CHWICHIYA 002

Numerous moderately weathered fragments and crusted stones having a combined weight of 779 g were found by Elho Sbiti in the Saguia el Hamra region of Western Sahara (Smith et al., 2022). Material was subsequently acquired by several meteorite dealers and a sample was sent to CEREGE (European Centre for Research and Teaching in Environmental Geosciences) for analysis and classification (J. Gattacceca et al.). The meteorite was designated Chwichiya 002 and determined to be an ungrouped carbonaceous chondrite of petrologic type 3.00. Additional fragments were found as early as 2016; one pairing designated NWA 12957 (43 g) was also classified at CEREGE (J. Gattacceca et al.) as C3.00-ungrouped (see diagrams below).

Chwichiya 002 was analyzed by Haenecour et al. (2022 #6425) and found to be composed of relatively large (480 [±300] µm; MetBull 109) chondrules (12.9 vol%) embedded in a dark, fine-grained matrix (73.4 vol%) containing magnetite (7.8 vol%) and sulfides (4.9 vol%). Accessory phases reported by Irving et al. (2022 #2046) include kamacite, pyrrhotite, pentlandite, chromite, magnetite, maghemite, taenite, calcite, and dolomite. An X-ray diffraction analysis was conducted at CEREGE (D. Borschnek), and both IR transmission and Raman spectroscopic analyses were conducted at the University of Grenoble, Institute of Planetology and Astrophysics (L. Bonal). With these techniques it was demonstrated that hydrous phases are virtually absent and that indigenous organic matter experienced peak heating less than that in Semarkona.

Ruggiu et al. (2021) employed multiple techniques in their study of incipient aqueous alteration in Chwichiya 002, including scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, infrared spectroscopy, Raman spectroscopy, electron microprobe, X-ray diffraction, position-sensitive-detector X-ray diffraction, thermogravimetric analysis, and secondary-ion mass spectrometry. They identified abundant unaltered GEMS-like (Glass with Embedded Metal and Sulfides) accretions and µm-sized TCIs (Tochilinite-Cronstedtite Intergrowths) in Chwichiya 002, as well as a low abundance of phyllosilicates, carbonates, sulfides, and magnetite. Therefore, they argue the meteorite is consistent with petrologic type 3.00. Using nanoscale secondary-ion mass spectrometry, Smith et al. (2022) identified two isotopically anomalous areas possibly representing organic matter, and six potential O-anomalous presolar grains.

Multiple analytical techniques were applied by Irving et al. (2022 #2046) and Garvie and Irving (2022 #2217; XRD data) to a number of previously classified carbonaceous chondrites including the ungrouped carbonaceous chondrite Chwichiya 002. High-precision oxygen isotope data obtained by Irving et al. (2022) for a broad sampling of carbonaceous chondrites enabled them to define two new CC region trend lines that are nearly aligned with those previously known (see top diagram below). The data points delineate a new carbonaceous group of at least 16 meteorites that was termed 'CT chondrites', a toponym for the type 3 group member Telakoast 001. This group currently comprises petrologic type 2 and type 3 members. With further analyses, including nucleosynthetic anomalous 54Cr values, it may be determined that other variously classified stones (e.g., NWA 13167, NWA 13249 [C2-ung], NWA 13456 [C2], NWA 13455 [CO3-an], and NWA 13479 [C2-ung]) with petrographic, chemical, mineralogical, magnetic susceptibility, and isotopic similarities to those in the initial study actually belong in the CT group.

The other new trend line demonstrated by Irving et al. (2022) delineates a potential carbonaceous grouplet of 4 that was termed 'CZ chondrites' (see diagram above). In addition, a trend line previously proposed by Irving et al. (2019 #2542) that they termed 'CX' initially comprised four meteorites from the CC region representing different textural types—Milton (pallasite), NWA 10503 (metachondrite), NWA 12264 (dunitic breccia), and NWA 11961 (unequilibrated chondrite). However, 54Cr isotope data obtained for all of these meteorites have resolved both NWA 11961 and NWA 12264 as potential new carbonaceous parent bodies distinct from that of NWA 10503 and Milton, the latter two previously considered possible members of the CV-clan (see diagram below). To date, no other meteorites have been demonstrated to have a genetic link to either NWA 11961 or NWA 12264.

It is noteworthy that this new CT group designation has somewhat of a history in and of itself, which is related to the unique meteorite NWA 2788. This meteorite has a metamorphic texture exhibiting ~120° triple junctions, elevated Fe/Mn and Ca/Na ratios, and an O-isotopic composition that plots very near to the Terrestrial Fractionation Line (TFL). NWA 2788 was considered by Bunch et al. (2006) to be a metachondrite associated with an unknown carbonaceous chondrite parent body. They conjectured that if a chondrule-bearing representative of this parent body is found and identified in the future, it should be termed a 'CT chondrite' (see NWA 2788 photos, abstract, and isotopic plots 1 [ref], 2 [ref]). Now in 2022, one of the co-authors of the 2006 abstract, A. J. Irving, has proposed that this new carbonaceous chondrite group be named 'CT', a toponym for the carbonaceous chondrite Telakoast 001. It is fitting that one of the members of the new CT group, Cimarron, was classified by T. Bunch.

Comparative analyses of the known asteroid types and a suite of ungrouped and rare meteorites in multiple forms (bulk, powder, polished section), including the ungrouped carbonaceous chondrite Chwichiya 002, were conducted by Krämer Ruggiu et al. (2021) utilizing petrographic, spectroscopic, and albedo data. They concluded that the best matches to this meteorite are the C-, Cg-, and B-type asteroids; this is the first meteorite match to Cg- or B-type asteroids (see diagram below).

The specimen of Chwichiya 002 shown in the photos above and below is a 0.84 g fragment showing a broken surface with a few protruding chondrules set in a dark matrix. The outer surface shows slight wind abrasion. These excellent photos are shown courtesy of Azelmat Nor Eddine.

Photo courtesy of Azelmat Nor Eddine—Saharock Meteorites