Abstract Samples returned by NASA's OSIRIS‐REx mission from the carbonaceous asteroid (101955) Bennu hold clues about conditions in the early solar system and the formation of planetary bodies. Initial investigation of Bennu samples found that they are rich in phyllosilicates and other secondary minerals formed during aqueous alteration of a larger parent body. To better understand the phyllosilicate minerals and constrain the extent and settings of alteration, we used X‐ray powder diffraction (XRD) to characterize the mineralogy of homogenized aggregate (unsorted) samples and an angular particle from Bennu. We find that these samples consist of abundant (>80 vol%) phyllosilicates and few (≤2 vol%) precursor anhydrous silicates, in excellent agreement with remote observations of the global asteroid surface, and consistent with extensive aqueous alteration of Bennu's parent body. The XRD patterns and modal mineralogy of the Bennu samples are similar to those of CI carbonaceous chondrites and particles returned from the carbonaceous asteroid (162173) Ryugu by JAXA's Hayabusa2 mission, offering further support for the close genetic relationship among these materials suggested by other studies. Bennu's phyllosilicates are dominated by a mixture of trioctahedral Mg‐rich clay minerals that formed from alkaline fluids under high water to rock ratios and likely evolved in response to changes in temperature and/or fluid chemistry as alteration progressed. Based on the XRD characteristics of the phyllosilicates, the Bennu samples did not experience a peak temperature at or above ~300°C after aqueous alteration. Finally, we show that the interlayer space of the clay minerals potentially contains up to ~2 wt% water and does not host trapped organic species. The abundance of interlayer water in Bennu samples is notably higher than reported for Ryugu samples (<0.3 wt%), which we speculate is due to differences in either the timing at which the asteroids decoupled from the Main Belt or the sampling depths and/or mechanisms of the respective spacecraft.
King et al. (Wed,) studied this question.