A single stone weighing 533 g was found in Northwest Africa and purchased by F. Kuntz. A sample was submitted for analysis and classification (Wittke and Bunch, NAU; Irving, UWS) and NWA 5131 was found to have a mineralogical composition similar to that of LL chondrites. However, following an O-isotopic analysis (Rumble III, Carnegie Institution; #5222), it could be demonstrated that the oxygen three-isotope plot falls within the CR chondrite field.
The meteorite is a highly metamorphosed assemblage with strong similarities to the CR6 (or CR7, or metachondrite) Tafassasset. Fine-grained portions exhibit 120° triple junctions, while other areas consist of mineral phases described as having a poikiloblastic texture, defining possible relict chondrules. These recrystallized chondrule relicts would be consistent with a porphyritic, metal-bearing, olivinepyroxene chondrule type.
Just as with Tafassasset, a more advanced stage of metamorphism compared to that exhibited by the CR6 chondrites NWA 7317 (and pairings) and NWA 3100 has been invoked to explain the recrystallized poikiloblastic texture in NWA 5131, and therefore the term metachondrite might be most appropriate for this meteorite as well (Wittke et al., 2011). It has also been argued that the similarity in O-isotopic compositions that is observed among the non-metamorphosed CR chondrites, the metamorphosed CR6 chondrites, and both NWA 5131 and Tafassasset, compared with the igneous achondrite NWA 011 (and pairings), is consistent with their derivation from a common, large parent body, one which experienced internal partial melting while retaining a chondritic regolith.
As with Tafassasset, NWA 5131 is a recrystallized meteorite that is petrographically consistent with a low-degree partial melt of Renazzo-like precursor material that has retained its metal component. The rock subsequently experienced equilibration processes through an extended period of thermal metamorphism. Additional genetic relationships that may exist among members of the large group of CR-related meteorites can be found on the Tafassasset page. The specimen of NWA 5131 shown above is a 2.73 g partial slice.