Phyllosilicates with embedded Fe‐based nanophases in Ryugu and Orgueil
Authors: Leroux Hugues , Le Guillou Corentin , Marinova Maya , Laforet Sylvain , Viennet Jean‐Christophe , Mouloud Bahae‐Eddine , Teurtrie Adrien , de la Peña Francisco , Jacob Damien , Hallatt Daniel , Fernandez Mario Pelaez , Troadec David , Noguchi Takaaki , Matsumoto Toru , Miyake Akira , Igami Yohei , Haruta Mitsutaka , Saito Hikaru , Hata Satoshi , Seto Yusuke , Miyahara Masaaki , Tomioka Naotaka , Ishii Hope A. , Bradley John P. , Ohtaki Kenta K. , Dobrică Elena , Langenhorst Falko , Harries Dennis , Beck Pierre , Phan Thi H. V. , Rebois Rolando , Abreu Neyda M. , Gray Jennifer , Zega Thomas , Zanetta Pierre‐M. , Thompson Michelle S. , Stroud Rhonda , Burgess Kate , Cymes Brittany A. , Bridges John C. , Hicks Leon , Lee Martin R. , Daly Luke , Bland Phil A. , Zolensky Michael E. , Frank David R. , Martinez James , Tsuchiyama Akira , Yasutake Masahiro , Matsuno Junya , Okumura Shota , Mitsukawa Itaru , Uesugi Kentaro , Uesugi Masayuki , Takeuchi Akihisa , Sun Mingqi , Enju Satomi , Takigawa Aki , Michikami Tatsuhiro , Nakamura Tomoki , Matsumoto Megumi , Nakauchi Yusuke , Abe Masanao , Nakazawa Satoru , Okada Tatsuaki , Saiki Takanao , Tanaka Satoshi , Terui Fuyuto , Yoshikawa Makoto , Miyazaki Akiko , Nakato Aiko , Nishimura Masahiro , Usui Tomohiro , Yada Toru , Yurimoto Hisayoshi , Nagashima Kazuhide , Kawasaki Noriyuki , Sakamotoa Naoya , Okazaki Ryuji , Yabuta Hikaru , Naraoka Hiroshi , Sakamoto Kanako , Tachibana Shogo , Watanabe Sei‐Ichiro , Tsuda Yuichi 
Published: November 2023

Samples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small‐sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine‐grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S‐ and Ni‐rich ferrihydrite is abundant. The coarse‐grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x‐ray microscopy at the Fe‐L3 edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe3+) than in Ryugu (<50% Fe3+). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine‐grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe‐based nanophases. On the other hand, the coarse‐grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long‐term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.