Iron valence state and mineralogy in particles from asteroid Ryugu
Jean-Christophe Viennet1, Mathieu Roskosz1, Pierre Beck2, Esen Ercan Alp3, Barbara Lavina3,4, Michael Hu3, Jiyong Zhao3,Tomoki Nakamura5, Kana Amano5, Mizuha Kikuiri5, Tomoyo Morita5 Hisayoshi Yurimoto6, Takaaki Noguchi7 Ryuji
Okazaki8 Hikaru Yabuta9, Hiroshi Naraoka8, Kanako Sakamoto10, Shogo Tachibana10 ,11, Seiichiro Watanabe12, Yuichi
Tsuda10, and the Hayabusa2-initial-analysis Stone team.
1

Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Museum National d’Histoire Naturelle, CNRS,Sorbonne Université, Paris, France. 2Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes,CNRS CNES, Grenoble, France. 3Center for Advanced Radiation Sources, The University of Chicago, Chicago, IL 60637,
USA.4Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA. 5Tohoku Univ. 6Hokkaido
Univ.7Kyoto Univ. 8Kyushu Univ. 9Hiroshima Univ. 10JAXA. 11Univ. of Tokyo, 12Nagoya Univ.
Introduction: Iron is a major element in rocky material from the solar system that can occur under multiple valence state. As
such it can be used to trace geological processes that occurred on asteroid Ryugu, including thermal metamorphism andaqueous alteration. Observations of the mineralogy of carbonaceous chondrites have revealed the presence of minerals
assemblages that are barely in thermodynamic equilibrium. Even at the micron-scale, iron is often present under multiple
valence state (most frequently 0, +2, or +3) in a wide range of minerals (primary or secondary in nature), including silicates,
oxy-hydroxide, sulfides, sulfates, clay minerals and carbonates. Such heterogeneous assemblages are the results of postaccretion processes that often triggered a partial textural and chemical equilibration, associated to a modification of primaryFe-bearing minerals and their valence state. It is therefore tricky to disentangle the different processes recorded by the ironmineralogy of chondritic materials. Furthermore, since most Fe-bearing phases are redox-sensitive, exposure to terrestrial
atmosphere may also induce iron oxidation and additional, late modifications of the iron mineralogy. In this context,
Hayabusa2 sampled Ryugu, an asteroid that did not suffer extensive thermal metamorphism, and returned rocks to Earth withminimal air exposure. It offers a unique opportunity to study the redox state of carbonaceous asteroids and evaluate the overall
redox state of the most oxidized primitive rocks of the solar system.
Samples and methods: Here, we determine the mineralogy and the redox state of Fe-bearing minerals from ten Ryugu
samples (five from the chamber A used for the first sampling and five from the chamber C for the second sampling) prior toand after exposure to air. We use Synchrotron Mössbauer Spectroscopy (SMS) technique that enables to probe the bonding
environment of iron at the microscopic scale. These measurements are combined with co-aligned X-ray diffraction, permitting
to assess locally the mineralogy and valence state of iron in Ryugu particles. We also apply conventional Mössbauer
spectroscopy on a couple of large (mm-size) Hayabusa returned samples from the chambers A and C.
Results and discussion: We will present the bulk proportions of iron-bearing minerals and the Fe redox state (Fe3+/Fetot) at the‘Stone’ scale. We will then provide the first estimate of the redox state of Ryugu as compared to the large array of bulk redox
data collected over several decades on meteorites. We will also describe the redox states of iron-bearing clay minerals before
any air contamination. The first clues on the ageing of these minerals after exposure to air will be discussed based on datacollected on the same sample before and after exposure to air. We will compare in greater details Ryugu samples to a series ofwell-known chondritic meteorites including Orgueil and Murchison. We will show, more specifically, that most of the iron isaccommodated in magnetite and sulfides (pyrrhotite and pentlandite). Clay minerals also contain a fraction of the total iron.
Overall, the investigated Ryugu samples appears to be (or are) more reduced than the Orgueil, both considering the bulk
composition and the clay minerals fraction. The redox state recorded in a pristine sample and in the same sample exposed to air
for more than two months do not show clear evidence of oxidation suggesting that, as far as iron is concerned, samples can beprepared and analyzed at ambient conditions at least for several weeks if stored in vacuum or in inert gas after preparation.