Sulfur‐bearing serpentine in carbonaceous chondrites

Abstract A correlative multi‐technique approach, including electron microscopy and X‐ray synchrotron work, has been used to obtain both structural and compositional information of a sulfur‐bearing serpentine identified in several carbonaceous chondrites (Winchcombe CM2, Aguas Zarcas CM2, Ivuna CI, and Orgueil CI), and in Ryugu samples returned by the Hayabusa2 mission. S‐K edge X‐ray absorption spectroscopy was used to determine the oxidation state of sulfur in the serpentine in all samples except Ryugu. The abundance of this phase varies across these samples, with the largest amount in Winchcombe; ~12 vol% of phyllosilicates are identified as sulfur‐bearing serpentine characterized by ~10 wt% SO 3 equivalent. HRTEM studies reveal a d 001 ‐spacing range of 0.64–0.70 nm across all sulfur‐bearing serpentine sites, averaging 0.68 nm, characteristic of serpentine. Sulfur‐serpentine has variable S 6+ /∑S total values and different sulfur species dependent on specimen type, with CM sulfur‐bearing serpentine having values of 0.1–0.2 and S 2− as the dominant valency, and CIs having values of 0.9–1.0 with S 6+ as the dominant valency. We suggest sulfur is structurally incorporated into serpentine as SH − partially replacing OH − , and trapped as SO 4 2− ions, with an approximate mineral formula of (Mg Fe 2+ Fe 3+ Al) 2‐3 (Si Al) 2 O 5 (OH) 5‐6 (HS − ) 1‐2 (SO 4 ) 2− 0.1‐0.7 . We conclude that much of the material identified in previous studies of carbonaceous chondrites as TCI‐like or PCPs could be sulfur‐bearing serpentine. The relatively high abundance of sulfur‐bearing serpentine suggests that incorporation of sulfur into this phase was a significant part of the S‐cycle in the early Solar System.