The iron mineralogy of Bennu sample: Similarities and variations among CI chondritic materials

Here, we present the iron mineralogy of the sample from the asteroid Bennu, representing a fresh CI material, determined by X-ray diffraction and Mössbauer spectroscopy, and compared to other CI materials, including Ryugu, Oued Chebeika 002, and Orgueil. Bennu grains contain substantial amounts of Fe sulfides (pyrrhotite, mackinawite, and pentlandite), as well as Fe oxide (magnetite) and Fe-bearing (Mg-rich) phyllosilicates, similar to Ryugu samples.

Cryogenic Mössbauer analysis indicates no significant contribution from ferric oxyhydroxides (i.e., typical terrestrial oxidation products observed in Orgueil) in Bennu and Oued Chebeika 002 sample, similar to previous observations on Ryugu samples. This strengthens the view that the actual CI materials and their formation environments are less oxidizing than estimated based on classical CI meteorites. Furthermore, we found two distinct phyllosilicate assemblages: (i) disordered phyllosilicates coexisting with mackinawite, a metastable Fe sulfide, and (ii) relatively well-crystalline phyllosilicates without mackinawite.

The origin and formation context of mackinawite remain elusive, yet, this mineralogical association opens a window into exploring the early stages of CI history.