A single meteorite weighing 56.2 g was found in Western Sahara and subsequently purchased by S. Ralew in Midelt, Morocco; a search for further pairings have proven unsuccessful. This small stone was analyzed over an extended period at the Museum für Naturkunde, Berlin, Germany (A. Greshake) and the Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, UK (I. Franchi and R. C. Greenwood). Northwest Africa 4042 is substantially composed of equigranular olivine crystals (~93.3 vol%), along with minor low-Ca pyroxene and accessory FeNi-metal, phyrrotite, and MgAlTi-chromite.
As a result of five test runs, the O-isotopic composition of NWA 4042 was determined to plot near several resolved fieldsthe aubrites, brachinites, and winonaites. Therefore, this meteorite was classified as an ungrouped achondrite. A subsequent O-isotopic analysis employing an acid leaching technique demonstrates that NWA 4042 plots within the brachinite field (Greenwood et al., 2007). A newly compiled O-isotope diagram for brachinites and other planetary achondrites, based on published data from D. Rumble, III et al. (2008), demonstrates that NWA 4042 (Δ17O value of 0.177) plots within a select grouping of brachinites including NWA 3151, NWA 595, and Zag (b); these investigators believe that this meteorite should probably be lumped with the brachinites (see also this linearized plot). However, through studies of highly siderophile element (HSE) abundances, and upon examining the metal-sulfide segregation processes, it was determined by Day et al. (2012) that NWA 4042 and similar brachinite-like achondrites were not likely genetically related (i.e. from the same parent body) to brachinites, but rather, originated on similar volatile-rich, oxidized, chondritic precursor asteroids while experiencing similar petrologic processes during their formation history. The measured HSE abundances are consistent with a partially melted parent body in which heating from short-lived radionuclides came to a halt before a core was fully formed.
Northwest Africa 4042 shows evidence of very weak shock (S2), and a low degree of weathering (W2). The specimen of NWA 4042 shown above is a 0.63 g partial slice. The top photo shown below is the remaining 16.19 g main mass, and below that is a 40× magnified view of a cut section depicting a coarse-grained, recrystallized texture.