Ilafegh 009

EL Melt Rock
(EL7 in MetBull 71; Primitive Enstatite AchondriteUsed in past as synonym for Aubrites. Present definition from the Meteoritical Bulletin states that this rare class is an "enstatite-rich  achondrite  that has not yet been classified into a group". Click on Term to Read More [Pilski et al., 2011])
(ELa7 in Weyrauch et al., 2018)

Found October 1989
21° 38′ N., 1° 16′ E. A single, relatively fresh (W0/1) stone weighing 421 g was found in the Algerian Sahara Desert. From initial chemical and petrologic analyses of Ilafegh 009, it was determined that the meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More was a highly metamorphosed enstatite chondriteType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More consistent with a classification of EL6/7 (Otto, 1992). Subsequent analyses by Bischoff et al. (1992) and McCoy et al. (1995) verified its genetic relationship to the EL chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More group, and brought to light its unique igneous texture reflecting both cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More and granoblastic characteristics, as well as its unusually large-sized enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More crystals (up to 0.75 cm) that impart a lighter color to the whole rock than is typically apparent in E chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More. Ilafegh 009 is considered by some to be a total shock-melted rock from the EL chondrite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More.

The mineralogy of Ilafegh 009 is similar to that of EL chondrites; in wt%, the modal mineralogy is as follows (McCoy et al., 1992, 1995): orthoenstatite (52.4), twinned enstatite (1.9–3.2), plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More (7.1), FeNi-metal + schreibersiteNi-Fe phosphide mineral, (Fe,Ni)3P, yellowish in color and predominantly found in iron and stony-iron meteorites. Schreibersite can also be found in a variety of other meteorites including some acapulcoites, aubrites, enstatite chondrites and achondrites, lunars, ureilites, winonaites and a smattering of other meteorite types like CM, CO and CB. Schreibersite Click on Term to Read More (27.2), Cr–Ti-troilite (8.1) in association with daubreélite and ferroan alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. Click on Term to Read More, CaS (oldhamiteMn-Ca sulfide, (Mn,Ca)S, is a pale to dark brown accessory mineral found in minor amounts in highly reduced meteorites such as many enstatite chondrites, and some aubrites and enstatite achondrites. Oldhamite in enstatite chondrites likely formed by solar nebular gas condensation. CaS Oldhamite was also found in the most fresh Click on Term to Read More), and trace TiN (osbornite). The plagioclase grains contain tiny fluid or gas inclusions. The FeNi-metal consists only of kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More that contains ~6 wt% Ni and up to 1.3 wt% Si. Nanoscale exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. Click on Term to Read More precipitates enriched in Ni, P, and Si are ubiquitous throughout the kamacite host phase and might characterize a combination of schreibersite and perryite.

Weyrauch et al. (2018) analyzed the mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More 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 troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More and the Fe concentration in niningerite and alabandite (endmembers of the [Mn,Mg,Fe] solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with 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).

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 metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More 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 Ilafegh 009 is a member of the ELa subgroup.

It has been suggested that Ilafegh 009 crystallized from a superheated impact-induced melt (peak pressures >75 GPa), and then was rapidly cooled from peak temperatures of ~5000°C/m.y. (McCoy et al., 1992). The Ar–Ar age of Ilafegh 009 was calculated to be 4.43 b.y., while an older I–Xe age of ~4.565 b.y. indicates this chronometer was not reset (Bogard et al., 2010). This petrogenetic scenario is consistent with the appearance of unfractionated metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More and sulfide, and the occurrence of magmatic inclusions within enstatite crystals. The large size of these crystals indicates that all of the original enstatite nuclei were eradicated in the superheated melt. At the same time, all of the trapped argonNoble gas represented by the atomic symbol Ar, that has Z = 18, and an atomic weight of 39.948. It is colorless, odorless, and very inert gas, comprising ~1 % of the Earth's atmosphere. Click on Term to Read More and many other volatile elementsChemical elements that condense (or volatilize) at relatively low temperatures. The opposite of volatile is refractory. Volatile elements can be divided into moderately volatile (Tc = 1230–640 K) and highly volatile (Tc < 640 K). The moderately volatile lithophile elements are: Mn, P, Na, B ,Rb, K, F, Zn. The moderately Click on Term to Read More were degassed during this event. The crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More sequence of the melt is believed to have been enstatite ⇒ plagioclase ⇒ silicaSilicon dioxide, SiO2. ⇒ FeNi-metal/sulfides. The mineral sinoite has also been identified, which is associated with crystallization of an impact melt.

Very fine-grained (few µm-sized), rounded, magmatic inclusions have been identified within the cores and rims of enstatites (Leroux et al., 1997). The inclusions consisting of plagioclase + metal + troilite are present in the cores, while those consisting of silica-rich glass (both K-rich and K-poor) are found in the enstatite rims as well as interstitially throughout the enstatite and plagioclase host grains—findings that are consistent with the proposed crystallization sequence. Besides these inclusions, undeveloped orthoenstatite nuclei are present within enstatite host grains.

It is considered that following solidification, the Ilafegh 009 lithology was impact-shocked to stage S4, as indicated by undulatory extinctionIn astronomy, the dimming of starlight as it passes through the interstellar medium. Dust scatters some of the light, causing the total intensity of the light to diminish. It is important to take this effect into account when measuring the apparent brightness of stars. The dark bands running across portions Click on Term to Read More, moderate mosaicism, and planar fractures in the orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More grains. In addition, microscopic striations are present in some pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More lamellae that were formed by the intergrowth of orthoenstatite and clinoenstatite phases resulting from shock (McCoy et al., 1995). In contrast to Ilafegh 009, the impact-melt derived EL chondrite Happy Canyon did not experience temperatures to the same high degree, as concluded from the presence in Happy Canyon of a fine-grained lithology, considered to result from crystallization sustained by pre-existing enstatite nuclei. Happy Canyon was also cooled more rapidly due to the incorporation of cold clastic material, which eventually solidified to form a brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More. In addition, Happy Canyon experienced less severe post-solidification shock than Ilafegh 009. Nevertheless, Pilski et al. (2011) propose that Ilafegh 009 may more properly be classified with Zakłodzie, and perhaps Happy Canyon, QUE 94204, and Y-8404, as a primitive enstatite achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More representing the residue from the rapid partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting Click on Term to Read More of an enstatite chondrite parent body.

In their advanced microscopic analyses of Ilafegh 009, Boesenberg et al. (2014) proposed a slightly different two-stage petrogenetic history. The first stage in their scenario involves formation as a melt rock, after which it would resemble the other fine-grained, granular-textured EL chondrites such as Zakłodzie and Happy Canyon. They invoke a subsequent heating event that resulted in the re-orientation and sintering (and/or Ostwald ripening) of these small enstatite grains into a coarse-grained, crystallographically-contiguous texture.

The O-isotopes for Ilafegh 009 plot with the E chondrites and aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as Click on Term to Read More. Interestingly, I–Xe radiochronometry demonstrates that Ilafegh 009, Happy Canyon, and the anomalous aubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as Click on Term to Read More Shallowater all share similar closure times for radiogenic xenon in the iodine host enstatite phase at ~4.565 b.y. ago. This suggests that each meteorite experienced essentially concurrent episodes of an I–Xe clock-resetting, impact-melting event in a common formation region (Kehm et al., 1993). The younger K–Ar age calculated for Ilafegh 009 of 4.34–4.44 b.y. may represent subsequent thermal resetting by impact, while the K–Ar age of 4.53 b.y. calculated for both Happy Canyon and Shallowater may reflect the actual closure of this particular chronometric systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with during metamorphic cooling following accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More.

The specimen shown above is a 1.55 g partial slice of Ilafegh 009, which highlights the metallic component of this meteorite. It was originally part of a 25.5 g partial slice in the collection of JNMC–Zurich (see photo below). The middle portion shown is being utilized for further thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. studies.

Photo courtesy of JNMC–Zurich