Apatite in Bennu samples indicates multiple stages of aqueous alteration

Abstract

Calcium phosphates are ubiquitous in planetary materials, including samples returned from asteroid Bennu by the OSIRIS-REx mission. We characterized apatite [Ca₅(PO₄)₃(F,Cl,OH)] grains in Bennu samples by scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy to investigate their compositions, mineral associations, and microstructures. We find that Bennu apatite is halogen-poor, consistent with a composition of hydroxyapatite, and can be separated into two main structural types: single crystals, which often exhibit etched crystal faces, and anhedral polycrystalline assemblages. Both types exhibit zoning in cathodoluminescence imaging that results from incorporation of trace Mn²⁺ and rare earth elements into the apatite structure during crystal growth. Transmission electron microscopy of a single phosphate crystal and a polycrystalline assemblage reveals close association between apatite and phyllosilicates in the surrounding matrix. Phyllosilicates are either oriented parallel to intact apatite crystal facets or radiating from altered crystal faces. We interpret that single crystals with or without etched crystal faces are among the least aqueously altered of the observed apatites, whereas polycrystalline assemblages exhibiting a porous texture, consistent with successive dissolution–reprecipitation reactions, represent assemblages that experienced more extensive aqueous alteration. These microstructural data suggest that several stages of aqueous alteration likely occurred on Bennu's parent body, leading to the mineral assemblages observed here.