  Northwest Africa (NWA) 5958 is a carbonaceous chondrite found in Morocco in
2009. Preliminary chemical and isotopic data leading to its initial
classification as C3.0 ungrouped have prompted us to conduct a multi-technique
study of this meteorite and present a general description here. The petrography
and chemistry of NWA 5958 is most similar to a CM chondrite, with a low degree
of aqueous alteration, apparently under oxidizing conditions, and evidence of a
second, limited alteration episode manifested by alteration fronts. The oxygen
isotopic composition, with $\Delta^{17}$O = -4.3 $\permil$, is more 16O-rich
than all CM chondrites, indicating, along with other compositional arguments, a
separate parent body of origin. We suggest that NWA 5958 be reclassified as an
ungrouped carbonaceous chondrite related to the CM group.
