A window into the early stages of aqueous alteration from mackinawite in samples of asteroid Bennu

We report the discovery of fibrous mackinawite in samples of asteroid Bennu returned by the OSIRIS-REx mission. Mackinawite occurs primarily in particles belonging to Bennu’s hummocky lithology, with fibers that range from 75 to 250 nm in length and 10 to 30 nm in width. In Bennu particles, mackinawite displays both fibrous and tabular habits and forms flower-like clusters that resemble the texture of coarse-grained phyllosilicates previously described. Energy-dispersive X-ray spectroscopy indicates an Fe/S ratio of 1, and four-dimensional scanning transmission electron microscopy reveals a tetragonal structure consistent with mackinawite. Similar to terrestrial occurrences, the activities of Fe 2+ and S 2– in aqueous solution are likely the main drivers of mackinawite precipitation within Bennu’s parent body. We suggest that mackinawite formed via precipitation from solution following the dissolution of accreted metal and sulfides when Fe and S activities were high enough to support mackinawite stability. Based on comparison to terrestrial Pourbaix diagrams, we hypothesize that mackinawite precipitation within Bennu’s parent body was possible at 7 < pH < 10, –0.5 < Eh < –0.15, 10 –9 ≤ a Fe ≤ 10 –6 , and temperatures up to 70°C.