# Phyllosilicates with embedded Fe-based nanophases in Ryugu and Orgueil

Hugues Leroux, Corentin Le Guillou, Maya Marinova, Sylvain Laforet, Jean-christophe Viennet, Bahae-eddine Mouloud, Adrien Teurtrie, Francisco de la Peña, Damien Jacob, Daniel Hallatt, et al.

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To cite this version:

Hugues Leroux, Corentin Le Guillou, Maya Marinova, Sylvain Laforet, Jean-christophe Viennet, et al. Phyllosilicates with embedded Fe-based nanophases in Ryugu and Orgueil. Meteoritics and Planetary Science, 2024, pp.1947-1965. 10.1111/maps.14101. hal-04295482

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Phyllosilicates with embedded Fe-based nanophases in Ryugu and Orgueil

Hugues LEROUX<sup>1*</sup>, Corentin LE GUILLOU<sup>1</sup>, Maya MARINOVA<sup>2</sup>, Sylvain LAFORET<sup>1</sup>, Jean-Christophe VIENNET<sup>1</sup>, Bahae-Eddine MOULOUD<sup>1</sup>, Adrien TEURTRIE<sup>1</sup>, Francisco de la PEÑA<sup>1</sup>, Damien JACOB<sup>1</sup>, Daniel HALLATT<sup>1,3</sup>, Mario Pelaez FERNANDEZ<sup>1</sup>, David TROADEC<sup>4</sup>, Takaaki NOGUCHI<sup>5</sup>, Toru MATSUMOTO<sup>5,6</sup>, Akira MIYAKE<sup>5</sup>, Yohei IGAMI<sup>5</sup>, Mitsutaka HARUTA<sup>7</sup>, Hikaru SAITO<sup>8,9</sup>, Satoshi HATA<sup>10,11</sup>, Yusuke SETO<sup>12</sup>, Masaaki MIYAHARA<sup>13</sup>, Naotaka TOMIOKA<sup>14</sup>, Hope A. ISHII<sup>15</sup>, John P. BRADLEY<sup>15</sup>, Kenta K. OHTAKI<sup>15</sup>, Elena DOBRICA<sup>15</sup>, Falko LANGENHORST<sup>16</sup>, Dennis HARRIES<sup>17</sup>, Pierre BECK<sup>18</sup>, Thi H. V. PHAN<sup>18</sup>, Rolando REBOIS<sup>18</sup>, Neyda M. ABREU<sup>19</sup>, Jennifer GRAY<sup>20</sup>, Thomas ZEGA<sup>21</sup>, Pierre-M. ZANETTA<sup>21</sup>, Michelle S. THOMPSON<sup>22</sup>, Rhonda STROUD<sup>23</sup>, Kate BURGESS<sup>24</sup>, Brittany A. CYMES<sup>25</sup>, John C. BRIDGES<sup>26</sup>, Leon HICKS<sup>26,27</sup>, Martin R. LEE<sup>28</sup>, Luke DALY<sup>28,29,30</sup>, Phil A. BLAND<sup>31</sup>, Michael E. ZOLENSKY<sup>32</sup>, David R. FRANK<sup>32</sup>, James MARTINEZ<sup>33</sup>, Akira TSUCHIYAMA<sup>34,35,36</sup>, Masahiro YASUTAKE<sup>37</sup>, Junya MATSUNO<sup>34</sup>, Shota OKUMURA<sup>5</sup>, Itaru MITSUKAWA<sup>5</sup>, Kentaro UESUGI<sup>37</sup>, Masayuki UESUGI<sup>37</sup>, Akihisa TAKEUCHI<sup>37</sup>, Mingqi SUN<sup>35,37,38</sup>, Satomi ENJU<sup>39</sup>, Aki TAKIGAWA<sup>40</sup>, Tatsuhiro MICHIKAMI<sup>41</sup>, Tomoki NAKAMURA<sup>42</sup>, Megumi MATSUMOTO<sup>42</sup>, Yusuke NAKAUCHI<sup>43</sup>, Masanao ABE<sup>43,44</sup>, Satoru NAKAZAWA<sup>43</sup>, Tatsuaki OKADA<sup>43,44</sup>, Takanao SAIKI<sup>43</sup>, Satoshi TANAKA<sup>43,44</sup>, Fuyuto TERUI<sup>45</sup>, Makoto YOSHIKAWA<sup>43,44</sup>, Akiko MIYAZAKI<sup>43</sup>, Aiko NAKATO<sup>43</sup>, Masahiro NISHIMURA<sup>43</sup>, Tomohiro USUI<sup>43</sup>, Toru YADA<sup>43</sup>, Hisayoshi YURIMOTO<sup>46</sup>, Kazuhide NAGASHIMA<sup>13</sup>, Noriyuki KAWASAKI<sup>46</sup>, Naoya SAKAMOTOA<sup>47</sup>, Ryuji OKAZAKI<sup>48</sup>, Hikaru YABUTA<sup>11</sup>, Hiroshi NARAOKA<sup>48</sup>, Kanako SAKAMOTO<sup>43</sup>, Shogo TACHIBANA<sup>49</sup>, Sei-Ichiro WATANABE<sup>50</sup>, and Yuichi TSUDA<sup>43</sup>

<footnote n="1"><sup>1</sup>Université de Lille, CNRS, INRAE, Centrale Lille, UMR 8207-UMET-Unité Matériaux et Transformations, Lille, France</footnote>

<footnote n="2"><sup>2</sup>Université de Lille, CNRS, INRAE, Centrale Lille, Université Artois, FR 2638-IMEC-Institut Michel-Eugène Chevreul, Lille, France</footnote>

<footnote n="3"><sup>3</sup>Department of Physics and Astronomy, University of Kent, Canterbury, Kent, UK</footnote>

<footnote n="4"><sup>4</sup>Université de Lille, CNRS, Centrale Lille, Junia, Univ. Polytechnique Hauts-de-France, UMR 8520 – IEMN – Institut d'Electronique de Microélectronique et de Nanotechnologie, Lille, France</footnote>

<footnote n="5"><sup>5</sup>Division of Earth and Planetary Sciences, Kyoto University, Kyoto, Japan</footnote>

<footnote n="6"><sup>6</sup>The Hakubi Center for Advanced Research, Kyoto University, Kyoto, Japan</footnote>

<footnote n="7"><sup>7</sup>Institute for Chemical Research, Kyoto University, Kyoto, Japan</footnote>

<footnote n="8"><sup>8</sup>Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="9"><sup>9</sup>Pan-Omics Data-Driven Research Innovation Center, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="10"><sup>10</sup>Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="11"><sup>11</sup>The Ultramicroscopy Research Center, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="12"><sup>12</sup>Department of Geosciences, Osaka Metropolitan University, Osaka, Japan</footnote>

<footnote n="13"><sup>13</sup>Department of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, Japan</footnote>

<footnote n="14"><sup>14</sup>Kochi Institute for Core Sample Research, X-Star, JAMSTEC, Nankoku, Kochi, Japan</footnote>

<footnote n="15"><sup>15</sup>Hawai'i Institute of Geophysics and Planetology, The University of Hawai'i at Mānoa, Honolulu, Hawaii, USA</footnote>

<footnote n="16"><sup>16</sup>Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany</footnote>

<footnote n="17"><sup>17</sup>European Space Resources Innovation Centre, Luxembourg Institute of Science and Technology, Belvaux, Luxembourg</footnote>

<footnote n="18"><sup>18</sup>Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), CNRS, Université Grenoble Alpes, Grenoble, France</footnote>

<footnote n="19"><sup>19</sup>NASA Langley Research Center, Hampton, Virginia, USA</footnote>

<footnote n="20"><sup>20</sup>Materials Characterization Lab, The Pennsylvania State University Materials Research Institute, University Park, Pennsylvania, USA</footnote>

<footnote n="21"><sup>21</sup>Department of Planetary Sciences, Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona, USA</footnote>

<footnote n="22"><sup>22</sup>Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA</footnote>
<sup>23</sup>Buseck Center for Meteorite Studies, Arizona State University, Tempe, Arizona, USA

<sup>24</sup>Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA

<sup>25</sup>U.S. Naval Research Laboratory, Washington, DC, USA

<sup>26</sup>Space Park Leicester, University of Leicester, Leicester, UK

<sup>27</sup>School of Geology, Geography and the Environment, The University of Leicester, Leicester, UK

<sup>28</sup>School of Geographical and Earth Sciences, The University of Glasgow, Glasgow, UK

<sup>29</sup>Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, New South Wales, Australia

<sup>30</sup>Department of Materials, The University of Oxford, Oxford, UK

<sup>31</sup>School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia

<sup>32</sup>ARES, NASA Johnson Space Center, Houston, Texas, USA

<sup>33</sup>Jacobs Engineering, Dallas, Texas, USA

<sup>34</sup>Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Shiga, Japan

<sup>35</sup>CAS Key Laboratory of Mineralogy and Metallogeny, Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou, China

<sup>36</sup>CAS Center for Excellence in Deep Earth Science, Guangzhou, China

<sup>37</sup>Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo, Japan

<sup>38</sup>University of Chinese Academy of Sciences, Beijing, China

<sup>39</sup>Department of Mathematics, Physics, and Earth Science, Ehime University, Matsuyama, Ehime, Japan

<sup>40</sup>Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan

<sup>41</sup>Faculty of Engineering, Kindai University, Hiroshima Campus, Hiroshima, Japan

<sup>42</sup>Department of Earth Science, Tohoku University, Sendai, Japan

<sup>43</sup>Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan

<sup>44</sup>The Graduate University for Advanced Studies, SOKENDAI, Hayama, Japan

<sup>45</sup>Department of Mechanical Engineering, Kanagawa Institute of Technology, Atsugi, Japan

<sup>46</sup>Department of Earth and Planetary Sciences, Hokkaido University, Sapporo, Japan

<sup>47</sup>Creative Research Institution Sousei, Hokkaido University, Sapporo, Japan

<sup>48</sup>Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan

<sup>49</sup>UTokyo Organization for Planetary and Space Science, The University of Tokyo, Tokyo, Japan

<sup>50</sup>Department of Earth and Environmental Sciences, Nagoya University, Nagoya, Japan

*Correspondence

Hugues Leroux, CNRS, INRAE, Centrale Lille, UMR 8207-UMET-Unité Matériaux et Transformations, Université de Lille, Lille F-59000, France.

Email: hugues.leroux@univ-lille.fr

(Received 20 April 2023; revision accepted 19 October 2023)

**Abstract**—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.

INTRODUCTION

Samples of the near-Earth carbonaceous asteroid (162173) Ryugu were delivered to Earth in December 2020 by the Japan Aerospace Exploration Agency (JAXA)'s Hayabusa2 mission. These samples offer us an exceptional opportunity to make a link between carbonaceous chondrites and C-type asteroids. The initial analysis started in June 2021 after 6 months of curation activities. The first studies of the samples, carried out with advanced electron microscopy techniques, showed that the dominant lithology was mainly composed of phyllosilicates (intimate mixture of serpentine and saponite), comparable to that of the CI (Ivuna-like) group of chondrites (Ito et al., 2022; Nakamura, Kobayashi, et al., 2022; Nakamura, Matsumoto, et al., 2022; Nakato et al., 2022; Noguchi et al., 2022, 2023; Viennet et al., 2023; Yada et al., 2021; Yamaguchi et al., 2023; Yokoyama et al., 2022). The Ryugu samples have a low bulk density (~1300 kg m⁻³) compared to other carbonaceous chondrites, were found to be very friable and would have had little chance of surviving atmospheric entry to Earth (Greenwood et al., 2023; Yada et al., 2021). Compared to CI chondrites, Ryugu samples are uncontaminated and provide the best available match to the bulk solar system composition (Ito et al., 2022; Yokoyama et al., 2022). Unlike the Orgueil CI chondrite, the Ryugu samples do not contain sulfates and ferrihydrite components. These preliminary studies also argued that Ryugu's material experienced aqueous alteration but has a low interlayer molecular water content in saponite than CI chondrites (Nakamura, Matsumoto, et al., 2022; Yokoyama et al., 2022). This corroborates the reflectance spectra obtained remotely which showed a weak 2.7 µm band assigned to hydroxyl (–OH) and no clear absorption of molecular water (Kitazato et al., 2019, 2021). Based on these remote data, it was suggested that the surface of Ryugu has been space weathered or, to some degree partially dehydrated (Kitazato et al., 2019, 2021; Sugita et al., 2019). The first results obtained on the samples by advanced characterization techniques did not confirm the occurrence of a significant thermal event. On the contrary, the mineralogy is compatible with formation during a low temperature (<100°C) hydrothermal alteration. No evidence of a higher temperature episode that would have caused partial dehydration was found (Nakamura, Matsumoto, et al., 2022; Yabuta et al., 2023). Space

weathering, on the other hand, has also been identified including evidence of micrometeoroid-induced impact melting and solar wind irradiation. The alteration of surface materials by space weathering could be responsible for the partial dehydroxylation of the surface layer but only the upper-most micrometers would have been affected (Noguchi et al., 2022, 2023).

In this article, we present results obtained by analytical transmission electron microscopy (TEM) and scanning transmission x-ray microscopy on the fine-grained phyllosilicate matrix of both Orgueil and Ryugu. These fine-grained matrices are the most abundant components in each sample. We leave aside the coarser-grained minerals such as carbonates (breunnerite and dolomite), large iron sulfides (pyrrhotite and pentlandite), magnetite, and hydroxyapatite, which have been described in other studies (Dobrică et al., 2023; Nakamura, Matsumoto, et al., 2022). The Orgueil mineralogy was initially studied between the 1960s and the 1980s (Boström & Fredriksson, 1966; Dufresne & Anders, 1962; Gounelle & Zolensky, 2014; Tomeoka & Buseck, 1988), and now the Ryugu samples have offered us an opportunity to revisit similar materials with modern instruments and from a standpoint of a sample that is more pristine and contextualized in the solar system. Our objective here is to compare these two pieces of CI material at the submicrometer scale. To achieve this, we focused on the chemistry and texture of serpentine and saponite, as well as on the nanosulfides and oxides that are intimately embedded within the matrix. We highlight the differences and similarities between them in order to consolidate our knowledge of their alteration conditions and to examine how their differences can be indicative of genetic links. In particular, it has been shown that Orgueil had been modified on Earth and that development of sulfates was the result of this alteration (Gounelle & Zolensky, 2001). Ryugu samples being well preserved, we aim to investigate the extent of these late terrestrial processes on the fine-grained phases. We also search for indications of variable hydrothermal conditions in Ryugu and Orgueil parent body(ies).

MATERIALS AND METHODS

Samples

The samples of Ryugu studied were in the framework of the Initial Analysis Min-Pet Fine “Sand” sub-team headed by T. Noguchi (Kyoto University). Portions of
them are from the first touchdown site (A0104-00600301, A0104-00600302, A104-00600303, A0104-01300102, A0104-01300103, A0104-01700604), while others are from the second touchdown site (C0105-034016b01). During our study of the phyllosilicate matrix, we did not find a significant difference in microstructure between the grains from the two collection sites. The grains, typically between 50 and 100 µm in size, were bonded to a gold plate using a small drop of epoxy. They were then observed by scanning electron microscopy at Kyoto University and Lille University to select grains of interest for this study. More details about the preliminary handling and observation are given in Noguchi et al. (2022). Regarding the samples of Orgueil, a polished section was lent to us by the Museum of Natural History of Paris (MNHN).

## FIB Preparation

For Ryugu samples, focused ion beam (FIB) sections were prepared both at Kyoto University using a Thermo Fisher Helios FIB-SEM (A0104-00600301, A0104-00600302, A104-00600303, A0104-01300102, A0104-01300103, A0104-01700604) and at the IEMN (Lille University) using a FEI Strata dual beam 235 (C0105-034016b01). For the Orgueil samples, the sections were prepared at the University of Lille. These sections are conventional, with a surface varying from 50 to 150 µm² and thickness between 100 and 200 nm, depending on the sample.

## Scanning TEM

Transmission electron microscopy observations have been performed at the Electron Microscopy Facility at the University of Lille. The data have been acquired on a Thermo Fischer high base TITAN Themis 80-300 kV in scanning TEM mode (STEM). This microscope is fitted with a high-brightness Schottky field effect gun, a Wien filter type monochromator, and a probe aberration corrector, allowing resolution down to 70 pm and flexibility in choosing the electron dose. The microscope is also equipped with a super-X windowless 4 quadrant silicon drift detection system for energy dispersive x-ray spectroscopy (EDXS) and several annual dark field detectors for imaging. High angle annular dark field (HAADF) imaging was performed at 300 kV, with a setup of the illumination system so that the probe semi-convergence was about 18 mrad, the probe size was about 500 pm, and the probe current was about 50 pA. The collection semi-angles for the HAADF images were between 50 and 200 mrad and the dwell time was 10 µs per pixel. For the STEM-EDXS mapping, the probe semi-convergence angle was either 5 or 10 mrad. The spot size was adjusted to 2 nm and the probe current was increased

between 300 and 500 pA to ensure a high count rate. In order to monitor and reduce beam damage, STEM-EDXS spectrum image acquisitions were performed in multi-frame mode with a dwell time of 5 µs per pixel and continuous scanning over a total integration time varying between 20 min to several hours. Quantification of the STEM-EDXS data was conducted by applying the absorption correction and using k-factors for major elements (O, Fe, Mg, Al, S, Ca, and K) experimentally determined from mineral and glass standards. k-factors for Na and Ni have been deduced from calculations. The EDXS data were processed using the Bruker Esprit software (version 1.9) and the open-source software python package HyperSpy (de la Peña et al., 2017).

## Scanning Transmission X-Ray Microscopy at the Fe_L2,3 Edge

XANES data were collected at the SOLEIL synchrotron using the scanning transmission x-ray microscope (STXM) of the HERMES beamline (Belkhou et al., 2015). Energy calibration was done using an internal hematite standard. XANES data were extracted from image stacks collected at energy increments of 0.15 eV over the critical Iron L₂,₃ edge energy range (703–715 eV) with a dwell time of 1 ms per pixel to prevent irradiation damage (Wang et al., 2009). Alignment of images of stacks, extraction of XANES spectra, and valency quantification were done using the Python HyperSpy library (de la Peña et al., 2017, https://hyperspy.org), that allows pixel by pixel analysis. Quantification of the iron valency was performed based on the protocol and calibration established by Le Guillou et al. (2015) following the work of Bourdelle et al. (2013). At each pixel of the hyperspectral data set, a background (680–700 eV) and a double arctangent function are fitted and subtracted to take into account the iron content. The spectra are then integrated over ranges specific to the Fe²⁺ and Fe³⁺ absorption energy. These integrated values are then derived into valency values using the standard-based calibration. As a result, quantified valency maps are produced, and averaged spectra can be displayed.

## RESULTS

### Main Characteristics of the Phyllosilicates

The phyllosilicate matrices of Ryugu and Orgueil display comparable characteristics but also significant differences. Both consist of a fibrous texture with small, elongated platy crystallites (Figure 1a,b). The phyllosilicates are often arranged in randomly oriented bundles, within which the stacks of layers are either parallel to each other or display a radial texture. In the Ryugu matrix, the finest
<figure><img src="image_3.png" /></figure>

FIGURE 1. Low magnification overview of the main characteristics of the phyllosilicate matrices of Ryugu and Orgueil. Both matrices display areas of coarser texture in which Fe-sulfides (Ryugu) or ferrihydrite (Orgueil) are present in lower abundance or absent. The interface between the fine and coarse regions is highlighted with a dashed white line. (a) STEM-HAADF image of a low magnification view of Ryugu phyllosilicates containing numerous Fe-sulfides inclusions, which appear bright. Sample A104-00600302 (b) STEM-HAADF image of the Orgueil phyllosilicates in which sulfur-rich ferrihydrite is finely dispersed. (c) Composition field of a representative area of Ryugu in the ternary diagram (Si + Al)-Mg-Fe with large ROI (125 × 125 nm²) giving the local composition of phyllosilicate areas including the nanosulfides embedded. The total surface scanned is ≈ 10 µm². Sample A104-01300102 (d) Same ternary diagram for Orgueil with the same methodology. Here the composition field includes the phyllosilicate and the intermixed ferrihydrite. (Color figure can be viewed at wileyonlinelibrary.com)

grained phyllosilicates comprise crystallites ~100 nm in length and ~20 nm in width and contains numerous Fe,Ni-sulfide nano-inclusions. This contrasts with Orgueil, where they are intimately mixed with S-Ni-bearing ferrihydrite and in which small pockets of calcium sulfate are occasionally present. In the coarse-grained phyllosilicate regions (several hundred nm in length and stacking layers of several tens of nm), iron sulfides are absent and ferrihydrite is less abundant, respectively, for Ryugu and Orgueil. Carbonaceous globules are frequent in both matrices, typically a few hundred of nanometers in size and sometimes found exhibiting a central hole. We also found diffuse carbon intimately mixed with the phyllosilicates, in agreement with results of Yabuta et al. (2023) for Ryugu and of Le Guillou et al. (2014) for Orgueil. EDXS techniques tend to show that the phyllosilicates in Ryugu and Orgueil are dominated by a finely intermingled assemblage of serpentine and saponite in various

proportions (Figure 1c,d). The chemical trend toward the Fe apex is due to the presence of nanometer Fe,Ni sulfides in Ryugu and nanometer ferrihydrite in Orgueil. Both matrices seem to be porous.

## Sulfide Nano-Inclusions in Ryugu

In Ryugu, areas with the finest phyllosilicate texture encompass numerous nano-inclusions of Fe,Ni sulfides (Figure 2). Magnetite nano-grains may occasionally be present but are rare. Most of these inclusions are heterogeneously distributed and clustered in various sizes. The size distribution of the sulfide grains is reasonably homogeneous within a cluster, however, between clusters, there are large differences in size. The sulfide grains are often euhedral. Unlike the large (>1 µm) sulfide grains as observed on polished section by SEM, strongly dominated by pyrrhotite grains which often have an
<figure><img src="image_4.png" /><figcaption>FIGURE 2. STEM-HAADF images of the Ryugu matrix. (a) Low magnification view showing several clusters. Note that the grain size of the sulfides is quite variable between clusters but rather homogeneous within a given cluster. Sample A104-01700604. (b) Higher magnification view of a sulfide cluster. Sample A104-00600301.</figcaption></figure>

elongated shape (e.g., Yokoyama et al., 2022), the nanosulfides distributed in the phyllosilicate matrix contain a significant pentlandite fraction. From hyperspectral EDXS chemical maps, on which the two phases are clearly distinguishable, we estimated that pentlandite represents about 40% of the sulfides. Sphalerite (Zn,Fe)S is found in minor abundance. Figure 3a shows the composition distribution of the sulfide grains in a Fe-Ni-S diagram. As mentioned earlier, the coarse phyllosilicate regions are found to be almost free of sulfides.

## Ferrihydrite in Orgueil

In Orgueil, the presence of ferrihydrite nano-grains was first revealed by the pioneering work of Tomeoka and Buseck (1988). Ferrihydrite grains are found mainly as interstitial to the fine-grained phyllosilicate crystallites (Figure 4). Some ferrihydrite is also found as layers parallel to the phyllosilicate layers. The amount of ferrihydrite is significantly lower in the coarse regions (dominated by serpentine), as already observed by Tomeoka and Buseck (1988). Ferrihydrite grains have irregular shapes and are S and Ni-rich (Figure 3b). As first evidenced by Kerridge (1977) and Tomeoka and Buseck (1988) thanks to electron microprobe analysis, there is a positive correlation between the concentration of Fe, Ni, and S, showing that these three elements are carried by the ferrihydrite (Figure 5). The Fe-S regression line leads to the approximate equation $Fe = 3S + 1.5$ (atom%). This equation is similar to the one established by Tomeoka and Buseck (1988), who used electron microprobe analysis data. Positive correlations are also found for Ni and S and Ni and Fe. Fe, Ni, and S are anti-correlated with Mg and Si (the latter elements are assumed to be present in the

phyllosilicates). Tomeoka and Buseck (1988) noted that ferrihydrite was poorly crystalline and suggested that the sulfur could be adsorbed at the edges of the domains. It could also be trapped in nanoporosity as ferrihydrite is known to be a highly defective nanomaterial with large surface area (Hiemstra & Van Riemsdijk, 2009). Nickel is also present with a Ni/Fe ratio close to 0.1.

## Composition Fields of the Phyllosilicate by EDXS

Several chemical STEM-EDXS maps were recorded in both matrices, with the same operating conditions. On each map the analyzed surfaces were of the order of µm². To process the data, we selected regions of interest (ROI) from 15 × 15 nm² to 50 × 50 nm² to sample the phyllosilicate composition fields. Going below this scale is not necessarily relevant due to the thickness of the samples (≈ 100 nm). Because of the characteristic size of the microstructure, which is much smaller than the thickness of the studied samples, it is extremely hard to extract the compositions of the endmember phases. Our reported composition field thus reflects mixing in different proportions. The composition fields are visualized here by ternary diagrams (Si + Al)-Mg-Fe. In contrast to the diagram in Figure 1c which shows a composition field on a broad scale, thus including nanosulfides, the composition fields of phyllosilicates shown in this section were measured by using smaller ROIs excluding nanosulfides. This was not possible for the Orgueil sample (see below).

## Ryugu Phyllosilicate Composition Field

The Ryugu matrix is dominated by Fe-poor phyllosilicates. The Fe/(Mg + Fe) atomic ratio is in
<figure><img src="image_5.png" /></figure>

FIGURE 3. Fe-Ni-S ternary diagram showing (a) the composition of the Fe-Ni sulfides enclosed in the Ryugu phyllosilicates. The composition of the vast majority of the sulfides is distributed between the FeS and (FeNi)S poles. Sample C0105-034016b01. (b) Composition field of ferrihydrite found within phyllosilicates in Orgueil. Because ferrihydrite is intimately mixed with phyllosilicates (which natively contain some Fe), its composition has been recalculated by subtracting the silicate contribution obtained through the Mg, Si versus Fe, S, and Ni plots. (Color figure can be viewed at wileyonlinelibrary.com)

<figure><img src="image_6.png" /></figure>

FIGURE 4. HAADF STEM images of the Orgueil matrix. (a) Ferrihydrite (brighter areas) within the fine-grained phyllosilicate matrix. (b) Fe-rich layers interstratified in the phyllosilicate.

the range of 0.08–0.15, depending on the samples studied. For a given sample, this ratio is remarkably constant (typically ±0.01). On the other hand, the variation in the Mg/Si ratio is more significant. Table 1 shows representative compositions obtained on the fine and coarse regions. In a (Si + Al)-Mg-Fe ternary diagram, the compositions of Ryugu phyllosilicates are distributed along a line passing through the Si apex, with the majority of points contained between the saponite and serpentine solid solution lines (Figure 6). This shows that the phyllosilicates are a mixture between serpentine and saponite.

In general, the coarse-grained regions have a higher (Mg + Fe)/Si ratio than the fine-grained regions. This ratio

is typically equal to ≈ 1.15 for coarse regions and ≈ 1.0 for fine regions. This (Mg + Fe)/Si ratio gives an estimate of the proportion between serpentine and saponite. This ratio is significantly different from one region to another and strongly dependent on the coarse or fine texture. Based on an estimate of endmember composition deduced from trends highlighted on ternary diagrams, for the coarse texture we estimate that the proportion of saponite–serpentine is about 50% each. For the finer texture, the barycenter suggests two third of the sample being saponite. From the point of view of charge compensating elements in clays, Na is dominant, whereas Ca and K are barely detectable. As expected, Na is also more abundant in region with a low Mg/Si ratio (e.g., with a higher fraction of saponite).
<figure><img src="image_7.png" /></figure>

FIGURE 5. Fe and S (atom%) showing a positive correlation in the phyllosilicate matrix of Orgueil. The amount of ferrihydrite is significantly lower in the coarse regions. This correlation allows estimating the concentration of Fe in the phyllosilicates (based on the extrapolation at S = 0), as well as that of the ferrihydrite composition itself. These data were extracted from a representative STEM-EDXS chemical map including a fine and a coarse region of phyllosilicates. Each point represents a ROI of 100 × 100 nm². (Color figure can be viewed at wileyonlinelibrary.com)

Some pixels plot above the saponite solid solution line and extend toward the Si apex (Figures 6 and 7). These silica-enriched regions correlate with the darkest regions of the HAADF images (low Z regions). This shows that silica is present in contact with porosity but also within carbonaceous material. Most often, these Si-rich areas are very small, and it was impossible to analyze them without including some signal from the adjacent phyllosilicate.

## Orgueil Phyllosilicate Composition Field

Compared to Ryugu, the composition fields of Orgueil's matrix are richer in iron and contain an appreciable amount of S and Ni due to the presence of finely intermixed ferrihydrite. We also observe a significant difference between the coarse and fine regions, with the coarse-grained regions being poorer in Fe than the fine-grained regions which contain ferrihydrite (Figures 8 and 9). The correlation Fe-S shown in Figure 5 allows us to estimate the amount of Fe carried by ferrihydrite and that carried by phyllosilicates. In fine-grained regions, about 25 atom% is carried by the ferrihydrite whereas only 5 atom% in the coarse-grained areas. The Fe-S correlation also permits an estimation of the compositional field of phyllosilicates themselves, without the contribution of ferrihydrite (Figure 9b). We found that the phyllosilicates in Orgueil are Fe-poor, and that their composition is comparable to that of Ryugu's phyllosilicates. The coarse-grained regions of Orgueil have Fe/(Fe + Mg) = 0.09 and

(Mg + Fe)/Si = 1.16. Without the ferrihydrite, the Fe/(Fe + Mg) of the fine-grained region was found to be equal to that of the coarse-grained regions. Similar to our Ryugu samples, the cation/Si ratio of Orgueil's fine-grained region is found to be significantly lower, with a value of 0.98. Remarkably, these values are very close to those of Ryugu's phyllosilicates, both with regard to the Fe content and the relative proportion of serpentine and saponite.

## Iron Valency of Phyllosilicates

We measured the valency of iron in phyllosilicates, which can be indicative of their conditions of formation. We quantified the Fe³⁺/ΣFe ratio of phyllosilicates from four different FIB sections of Ryugu and Orgueil each. We first obtained a proxy of the iron content at each pixel by integrating the optical density up to 715 eV. This can be plotted versus the Fe³⁺/ΣFe (Figure 10), allowing the identification of different components. For each data set, we then isolated the most different spectra (sulfides, oxides, and phyllosilicates with higher Fe³⁺ content and/or lower Fe³⁺ content) and ran a multiple linear least square fit to obtain the proportion of each component at each pixel. In Ryugu, phyllosilicates are mixed with nanosulfides which carry Fe²⁺. The latter are large enough and sufficiently isolated to be masked out directly based on the results of the linear least square fitting. After masking all pixels containing more than 1% sulfides, we obtained Fe³⁺/ΣFe maps (Figure 10). We find that the values of Fe³⁺/ΣFe are heterogeneously distributed between 35% and 55%, with an average value below 50% for three of the four FIB sections from Ryugu. One FIB section, however, has a higher value of 70% Fe³⁺/ΣFe.

In Orgueil, ferrihydrite and phyllosilicates cannot be spatially resolved and a mixing line is observed in the Fe content versus Fe³⁺/ΣFe diagram (Figure 11). To determine the Fe³⁺/ΣFe ratio of the phyllosilicates, we calculated a regression line for each of the four FIB sections. Each of them revealed a similar value of about 70%. The spread is lower than in Orgueil showing a more homogeneous valency.

## DISCUSSION

### The Ryugu and Orgueil Phyllosilicate Share Many Similarities

Our comparative study revealed a strong similarity between the Ryugu and Orgueil phyllosilicates. Indeed, both matrices show fine and coarse textures with comparable morphologies. It should, however, be mentioned that this fine and coarse duality is more marked for Orgueil than for Ryugu. In both cases, the finer fraction is dominated by saponite while the coarser
<table><caption>TABLE 1. STEM-EDXS representative compositions of the main phases in phyllosilicate matrices of Ryugu and Orgueil.</caption><thead><tr><th>Sample</th><th>O</th><th>Na</th><th>Mg</th><th>Al</th><th>Si</th><th>S</th><th>K</th><th>Ca</th><th>Cr</th><th>Mn</th><th>Fe</th><th>Ni</th><th>Fe/(Mg + Fe)</th><th>(Mg + Fe)/Si</th></tr></thead><tbody><tr><td rowspan="3">Ryugu CG</td><td>63.1</td><td>0.91</td><td>16.7</td><td>1.11</td><td>16.1</td><td>bd</td><td>0.03</td><td>bd</td><td>0.08</td><td>0.02</td><td>1.92</td><td>bd</td><td>0.10</td><td>1.16</td></tr><tr><td>62.6</td><td>0.97</td><td>17.4</td><td>1.15</td><td>16.0</td><td>0.02</td><td>bd</td><td>bd</td><td>0.05</td><td>0.02</td><td>1.63</td><td>bd</td><td>0.08</td><td>1.19</td></tr><tr><td>62.7</td><td>0.92</td><td>17.0</td><td>1.12</td><td>16.3</td><td>bd</td><td>0.05</td><td>bd</td><td>0.14</td><td>0.02</td><td>1.74</td><td>bd</td><td>0.09</td><td>1.15</td></tr><tr><td rowspan="3">Ryugu FG</td><td>62.2</td><td>1.10</td><td>15.9</td><td>1.33</td><td>17.3</td><td>0.02</td><td>0.04</td><td>bd</td><td>0.12</td><td>0.02</td><td>2.01</td><td>bd</td><td>0.11</td><td>1.03</td></tr><tr><td>61.7</td><td>0.79</td><td>16.1</td><td>1.34</td><td>18.1</td><td>0.04</td><td>0.04</td><td>bd</td><td>0.08</td><td>0.04</td><td>1.66</td><td>bd</td><td>0.09</td><td>0.98</td></tr><tr><td>62.9</td><td>0.99</td><td>15.2</td><td>1.26</td><td>17.9</td><td>0.06</td><td>0.07</td><td>bd</td><td>0.40</td><td>bd</td><td>1.52</td><td>bd</td><td>0.09</td><td>0.94</td></tr><tr><td rowspan="3">Ryugu SP</td><td>63.5</td><td>bd</td><td>4.90</td><td>0.43</td><td>29.5</td><td>bd</td><td>0.11</td><td>0.08</td><td>0.21</td><td>bd</td><td>1.14</td><td>bd</td><td>—</td><td>—</td></tr><tr><td>60.9</td><td>0.30</td><td>5.90</td><td>0.54</td><td>31.0</td><td>0.16</td><td>0.09</td><td>bd</td><td>Bd</td><td>0.06</td><td>1.03</td><td>bd</td><td>—</td><td>—</td></tr><tr><td>62.2</td><td>bd</td><td>8.15</td><td>0.10</td><td>29.4</td><td>0.20</td><td>bd</td><td>bd</td><td>0.07</td><td>bd</td><td>0.78</td><td>bd</td><td>—</td><td>—</td></tr><tr><td rowspan="3">Orgueil CG</td><td>62.3</td><td>0.21</td><td>17.3</td><td>1.23</td><td>14.8</td><td>0.70</td><td>bd</td><td>0.02</td><td>0.10</td><td>0.03</td><td>3.01</td><td>0.24</td><td>0.15</td><td>1.37</td></tr><tr><td>60.6</td><td>0.37</td><td>18.1</td><td>1.12</td><td>15.8</td><td>0.54</td><td>bd</td><td>bd</td><td>0.17</td><td>0.03</td><td>3.09</td><td>0.22</td><td>0.15</td><td>1.34</td></tr><tr><td>62.1</td><td>0.22</td><td>17.0</td><td>0.92</td><td>15.5</td><td>0.66</td><td>0.04</td><td>bd</td><td>0.10</td><td>0.02</td><td>3.57</td><td>0.19</td><td>0.17</td><td>1.32</td></tr><tr><td rowspan="3">Orgueil FG</td><td>58.8</td><td>0.23</td><td>12.9</td><td>0.64</td><td>13.6</td><td>2.35</td><td>0.04</td><td>bd</td><td>0.018</td><td>0.12</td><td>10.3</td><td>0.91</td><td>—</td><td>—</td></tr><tr><td>61.0</td><td>0.55</td><td>11.9</td><td>0.55</td><td>11.8</td><td>4.15</td><td>0.10</td><td>0.04</td><td>0.13</td><td>0.12</td><td>8.90</td><td>0.65</td><td>—</td><td>—</td></tr><tr><td>59.7</td><td>0.42</td><td>11.5</td><td>0.78</td><td>12.3</td><td>2.83</td><td>bd</td><td>bd</td><td>0.15</td><td>0.09</td><td>11.3</td><td>0.86</td><td>—</td><td>—</td></tr><tr><td rowspan="3">Orgueil SP</td><td>65.2</td><td>bd</td><td>1.72</td><td>0.55</td><td>30.0</td><td>0.49</td><td>bd</td><td>bd</td><td>0.13</td><td>0.08</td><td>1.56</td><td>0.14</td><td>—</td><td>—</td></tr><tr><td>64.2</td><td>0.29</td><td>1.11</td><td>0.06</td><td>30.6</td><td>1.52</td><td>bd</td><td>0.17</td><td>0.06</td><td>0.05</td><td>1.57</td><td>0.08</td><td>—</td><td>—</td></tr><tr><td>62.3</td><td>0.24</td><td>2.42</td><td>0.82</td><td>27.3</td><td>1.95</td><td>bd</td><td>bd</td><td>0.03</td><td>0.05</td><td>4.69</td><td>0.23</td><td>—</td><td>—</td></tr></tbody></table>

Note: The fine-grained areas of Orgueil, the compositions include the phyllosilicate and ferrihydrite. Data are given in atom% and are normalized at 100%.

Abbreviations: bd, below detection; CG, coarse-grained; FG, fine-grained; SP, silica-rich pockets.

fraction is dominated by serpentine. The composition of the phyllosilicates is comparable, with an atomic ratio Fe/ (Mg + Fe) ranging from ≈ 0.1 to 0.15. In both cases, the intergranular spaces are often enriched in silica (see next section). Given the similarities, our study confirms that Ryugu material resembles the CI (Ivuna type) chondrites, as not only revealed during the initial analysis period of the Ryugu grains, but also predicted from the remote sensing data from 2018 to 2020 (Ito et al., 2022; Nakamura, Kobayashi, et al., 2022; Nakamura, Matsumoto, et al., 2022; Noguchi et al., 2022; Yada et al., 2021; Yamaguchi et al., 2023; Yokoyama et al., 2022; Yabuta et al., 2023). In addition to mineralogical similarities, Ryugu also shares a common oxygen isotopic signature with CI chondrites (Greenwood et al., 2023; Ito et al., 2022; Yokoyama et al., 2022). Mineralogy (including multi-micrometric phases such as carbonates and magnetite that have not been studied here) and isotopic compositions allowed a low temperature (<100°C) alteration history to be deduced (McCain et al., 2023; Nakamura, Matsumoto, et al., 2022; Ito et al., 2022; Yokoyama et al., 2022; Yamaguchi et al. 2023). While it is clear that the Ruygu samples contain less water than the CI chondrites (e.g., Nakamura, Matsumoto, et al., 2022; Yokoyama et al., 2022), we did not detect any evidence of reheating in the Ryugu microstructures that could explain the weak hydroxyl (OH) absorption feature at 2.7 µm from remote sensing instruments (e.g., Kitazato et al., 2019, 2021; Sugita et al.,

2019). The weak spectral band may originate from space weathering (Noguchi et al., 2022, 2023), but the presence of numerous nanosulfides in phyllosilicates should also be considered, both on the albedo and the OH band depth.

## Origin of Saponite and Serpentine-Rich Submicrometer Regions

Early studies have shown that the Ryugu asteroid has undergone several geological stages, including aqueous alteration, disruption and re-accretion, and brecciation (e.g., Ito et al., 2022; Nakamura, Matsumoto, et al., 2022; Yamaguchi et al., 2023; Yokoyama et al., 2022). In the context of aqueous alteration, interpretation of chemical and isotopic signatures can be difficult due to temporal variations in temperature, water/rock ratio, or pH. Under these complex conditions, it is challenging to decipher a clear path of mineralogical evolution. The microstructures show evidence of obvious aqueous alteration, most of the solid in the major lithology having been transformed. Fluid circulation is highlighted by the presence of micron-sized carbonates, magnetite and pyrrhotite. Their formation requires dissolution of primary phases, transport, and subsequent precipitation. However, it concerns the most soluble elements, such as Fe²⁺ and Ca²⁺. In contrast, there is little evidence that fluid circulation massively affected the silicates. The only evidence is the presence of a few Na-rich phyllosilicates that indicate that the fluid was locally
<figure><img src="image_8.png" /><figcaption>FIGURE 6. (Si + Al)-Mg-Fe ternary diagram of a representative phyllosilicate matrix of Ryugu. The compositions are distributed along a line pointing toward the Si apex. Note that some compositions are SiO₂-rich. For each point on the ternary diagram, the selected surface area was 50 × 50 nm², probing a total surface of about 500 × 600 nm². Sample A0104-01300103. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

enriched in Na (Nakamura, Matsumoto, et al., 2022; Yamaguchi et al., 2023).

Our study highlights a duality of the microstructure of the phyllosilicate. The Ryugu matrix is composed of regions dominated by saponite embedding numerous Fe, Ni nanosulfides, adjacent to coarser regions richer in serpentine than saponite, where nanosulfides are rare or totally absent. In the Orgueil matrix, comparable coarse-grained areas are found, but in the fine-grained regions, the saponite is intimately mixed with Ni- and S-bearing ferrihydrite. The interface between the two types of regions is often sharp. We do not detect any evidence of an alteration front that would suggest a relationship between them. As such, this fails to clarify if one originates from the other. For Orgueil, Tomeoka and Buseck (1988) interpreted the finer textures as the product of the destabilization of previously formed coarse textures. During this stage, a first generation of phyllosilicates (coarse texture) would have been progressively altered under oxidizing conditions to form finer phyllosilicates. Within this fine-grained saponite, iron-rich phases originating from magnetite and iron sulfide dissolution would have precipitated between the phyllosilicate laths. This scenario poses a difficulty of mass balance because the coarse regions are rather Mg-rich serpentine dominated, whereas the fine regions are rather saponite type. The chemical difference is significant, and there are no other phases associated with saponite to carry the excess Mg in the case of a serpentine-type reaction that gives saponite. Another scenario to explain the intergrowth of ferrihydrite and Mg-rich saponite involves the transformation of an iron-rich saponite formed during an

earlier stage of alteration (Brearley, 1997; Brearley & Prinz, 1992; Zolensky et al., 1993). But in this case, the high concentration of Ni and S associated with the Orgueil ferrihydrite is difficult to explain.

In the following, we propose another scenario to explain the fine-coarse duality. This scenario takes into consideration the recent results of Nakamura, Matsumoto, et al. (2022), who observed weakly altered fragments in the Ryugu samples. They observed fragments with different lithologies that seem to have experienced a much lower degree of alteration. These samples have a porous texture, contain submicrometer Mg-rich olivine and Mg-rich pyroxene, and rounded amorphous to poorly crystalline domains that are close in terms of texture and composition to the glass with embedded metal and sulfide (GEMS) found in chondritic-porous interplanetary dust particles (IDPs) and carbonaceous micrometeorites (e.g., Bradley, 1994; Dobrică et al., 2012; Keller & Messenger, 2011; Noguchi et al., 2017). The microstructures of the weakly altered Ryugu fragments are also close to those observed in the fine-grained matrix of the least altered primitive chondrites, in which amorphous silicates embed numerous nanosulfides (e.g., Abreu & Brearley, 2010; Brearley, 1993; Greshake, 1997; Le Guillou et al., 2015; Leroux et al., 2015; Ohtaki et al., 2021). However, in this case, the amorphous silicates are richer in Fe. The silicate fraction of the GEMS-like objects in Ryugu is Mg-rich, shows signs of alteration, and is partly transformed into poorly crystallized phyllosilicates (Nakamura, Matsumoto, et al., 2022). We interpret these objects as the precursors of the fine-grained phyllosilicates dominated by saponite containing nanosulfide inclusions. This hypothesis therefore implies the alteration of primitive precursors with minor redistribution of silicate forming elements during alteration. The nanosulfides (initially likely troilite but converted into pyrrhotite due to the action of water; e.g., Brearley, 2006; Schrader et al., 2021) of the fine-grained regions would have been already present, encapsulated within the amorphous silicate precursor. From a chemical perspective, this scenario is also attractive. Indeed, in addition to the presence of the sulfides, the amorphous silicate precursors are enriched in silica compared to the chondritic composition, with a bulk composition which compares well with the fine-grained Ryugu domains which have a composition close to saponite. GEMS in IDPs and micrometeorites have subsolar element/Si ratios (e.g., Bradley, 1994; Keller & Messenger, 2011). This is also the case for amorphous silicates in the most primitive chondrites, as these frequently also enclose many nanosulfides (e.g., Abreu & Brearley, 2010; Brearley, 1993; Greshake, 1997; Le Guillou et al., 2015; Le Guillou & Brearley, 2014; Leroux et al., 2015; Matsumoto et al., 2019; Vollmer et al., 2020; Zanetta et al., 2021, 2022). These compositions enriched in
<figure><img src="image_9.png" /></figure>

FIGURE 7. Ryugu (a) STEM-HAADF image of an area containing SiO₂ enrichments interstitial to the phyllosilicates (b) associated x-ray intensity map: blue: Si, green: Mg. The bluer regions correspond to the most silica-rich points in the ternary diagram in Figure 6. (c) X-ray intensity map showing elongated SiO₂-rich areas. Fe in red, Si in blue, Mg in green, and yellow is Ni. (d) Associated ternary diagram (Si + Al)-Mg-Fe with grouping colors corresponding to that of the chemical map of (c) (SiO₂-rich areas circled in blue, phyllosilicates in green and mixture of phyllosilicate and Fe,Ni nanosulfides in red). The pixel size is here 17 × 17 nm². Sample A0104-01300103. (Color figure can be viewed at wileyonlinelibrary.com)

silica are compatible with the composition of the fine-
grained phyllosilicate which is dominated by saponite,
both in Ryugu and Orgueil. The fine texture is likely due
to the presence of numerous inclusions initially present in
the amorphous silicate precursors. These inclusions could
have acted as obstacles to the development of an extended
fibrous structure during the crystallization. Figure 12
repositions the chemical composition fields of the fine-
grained areas obtained in this work with those of the
altered GEMS-like object described by Nakamura,
Matsumoto, et al. (2022). The compositions are aligned
along a straight line that encapsulates the composition of
the fine textures with the GEMS-like objects, and points
toward the Fe apex. The observed continuum of the bulk
composition of the fine-grained fraction, highlighted in
Figure 1c, suggests that the fine-grained objects could not
retain all of the initial Fe. Part of the Fe was removed by
the fluids and certainly redeposited to form the large
magnetite and pyrrhotite grains which are abundant in
Ryugu.

The most primitive material of the solar system is
mainly composed of amorphous silicates containing Fe-
rich nanophses. It also contains abundant crystalline
silicates, often submicrometric and rich in magnesium
(e.g., Brearley, 1993; Greshake, 1997; Leroux et al., 2015).
Nakamura, Matsumoto, et al. (2022) showed that the least
altered fragments of Ryugu contain a number of Mg-rich
olivine and pyroxene grains scattered through the
amorphous silicate groundmass. In the main lithology of
Ryugu, these micrometric to submicrometric crystalline
silicates are absent. This suggests that they were altered
and replaced by phyllosilicates. Following Nakamura,
Matsumoto, et al. (2022), we interpret the coarse-grained
areas, which are contiguous with the fine textures and
dominated by serpentine, as the result of the alteration of
these anhydrous silicates. This alteration would not have
generated a large redistribution of elements carried by
fluids and was certainly operated by interface reactions
leaving most of the material involved in place. This
process of replacement of olivine (or pyroxene) by
<figure><img src="image_10.png" /></figure>

FIGURE 8. Orgueil matrix comparing the coarse- and fine-grained areas (a) HAADF STEM image (b) x-ray intensity map in RGB color (R = Fe, G = Mg, and B = Si). The central coarse-grained area is Mg-rich and is dominated by serpentine. The red areas correspond to the finer grained material, which contains more saponite. Note the presence of blue areas enriched in Si and located in the porosity. The blue areas seem mainly located at the periphery of the serpentine pockets. (c) Ternary (Si + Al)-Mg-Fe diagram. Not all the surface was scanned, but only the areas which appear to represent the endmembers (the central green area which corresponds to serpentine, the left red area which is representative of a fine-grained saponite containing ferrihydrite, and the small blue Si-rich pockets mainly located at the interface with the serpentine area (green)—colored boxes on the HAADF image). For the fine-grained material, the composition field is found elongated along a line that joins the Fe apex with a saponite-rich phyllosilicate. (Color figure can be viewed at wileyonlinelibrary.com)

<figure><img src="image_11.png" /></figure>

FIGURE 9. (Si + Al)-Mg-Fe ternary diagrams showing the composition field of Orgueil's phyllosilicates obtained by STEM-EDXS. The microstructure is comparable to the one in Figure 8, with coarse-grained serpentine-rich areas close to saponite-rich areas containing ferrihydrite in abundance. The ROI used to generate data for this diagram is 100 × 100 nm for a sample about 100 nm thick (cube of about $10^{-3}$ µm$^3$). (a) Phyllosilicates together with the ferrihydrite nano-domains enclosed. Note the significant difference between the coarse (in green) and fine textures (in red), illustrating that the ferrihydrite is preferentially located in the fine-grained fraction. (b) Same composition as (a), but with subtraction of the amount of Fe estimated to be associated with the ferrihydrite. This was done assuming an equation $Fe = 3.38S$, deduced from a Fe-S correlation plot such as the one shown in Figure 5. After this subtraction, the composition of the phyllosilicate in fine-grained areas is revealed to be Fe-poor and close to the saponite composition. (Color figure can be viewed at wileyonlinelibrary.com)

serpentine is widely documented in different contexts (e.g., Boudier et al., 2010 in a harzburgite rock; Hanowski & Brearley, 2001; Velbel et al., 2012 in CM chondrites). Serpentinization resulting from a direct replacement of a parent mineral is fluid assisted and involves an interfacial dissolution-precipitation mechanism (e.g., Putnis, 2009; Ruiz-Agudo et al., 2014). A replacement, without implicating fluid transport of elements over a long distance (further precipitation in some porosity), would also explain why the serpentine-rich, coarse-grained areas

contain no or very little sulfide nano-inclusions. In their supplementary materials, Nakamura, Matsumoto, et al. (2022) also showed the microstructure of an altered chondrule which is replaced by serpentine. It is striking to note how similar its microstructure is compared to that of our reported coarse-grained phyllosilicate regions, both in terms of texture and composition.

For the Orgueil matrix, which shows the same fine and coarse duality, it is likely that the alteration history is similar to that of Ryugu considering the close parallel in
<figure><img src="image_12.png" /><figcaption>FIGURE 10. Combined STXM and STEM-EDXS data obtained on one of the studied Ryugu FIB section: STEM-HAADF image; phase maps obtained by linear least square fit of the EDXS data. In particular, it illustrates the existence of two populations of phyllosilicates containing more or less silicon, and interpreted as serpentine and saponite dominated areas; corresponding map of the Fe³⁺/ΣFe % ratio obtained by processing of the STXM hyperspectral data set. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

microstructure that we have highlighted. In the case of Orgueil, the conditions were, however, more oxidizing, beyond the stability limit of the sulfides, which were consequently corroded and led to the formation of ferrihydrite. This latter oxidation step may have been the consequence of a prolonged, unprotected stay on Earth before being collected (see last section). The fine intercalation of ferrihydrite between phyllosilicate crystallites, sometimes forming thin layers parallel to those of the phyllosilicates, reinforces our scenario of an initial presence of nanosulfides finely dispersed in the precursor silicate.

## Origin of the SiO₂-Rich Interstitial Pockets

Our study showed that silica was present between the phyllosilicate laths, in both fine and coarse textures. Both Ryugu and Orgueil phyllosilicates have approximately equivalent, yet low, interstitial nanosized pockets of silica-rich material, supporting similar alteration conditions. Their most direct origin would be that the silica pockets are inherited from the alteration mechanism. On contact with water, a hydrated and amorphous film (called a gel) is formed on the surface of the parent mineral as a result of hydrolysis and condensation reactions (e.g., Gin et al.,

2021; Pokrovsky & Schott, 2000). The layer is enriched in silica due to the release of the most soluble elements. In Ryugu and Orgueil, these silica-rich nano-pockets are associated with porosity, suggesting that the walls of the phyllosilicate laths are lined with a silica-rich layer of varying thickness. Another hypothesis is that the enrichment in silica is the consequence of a circulation of water that carried away the soluble elements, precipitating the large magnetites or carbonates present in both the Ryugu and Orgueil matrices. If the soluble elements were initially in the silicate precursors (amorphous or crystalline), this leads to an enrichment in silica of this silicate following the leaching processes. Silica-rich interstitial spaces have also been observed in the pores or laths of magnetite (Dobrică et al., 2023).

## Two Possible Scenarios to Explain the Nanosulfides in Ryugu and the Ferrihydrite in Orgueil

Despite the strong similarities, the two phyllosilicate matrices show notable differences. In Ryugu, the matrix contains numerous Fe,Ni nanosulfides (pyrrhotite, pentlandite, and sphalerite), whereas in Orgueil, the phyllosilicates are mixed with S- and Ni-bearing
<figure><img src="image_13.png" /><figcaption>FIGURE 11. (Left) XANES spectra at the Fe_L3 edge from Ryugu and Orgueil and illustrating the range of measured valency (center). In Ryugu, Fe³⁺/ΣFe ratio versus a proxy of the iron content after masking the nanosulfides. Each point represents one pixel of the corresponding STXM map; and (right) similar data for Orgueil representing the mixing of phyllosilicates (low iron content) and ferrihydrite (higher iron content). Regression lines are calculated to extrapolate the Fe³⁺/ΣFe ratio of phyllosilicates. Sample A0104-01700604, A0104-01300102, A0104-01300103, A0104-00600303. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

ferrihydrite. In both matrices, the distribution of Fe-, Ni-, and S-rich phases is heterogeneous. They are abundant in the fine fraction but much rarer or absent in the coarse fraction. From these observations, it is tempting to draw a direct link between the two objects.

One hypothesis is that terrestrial weathering altered the nanosulfides into ferrihydrite and produced calcium sulfates. Pyrrhotite is not stable under oxidizing conditions (e.g., Belzile et al., 2004; Harries et al., 2013; Mikhlin et al., 2002; Steger, 1982). At low temperatures, typically at Earth's ambient temperature, pyrrhotite exposed to water and higher oxygen fugacity is dissolved in the fluid in the initial form of ferrous ions which becomes ferric if oxygen is present. Ferric hydroxide Fe(OH)₃ then

precipitates when the fluid becomes rarefied to form ferrihydrite. A generic reaction can be written as follows:

$$Fe_{1-x}S + 2O_2 + (3-3x)H_2O = (1-x)Fe(OH)_3 + SO_4^{2-} + (3-3x)H^+$$

The sulfuric acid byproduct could then react with solubilized elements such as Ca²⁺ and form calcium sulfates (gypsum CaSO₄·2H₂O), as suggested before (Gounelle & Zolensky, 2001). Moreover, both serpentine and saponite are more oxidized in Orgueil (70% Fe³⁺) than in Ryugu (50% Fe³⁺), as attested by our STXM results which corroborate earlier results obtained on bulk grains by Mossbauer spectroscopy (Nakamura, Matsumoto, et al.,
<figure><img src="image_14.png" /></figure>

FIGURE 12. Schematic (Si + Al)-Mg-Fe ternary diagram showing the composition field of the fine-grained material in Ryugu (including the nanosulfides), in Orgueil (including the ferrihydrite) as well as the GEMS-like objects in Ryugu (from Nakamura, Matsumoto, et al., 2022). Note that the coarse-grained domains are not included. For Ryugu and Orgueil, we used the data from Figure 1c (ROI 125 × 125 nm) and Figure 9a (ROI 100 × 00 nm), respectively. This ROI size is close to the one of the GEMS-like domains of the least altered samples of Nakamura, Matsumoto, et al. (2022). (Color figure can be viewed at wileyonlinelibrary.com)

2022). Bulk analysis revealed also the Ryugu grains contain less oxygen than Orgueil (Nakamura, Matsumoto, et al., 2022).

It should also be noted that ferrihydrite is not a common alteration phase in carbonaceous chondrites. However, it develops in meteorites on Earth when they are exposed to an aqueous environment, most often in the form of alteration veins. This is especially the case for meteorites discovered a long time after their fall (e.g., Nishiizumi et al., 1989 for the alteration of Antarctic meteorites; Lee & Bland, 2004 for the alteration in cold and hot deserts; Bland et al., 2006 for a review; King et al., 2020 for a review and a focus on the Ivuna CI chondrite; van Ginneken et al., 2022 for an experimental approach). Furthermore, it is interesting to note that the formation of sodium and calcium sulfates was detected in Ryugu samples shortly after they were prepared for analysis. These are interpreted as resulting from a reaction between calcite and sulfuric acid, the latter being produced by the oxidation of pyrrhotite grains in samples exposed to the Earth's atmosphere (Nakamura, Matsumoto, et al., 2022).

However, some of the differences we observed are more difficult to explain by terrestrial weathering and lead us to raise the possibility that Orgueil was altered under different parent body conditions. First, the iron valency of phyllosilicates is significantly higher in Orgueil than in Ryugu (Figures 10 and 11). In serpentine, Fe³⁺ can be found in octahedral and tetrahedral sites while

Fe²⁺ can only be in octahedral sites. In order to keep electroneutrality of the 1:1 serpentine layer, the terrestrial oxidation of Fe²⁺ would necessarily lead to Si substitution by Fe³⁺ in tetrahedral sites. Silicon would have to be hydrolyzed, leached away and replaced by Fe³⁺. Dissolution-precipitation reaction would thus have to occur, which seems to be difficult during curation under atmospheric conditions. In particular since the 1:1 structure does not allow interlayer water to enter grains, in contrast to saponite (2:1 structure). Another observation is that one of our Ryugu FIB sections does show an iron valency value comparable to that of Orgueil, whereas it was measured soon after its recovery and prepared in a similar manner as the other. We interpret this as the result of heterogeneous oxidation conditions within the Ryugu parent body. It also indicates that these conditions could also have occurred in the Orgueil parent body(ies). Variable water to rock ratio is a known factor influencing the phyllosilicate Fe³⁺ content (e.g., McCollom & Bach, 2009).

Under these conditions, we raise the hypothesis that the nanosulfides (observed in Ryugu) would not have been stable and that an alternative phase could have formed. Deciphering whether ferrihydrite formed directly under these conditions or whether another phase formed that was later weathered into ferrihydrite is beyond the scope of this study. Possible candidates could be tochilinite, for instance, which was observed in some Ryugu fragments (Nakamura, Matsumoto, et al., 2022), or goethite, which has potential stability fields at low temperatures under CI and/or Ceres-like conditions (Zolotov, 2012, 2014). In addition, ferrihydrite in Orgueil is poorly crystalline (Tomeoka & Buseck, 1988). It could thus have formed out of equilibrium and is susceptible to variations of pH, fluid activity, and metal-Fe ligand, which could have driven kinetically driven reactions leading to the precipitation of a metastable phase. Its formation on a parent body thus cannot be excluded. At some point of the fluid evolution sequence, for instance, sulfur activity could have become too low for sulfide to be stable. Zolotov (2014) also finds that a decreasing silicon activity in the fluid could displace the thermodynamic equilibrium from cronstedtite (Fe-rich oxidized serpentine) to goethite. The terrestrial weathering scenario implies the evolution of 50–150 nm large nanosulfide grains forming clusters into smaller and more finely disseminated grains of ferrihydrite. This implies sulfur, nickel, and iron mobility as well as dissolution-precipitation reactions. In contrast, the parent body precipitation of ferrihydrite (or its precursor) could have occurred directly. This hypothesis, however, does not contradict (and would even facilitate) the formation of sulfates on Earth. The sulfur would then be trapped within the poorly crystalline ferrihydrite and would be easily available.
Finally, other comparisons of Ryugu and CI
chondrites, based on characteristics such as bulk carbon
and nitrogen contents, carbonate abundance, amino acids,
PAHs, and insoluble organic matter, have also
demonstrated differences (e.g., Aponte et al., 2023; Parker
et al., 2023; Yabuta et al., 2023; Yamaguchi et al., 2023).
Differences in mineralogical phase abundance are also
observed within CI chondrites, suggesting that they may
have recorded different alteration paths (e.g., Endress &
Bischoff, 1996; King et al., 2015). Terrestrial weathering
could not always easily explain these differences and
variable alteration conditions could be expected for CI-
type parent body(ies).

CONCLUSION

Our study of the Ryugu and Orgueil phyllosilicate
matrices has consolidated our knowledge of the
mineralogy of these two objects at the submicrometer
scale. Conducted on representative and dominant
lithologies, we have highlighted a duality of
microstructure. One half of this dichotomy is composed of
areas with a fine-grained texture dominated by saponite
containing numerous nanosulfide inclusions for Ryugu
and ferrihydrite for Orgueil. The other morphology is
represented by coarser textured areas, dominated by
serpentine and containing no or very little nanosulfides or
ferrihydrite. These textures are evidence of intense aqueous
alteration, but to explain this duality, it is difficult to
consider the complete redistribution of the primary
material as dissolution and re-precipitation. We favor an
explanation based on the replacement by hydrated phases
of precursor compounds of a fundamentally different
nature. We propose that the fine texture of both samples
would be the alteration product of GEMS-like objects or
any other material with a fine-grained silicate texture
containing Fe-based nanophases, which would explain
both the saponite-like chemistry and the presence of
numerous nanosulfides. Contrarily, the coarse textures
would correspond to the replacement of anhydrous
silicates (olivine and pyroxene), leaving altered regions
lacking nanosulfides or ferrihydrite. A large part of the
material was not transported and homogenized but was
instead transformed at the micrometer scale. It does not
exclude that dissolution and/or leaching of the most
soluble elements led to the precipitation of minerals
(magnetites, carbonates, phosphates, pyrrhotite, or Na-
rich phyllosilicates) in the pores.

The conclusion of our study is largely based on the
discovery of GEMS-like objects by Nakamura,
Matsumoto, et al. (2022) in the least altered fragments of
Ryugu samples. It would be appreciated if future studies
would be oriented toward the search for intermediate

microstructures located between these altered GEMS-like
domains and the phyllosilicate major lithology described in
this study in order to clarify the presence of a continuum
between the two. It may also be possible to discover
fragments even more primitive than those described by
Nakamura, Matsumoto, et al. (2022). Such a search would
provide major advances in our understanding of the
mineralogical evolution in the presence of water of small
carbonaceous asteroids in our solar system. It would also
help to advance the debate on the possible relationship that
may exist between the GEMS of IPDs and carbonaceous
micrometeorites and the GEMS-like objects that are often
observed in the most primitive chondrites.

Finally, our study has shown that there is a direct link
between Ryugu and Orgueil microstructures. One likely
explanation is that Orgueil was initially similar to Ryugu
and their differences result from terrestrial weathering.
However, we cannot exclude that the two objects have
experienced different aqueous alteration conditions on
their parent body. The conditions for Orgueil would have
been more oxidizing, beyond the stability limit of
pyrrhotite, which would no longer have been stable. In
order to decipher between these two hypotheses,
experimental studies are needed to explore the evolution of
the Ryugu microstructure at different conditions of
temperature, water/rock ratio, humidity, pH, and partial
pressure of oxygen.

Acknowledgments—The Hayabusa2 project has been
developed and led by JAXA in collaboration with
Deutsches Zentrum für Luft- und Raumfahrt (DLR) and
Centre national d'études spatiales (CNES), and supported
by NASA and Australian Space Agency (ASA). We thank
all the members of the Hayabusa2 project for their
technical and scientific contributions. This work was
carried out on the electron microscopy facility of the
Advanced Characterization Platform of the Chevreul
Institute, University of Lille - CNRS. It has been funded
by ISITE ULNE and the "Métropole Européenne de
Lille" through the "TEM-Aster projet". The Chevreul
Institute is thanked for its help in the development of this
work through the CHEMACT project supported by the
"Ministère de l'Enseignement Supérieur de la Recherche et
de l'Innovation", the region "Hauts-de-France" and the
"Métropole Européenne de Lille". HAI and JPB are
supported by NASA's Laboratory Analysis of Returned
Samples Program. JSPS KAKENHI Grant Numbers
related to this study are 19H00725, 19K0094, and
21H05424. This research was supported by the H2020
European Research Council (ERC) (SOLARYS ERC-
CoG2017_771691). Reviews by A. J. King and an
anonymous reviewer helped to improve the manuscript
and are gratefully acknowledged.
*Data Availability Statement*—Data available on request from the authors.

*Editorial Handling*—Dr. Yves Marrocchi

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