A single 407 g meteorite was found in the Sahara Desert, possibly near Tan Tan, Morocco, and subsequently purchased by F. Kuntz in Zagora, Morocco on behalf of Planetary Studies Foundation in Galena, Illinois. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 10503 was determined to be a texturally-evolved meteorite exhibiting ~120° triple junctions with no relict chondrules visible. The elevated silicate FeO/MnO ratios are higher than those for ordinary chondrites and are consistent with a carbonaceous chondrite classification. In May 2016, a lot of smaller stones having a combined weight of 215 g was purchased by B. Hoefnagels. This group of stones was designated NWA 10859, and results of petrographic and isotopic analyses (A. Irving and S. Kuehner, UWS; K. Ziegler, UNM) led to the determination that they are paired with NWA 10503.
On an oxygen three-isotope diagram the values plot away from all other analyzed meteorites and along an extension of the trend line for the ungrouped pallasite Milton (K. Ziegler, UNM; see diagram below). NWA 10503 is classified as an ungrouped carbonaceous metachondrite and might be related to Milton; results from 54Cr analyses will help resolve the basis for any genetic relationship.
Diagram credit: Irving et al., 79th MetSoc, #6461 (2016)
Another ungrouped metachondrite with affinities to carbonaceous chondrites is 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 TFL. Bunch et al. (2006) suggest 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 and abstract #P51E-1246).
Northwest Africa 10503 is a somewhat friable meteorite with features indicating a low degree of terrestrial weathering and a low shock stage. Two views of a 3.79 g fragment of NWA 10503 are shown above, while some of the larger stones representing NWA 10859 are shown below.