NORTHWEST AFRICA 16493

A single relatively fresh stone weighing 156 g was purchased from a Moroccan dealer by F. Kuntz. A type sample was sent for analyses and classification to the University of Washington in Seattle, WA (A. Irving), Washington University in St. Louis, MO (P. Carpenter), Brown University in Providence, RI (J. Boesenberg), and the University of New Mexico Institute of Meteoritics in Albuquerque, NM (oxygen isotope analysis; K. Ziegler). It was determined that NWA 16493 is an ungrouped, thermally equilibrated carbonaceous achondrite.

Northwest Africa 16493 is a fine-grained equilibrated meteorite (olivine Fa40.1 [±0.4]) with no identification of relict chondrules or matrix, and exhibits 120° triple junctions between grains. Minor augite and plagioclase are present, along with accessory phases which include low-Ca pyroxene, merrillite, nepheline, chromite, magnetite, pentlandite, and troilite (Irving et al., 2024). The oxygen isotope values for NWA 16493 plot on the trend line for the CT chondrite group (see the following paragraph).

Previously, multiple analytical techniques were applied by Irving et al. (2022 #2046) and Garvie and Irving (2022 #2217; XRD data) to a number of carbonaceous chondrites which had been classified earlier. 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 which are nearly aligned with those known before (see diagram below). The data points delineate a new carbonaceous group of at least 16 meteorites which was termed 'CT chondrites', a toponym for the type 3 group member Telakoast 001.

The CT group was previously comprised of only petrologic type 2 and type 3 members, but it has been proposed by Irving et al. (2024 #6221) that the petrography, geochemistry, and O-isotopic composition of ungrouped achondrite NWA 16493 is consistent with it becoming the first equilibrated member of the group (see oxygen isotope diagram below). With further analyses, including nucleosynthetic anomalous 54Cr values, it may soon be determined that other variously classified stones (e.g., NWA 13167 [C2-ung], 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). More recently, Cuppone et al. (2025) conducted in-depth analyses of NWA 13489, the results of which led them to conclude that a genetic relationship may exist between four meteorites—NWA 13489, NWA 11961, NWA 10503/10859, and Milton (see the NWA 13489 page for further details).

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.

Information about other members of the CT group can be found on the following pages: Chwichiy 002 (CT3.00), NWA 8781 (CT3), Qued Mya 002 (CT2[3]), and tentative members NWA 13167 (CT2) and NWA 16493 (CT7). The specimen of NWA 16493 shown above is a small 1.08 g cut fragment acquired from F. Kuntz.