NWA 5958

C2-ungrouped (CM-like)
(C3.0-ung [2012], C2-ung in MetBull 99)

Purchased September 2009 Numerous fragments of a fusion-crusted carbonaceous chondriteCarbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than having a combined weight of 286 g were found in Algeria and purchased by G. Hupé in Morocco. A sample was submitted to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington at Seattle (A. Irving and S. Kuehner) for analysis and classification. Northwest Africa 5958 was designated an ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). carbonaceous 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 of petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes. 3.0, with a low shock stageA petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level".               of S1 consistent with other carbonaceous 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.

In their study of magnetic susceptibility, Elmaleh et al. (2012) identified abundant Fe-rich phyllosilicatesClass of hydroxyl-bearing silicate minerals with a sheet-like structure. They result from aqueous alteration are dominantly serpentine and smectite in meteorites; found in the matrixes of carbonaceous chondrites. Phyllosilicates consist of repeating sequences of sheets of linked tetrahedra (T) and sheets of linked octahedra (O). The T sheet consists of such as the serpentineName used for a large group of phyllosilicate minerals with the generalized formula X2-3 Y2 O5 (OH)4. Due to their various structures (meteoritics focuses primarily on (Fe, Mg)3Si2O5(OH)4), serpentine can be used to understand the chemistry and progress of aqueous alteration (hydration) of olivine, amphibole, or pyroxene dating back to cronstedtite, indicative of a low degree of 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. hydrothermal alteration. Moreover, the observation of partially altered chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine led them to consider revising the petrologic type from 3.0 to 2.9. Although 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 experienced low terrestrial weathering (W1), NWA 5958 has experienced significant loss or gain of some elements during its residence in the desert, with more extreme alteration observed toward the surface (Ash et al., 2011). Subsequent analyses of the magnetic properties of NWA 5958 conducted by Jacquet et al. (2016) indicate the presence of significant magnetiteFe oxide, FeFe2O4, containing oxidized iron (Fe) found in the matrix of carbonaceous chondrites and as diagnostic component in CK chondrites. In CK chondrites, magnetite is typically chromian, containing several wt. % Cr2O3. (1.2 wt%) compared to 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 (0.42 wt%), which attests to a degree of parent body aqueous alteration. Utilizing infrared spectroscopyTechnique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in to investigate the hydrous phases in NWA 5958, Jacquet et al. (2016) identified a significant abundance of phyllosilicates indicative of weak to moderate aqueous alteration, presenting a spectral range that matches well that of CM2 LEW 85311.

In the first compositional anaysis conducted by Bunch et al. (2011), it was found that NWA 5958 consists of a wide variety of small objects (0.05–2.5 mm) set in a dark, porous, fine-grained matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents.. Notably, matrix material constitutes 76 vol% of the meteorite, a higher abundance than in most CM chondrites, and it contains a relatively low abundance of chondrules composing 19 vol% (Jacquet et al., 2016). Silicates, phyllosilicates (e.g., the Fe-serpentine cronstedtite), and partially equilibrated Fe-sulfides were observed in the matrix by Stroud et al. (2014). Similar to CM chondrites, relatively small chondrules of mostly Mg-rich POP, 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). PP, and forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) PO types are present and host multiple 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 rims (up to five), with each successive layer composing material of larger grainsizes. In their study, Jacquet et al. (2016) identified accessory 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, sulfides, and poorly characterized phases (PCP, now determined to be tochilinite-cronstedtite intergrowths [TCI]). The TCI composition in NWA 5958 reflects a relatively high mean ‘FeO’/SiO2 ratio of 5, which is consistent with very minimal aqueous alteration compared to other CM group members (Rubin et al., 2007). Contrariwise, the relatively low mean S/SiO2 ratio (0.05) for NWA 5958 is consistent with a high degree of aqueous alteration. At the same time, the low abundance of FeNi-metal (< ~0.2 vol%) is more consistent with a moderate degree of aqueous alteration comparable to petrologic type 2.2–2.5 in the Rubin et al. (2007) scheme.

Bunch et al. (2011) reported µm-sized hexagonal carbonElement commonly found in meteorites, it occurs in several structural forms (polymorphs). All polymorphs are shown to the left with * indicating that it been found in meteorites and impact structures: a. diamond*; b. graphite*; c. lonsdalite*; d. buckminsterfullerene* (C60); e. C540; f. C70; g. amorphous carbon; h. carbon nanotube*. grains present in some chondrules with larger C aggregates in the matrix and olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion in NWA 5958. They found only a few small CAIsSub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they in the meteorite (2 vol%), and it was ascertained that these do not contribute significantly to the unique 16O-rich composition of the bulk meteorite (Ash et al., 2011). A larger number of AOAs were observed in the meteorite by Jacquet et al. (2016). Rare fine-grained, carbonaceous chondrite xenoliths were only observed by Bunch et al. (2011).

An oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only 3-isotope diagram based on the initial values determined by Bunch et al. (2011) is shown below. Given these values, NWA 5958 falls along an extension of the carbonaceous chondrite anhydrous 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 (CCAM) line of slope-0.94, but is distinct from other C chondrites in having values even closer to initial solar values exemplified by the slope-1 line (D. Rumble III, CIW; see O-isotope plot). The CCAM line represents a linear array on an oxygen three-isotope diagram that is defined by the ratio plots for a mixture of all minerals that constitute CAIs. It was determined by Young and Russell (1998) that the most primitive Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. materials defined a linear array with a slope of 1.00 (Y&R line). These primary materials initially had heterogeneous 16O contents, but later mass fractionationFractionation of isotopes or elements that is dependent on their masses. and oxygen exchange processes resulted in material with higher 17O and 18O contents, generally evolving towards the CCAM line.
Diagram credit: Bunch et al., 42nd LPSC, #2343 (2011) Further analyses of NWA 5958 were conducted by Jacquet et al. (2016), and they obtained O-isotopic values (Δ17O = –4.26‰) different from those determined previously (Δ17O = –7‰). A new oxygen 3-isotope diagram based on these new values is shown below. It demonstrates that NWA 5958 plots near some C2-ungrouped meteorites such as Acfer 094 rather than with C3-ungrouped meteorites. Together with other similar meteorites, NWA 5958 could sample a separate CM-like parent body (Jacquet et al., 2017).
click on image for a magnified view

Diagram credit: Jacquet et al., MAPS, vol. 51, #5, p. 862 (2016)
‘Northwest Africa 5958: A weakly altered CM-related ungrouped chondrite, not a CI3’ (http://dx.doi.org/10.1111/maps.12628) Sanborn et al. (2015) presented a Δ17O vs. ε54Cr coupled diagram in their analyses of NWA 5958. Utilizing the initial Δ17O value determined by Bunch et al. (2011), along with the 54Cr value determined by Göpel et al. (2013) of +0.973 (± 0.153)—this value being close to the mean for all carbonaceous chondrites—they demonstrated that the meteorite plots in a distinct region (bottom orange circle in diagram below). However, given the new O-isotope value determined by Jacquet et al. (2016) of Δ17O = –4.26‰, the meteorite is within the trend of other carbonaceous chondrite groups (top orange circle in diagram below); NWA 5958 plots slightly below the ungrouped 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 Al (aluminum isotope) and gravitational (CV-clan-related) NWA 7822 (Δ17O = ~ –4‰).
Diagram adapted from Sanborn et al., 46th LPSC, #2259 (2015) The initial bulk chemical and trace elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of compositions calculated for NWA 5958 were found to resemble that of CI chondrites, including a high volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). content, with the 187Os/188Os ratios having the largest value (the most radiogenic) of any measured carbonaceous chondrite (Ash et al., 2011). Subsequent bulk chemical and trace element compositional analyses were conducted by Jacquet et al. (2014). After accounting for the typical altered signature caused by an extended residence in a desert environment, NWA 5958 shows very close similarities to the CM chondriteClass of carbonaceous chondrites named after the Mighei meteorite that fell in Ukraine in 1889. They represent samples of incompletely serpentinized primitive asteroids and have experience extremely complex histories. CM meteorites are generally petrologic level type 2 though a few examples of CM1 and CM1/2 also exist. Compared to CI Paris. Measurement of the Cr2O3 content in chondruleRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine olivine for NWA 5958 is comparable to that for the CO3.03 ALHA77307 and the CM2.7–2.9 Paris, which is indicative of very limited thermal metamorphism at temperatures <300°C.

In a study of presolar grainsMineral grains that formed before our solar system. These tiny crystalline grains are typically found in the fine-grained matrix of chondritic (primitive) meteorites. Most grains probably formed in supernovae or the stellar outflows of red giant (AGB) stars before being incorporated in the molecular cloud from which the solar system present in NWA 5958 matrix conducted by Nittler et al. (2012), a small abundance of 13C-rich grains (45 ppmParts per million (10).) and O-anomalous grains (~100 ppm) were identified. The low abundance of O-rich presolar silicates (~50 ppm) observed by Stroud et al. (2014), including an Al,Mg-spinel and an enstatite grain, is thought to be due to loss as a result of parent body hydrothermal alteration. These values suggest a slightly higher degree of hydrothermal metamorphism compared to typical type 3.0 carbonaceous chondrites, more consistent with a type 2. Northwest Africa 5958 is a unique primitive sample from the early solar 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, having many characteristics intermediate between CM and CO chondrites. The specimen of NWA 5958 shown above is a 1.62 g crusted fragment, while the image below shows an excellent interior close-up, shown courtesy of Greg Hupé.
click on image for a magnified view
Photo courtesy of Greg Hupé—Nature’s Vault