of its enstatite and plagioclase composition (MetBull 92), but the documented existence of terrestrially weathered chondrules in this meteorite should disqualify this classification (see The Al Haggounia ‘Fossil or Paleo’ Meteorite Problem).

Subsequent studies of Al Haggounia by Devaux et al. (2011) found that the ordering of carbon in the matrix indicates that it has experienced significant metamorphism. This crystalline carbon as well as the overall textures of this material prompted them to classify this meteorite as a type 6. Contrariwise, the lack of recrystallization/equilibration in this meteorite has been cited by some investigators as evidence against a petrologic type as high as 6. It was proposed by T. Bunch (pers. comm.) that this meteorite could represent an unequilibrated primitive regolith that formed without chaotic, turbulent mixing on an E-type asteroid. However, in his study of the Al Haggounia 001 pairing, A. Rubin (2016) noted multiple features which indicate this meteorite is an impact-melt breccia, including the following: vesicles produced by troilite evaporation and preserved through quenching, euhedral lath-like graphite grains, kamacite-rich veins; melt globules; partially resorbed chondrules; enstatite nucleation on relict silicate grains and relict chondrules; shock-induced silicate darkening; shock deformation and mosaicism of silicates; quenched feldspathic glass containing trapped, rounded enstatite inclusions; depletion of siderophile elements; and depletion in elements associated with various sulfides.

<!– with millimeter-sized, rounded inclusions consisting of coarse-grained orthopyroxene, sparsely distributed in this meteorite (see photo below). These inclusions are highly recrystallized and equilibrated features, and while they offer no petrographic evidence as to their nature, they are likely remnants of pre-existing chondrules. Northwest Africa 2965 has undergone heavy oxidation since its arrival on Earth, as attested by the many calcite veins intruding the stone; it has a weathering grade of W2–W5.

–> Images of distinct chondrules present in this meteorite can be seen on J. Kashuba’s NWA 2965 page. In their study of multiple samples of this meteorite, Leili et al. (2018, #6263) identified a wide range of chondrule types including rimmed macrochondrules/clasts up to 11 × 6 mm in size. The specimen of NWA 2965 shown above is a 30 g slice exemplifying the disparate weathering grades in contact with each other (e.g., a small portion of the less weathered, bluish-colored clast is attached on the right side. The top photo below is a close-up of a 7.4 g slice of NWA 2965 exhibiting a dense web of oxide-filled fractures and an oval enstatite or plagioclase aggregate. The bottom photo shows a 4.1 g partial end section exhibiting a distinct bluish coloration representing the less altered material of this extensive find. *Relict meteorites, defined as those highly altered meteorites which are >95% replaced by secondary phases since their fall, comprise a new category adopted by the Committee on Meteorite Nomenclature in October 2006. standby for nwa 2965 photo

standby for nwa 2965 photo


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NWA 1222

EL5
standby for northwest africa 1222 photo
purchased 1999
no coordinates recorded A single stone weighing 2,800 g was found in the Moroccan Desert. This meteorite was analyzed by T. Mikouchi and K. Kentaro at the University of Tokyo, Japan, and determined to be a rare EL5 chondrite, one of only three found to date. This meteorite has been heavily weathered to grade W3, but this is most obvious very near the crust. Enstatite chondrites were formed in a highly reducing environment and all contain abundant metallic FeNi.

In contrast to the EL6 meteorites, all very weakly shocked to S2, NWA 1222 is more strongly shocked to stage S3 (corresponding to a peak pressure of 10–20 GPa). The mineral sinoite (silicon oxynitride) has been found to occur in many EL chondrites, associated with crystallization of an impact melt. Presumably, these initial intense shock effects were erased during an annealing phase, but sometime later, a minor shock event left its characteristic signature in the rock.

The EL5 subgroup presently comprises only a few confirmed members, including Tanezrouft 031 (28 g), TIL 91714 (163.9 g), and NWA 1222 (2,800 g). Another E chondrite of petrologic type 5, the ambiguously grouped RKPA80259 (20.2 g), was initially included in the EL group based on its low siderophile element content. However, further studies determined that it has certain refractory element ratios more consistent with the EH group, and that its low siderophile content was probably the result of terrestrial weathering. Additional support for inclusion into the EH group comes from the presence of niningerite [(Mg,Fe)S] or keilite [(Fe,Mg)S] rather than alabandite [(Mn,Fe)S], a Si content in kamacite of 2.1 wt% (EH: 1.9–3.8 wt%; EL: 0.3–2.1 wt%), and a small chondrule size. Paradoxically, RKP A80259 displays the characteristic cathodoluminescence of enstatite found in the EL group.

The specimen of NWA 1222 pictured above is a 5.35 g polished partial slice, while the photo below shows the cut face of the main mass. Although not obvious from the angle of the lighting in these pictures, FeNi-metal is pervasive throughout the rock.

standby for northwest africa 1222 photo
Photo courtesy of R. A. Langheinrich Meteorites