Mineralogy and petrology of fine-grained samples recovered from the asteroid
(162173) Ryugu

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

<footnote n="1">¹Division of Earth and Planetary Sciences, Kyoto University, Kyoto, Japan</footnote>

<footnote n="2">²The Hakubi Center for Advanced Research, Kyoto University, Kyoto, Japan</footnote>

<footnote n="3">³Institute for Chemical Research, Kyoto University, Kyoto, Japan</footnote>

<footnote n="4">⁴Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="5">⁵Pan-Omics Data-Driven Research Innovation Center, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="6">⁶Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="7">⁷The Ultramicroscopy Research Center, Kyushu University, Fukuoka, Japan</footnote>

<footnote n="8">⁸Department of Geosciences, Osaka Metropolitan University, Osaka, Japan</footnote>

<footnote n="9">⁹Department of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, Japan</footnote>

<footnote n="10">¹⁰Kochi Institute for Core Sample Research, X-Star, JAMSTEC, Nankoku, Kochi, Japan</footnote>

<footnote>Hawaiʻi Institute of Geophysics and Planetology, The University of Hawaiʻi at Mānoa, Honolulu, Hawaii, USA</footnote>

<footnote n="12">¹²Université de Lille, CNRS, INRAE, Centrale Lille, UMR 8207-UMET-Unité Matériaux et Transformations, Lille, France</footnote>

<footnote n="13">¹³Université de Lille, CNRS, INRAE, Centrale Lille, Université Artois, FR 2638-IMEC-Institut Michel-Eugène Chevreul, Lille, France</footnote>

<footnote n="14">¹⁴Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany</footnote>

<footnote n="15">¹⁵European Space Resources Innovation Centre, Luxembourg Institute of Science and Technology, Belvaux, Luxembourg</footnote>

<footnote n="16">¹⁶Université Grenoble Alpes, CNRS, IPAG, Grenoble, France</footnote>

<footnote n="17">¹⁷NASA Langley Research Center, Hampton, Virginia, USA</footnote>

<footnote n="18">¹⁸Materials Characterization Lab, The Pennsylvania State University Materials Research Institute, University Park, Pennsylvania, USA</footnote>

<footnote n="19">¹⁹Lunar and Planetary Laboratory, Department of Planetary Sciences, The University of Arizona, Tucson, Arizona, USA</footnote>
²⁰Department of Earth, Atmospheric and Planetary Sciences, Purdue University, West Lafayette, Indiana, USA

²¹Buseck Center for Meteorite Studies, Arizona State University, Tempe, Arizona, USA

²²Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA

²³Jacobs Engineering, Dallas, Texas, USA

²⁴Space Park Leicester, University of Leicester, Leicester, UK

²⁵School of Geology, Geography and the Environment, The University of Leicester, Leicester, UK

²⁶School of Geographical and Earth Sciences, The University of Glasgow, Glasgow, UK

²⁷Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, New South Wales, Australia

²⁸Department of Materials, The University of Oxford, Oxford, UK

²⁹School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia

³⁰Materials and Condensed Matter Physics, School of Physics and Astronomy, University of Glasgow, Glasgow, UK

³¹John de Laeter Centre, Curtin University, Perth, Western Australia, Australia

³²ARES, NASA Johnson Space Center, Houston, Texas, USA

³³Earth and Space Exploration Center, Ritsumeikan University, Kusatsu, Shiga, Japan

³⁴CAS 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

³⁵CAS Center for Excellence in Deep Earth Science, Guangzhou, China

³⁶Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo, Japan

³⁷University of Chinese Academy of Sciences, Beijing, China

³⁸Department of Mathematics, Physics, and Earth Science, Ehime University, Matsuyama, Ehime, Japan

³⁹Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan

⁴⁰Faculty of Engineering, Kindai University, Hiroshima, Japan

⁴¹Department of Earth Science, Tohoku University, Sendai, Japan

⁴²Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan

⁴³The Graduate University for Advanced Studies, SOKENDAI, Hayama, Japan

⁴⁴Department of Mechanical Engineering, Kanagawa Institute of Technology, Atsugi, Japan

⁴⁵Department of Earth and Planetary Sciences, Hokkaido University, Sapporo, Japan

⁴⁶Creative Research Institution Sousei, Hokkaido University, Sapporo, Japan

⁴⁷Abteilung Partikelchemie, Max Planck Institut für Chemie, Mainz, Germany

⁴⁸Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan

⁴⁹UTokyo Organization for Planetary and Space Science, The University of Tokyo, Tokyo, Japan

⁵⁰Department of Earth and Environmental Sciences, Nagoya University, Nagoya, Japan

*Correspondence

Takaaki Noguchi, Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake-cho, Sakyoku, Kyoto 606-8502, Japan.

Email: noguchi.takaaki.2i@kyoto-u.ac.jp

*(Received 19 April 2023; revision accepted 20 September 2023)*

**Abstract**—Samples returned from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 mission revealed that Ryugu is composed of materials consistent with CI chondrites and some types of space weathering. We report detailed mineralogy of the fine-grained Ryugu samples allocated to our “Sand” team and report additional space weathering features found on the grains. The dominant mineralogy is composed of a fine-grained mixture of Mg-rich saponite and serpentine, magnetite, pyrrhotite, pentlandite, dolomite, and Fe-bearing magnesite. These grains have mineralogy comparable to that of CI chondrites, showing severe aqueous alteration but lacking ferrihydrite and sulfate. These results are similar to previous works on large Ryugu grains. In addition to the major minerals, we also find many minerals that are rare or have not been reported among CI chondrites. Accessory minerals identified are hydroxyapatite, Mg-Na phosphate, olivine, low-Ca pyroxene, Mg-Al spinel, chromite, manganese chromite, eskolaite, ilmenite, cubanite, polydymite, transjordanite, schreibersite, calcite, moissanite, and poorly crystalline phyllosilicate. We also show scanning transmission electron microscope and scanning electron microscope compositional maps and images of some space-weathered grains and severely heated and melted grains. Although our mineralogical results are consistent with that of millimeter-sized grains, the fine-grained fraction is best suited to investigate impact-induced space weathering.
INTRODUCTION

The Japan Aerospace Exploration Agency (JAXA)'s Hayabusa2 mission successfully brought back 5.4 g of samples of the near-Earth carbonaceous (C-type) asteroid (162173) Ryugu to the Earth on December 6, 2020. After about 6 months of curation activities at the Planetary Material Sample Curation Facility of JAXA, Ryugu grains were allocated for initial analysis and the advanced stage of curation. Many papers have described the mineralogy and petrology of the Ryugu grains investigated during the initial analysis and the advanced curation (Greenwood et al., 2023; Ito et al., 2022; Kawasaki et al., 2022; Liu et al., 2022; Nakamura et al., 2022, 2023; Paquet et al., 2022; Yada et al., 2021; Yamaguchi et al., 2023; Yokoyama et al., 2023). These studies consistently show that the mineralogy of the mm-sized Ryugu grains experienced severe aqueous alteration. Their essential minerals are saponite, serpentine, magnetite which often occurs as framboidal, plaquette, and spherulitic aggregates, pyrrhotite, pentlandite, dolomite, and breunnerite. Their bulk oxygen isotopic composition most closely resembles that of CI chondrites. Both chemical and oxygen isotopic compositions of olivine crystals in the mm-sized Ryugu grains completely overlap with those of olivine crystals in CI chondrites. These results consistently show that Ryugu samples have a mineralogy comparable to that of CI chondrites. These results also revealed that some basic features of CI chondrites, in particular, abundant ferrihydrite and sulfates are not intrinsic but instead terrestrial weathering products because they were not identified by both synchrotron x-ray diffraction (XRD) and (scanning) transmission electron microscope ((S)TEM) observation methods. Even interlayer water molecules in saponite in CI chondrites have been shown to include terrestrial absorbed moisture (e.g., Yokoyama et al., 2023). The onboard near-infrared spectrometer NIRS3 indicated that Ryugu's reflectance spectra are in good agreement with those of heated CM or CI carbonaceous chondrites (e.g., Kitazato et al., 2021) that present a significantly reduced 2.7 µm absorption band compared to unheated CM and CI chondrites. However, as described above, the returned samples are severely hydrated CI-like materials. Space weathering, especially induced by meteoroid impact heating and melting on the top surface (~3 µm thick), caused the spectral features consistent with dehydration (Noguchi et al., 2023). Although many other reasons could contribute to the low intensity of the ~2.7 µm absorption band, we think that the dehydroxylation by space weathering is one of the most important reasons for the low intensity. In this paper, we describe the detailed mineralogy of space weathering induced by meteoroid impact heating and melting found in the fine-grained samples allocated to the

Min-Pet Fine “Sand” subteam (hereafter the “Sand” team) and compare the mineralogy of the fine-grained Ryugu samples with the mm-sized Ryugu samples.

MATERIALS AND METHODS

Because the details of the samples and methods were described in detail by Noguchi et al. (2023), we describe the methods briefly here.

Samples

Fine-grained Ryugu samples collected from touchdown (TD) #1 site A0104 and TD #2 site C0105 were allocated to the “Sand” team (Figure A1). The weight of the sand-like sample is 0.3 and 0.4 mg, respectively, from the two sites. Their total weight is ~0.2% of the samples allocated to the initial analysis team composed of six subteams. The samples were fixed on a gold plate using a very small amount of epoxy glue on an SEM sample stub in an N₂-filled glove box at Kyoto University, and all the samples were sent to each university or institute after being packed in N₂-filled sealed aluminum-coated plastic bags to avoid alteration by the terrestrial environment. In total, our team observed >1000 grains by field-emission scanning electron microscopes (FE-SEMs). More than 700 grains were observed at Kyoto and Kyushu Universities. About 20–40 grains were observed each at Kobe University, Hiroshima University, the University of Hawai‘i at Mānoa, Université de Lille, Université Grenoble Alpes, the University of Arizona, U.S. Naval Research Laboratory, the University of Leicester, and the electron Physical Science Imaging Centre (ePSIC) facility at Diamond Light Source, the University of Glasgow, Molema Building, Curtin University, and the University of Tokyo. The fine-grained sample grains were typically <70 µm across for TD#1 site samples and <50 µm across for TD#2 site samples.

Sample Processing

To distribute TEM samples, focused ion beam (FIB) sections were prepared using a Thermo Fisher Helios FIB-SEM at Kyoto University. We prepared ~80 FIB sections from 40 grains and >30 FIB sections were distributed to the “Sand” team members. In addition, ~30 FIB sections and ~20 TEM grids on which ultramicrotomed sections were placed were prepared at universities and institutes other than Kyoto University from the grains on SEM sample stubs. The instruments and the processing conditions were described in Noguchi et al. (2023).
The conventional method to identify expansive clay minerals such as smectite is the ethylene glycol treatment (e.g., Moore & Reynolds Jr., 1997). If ethylene glycol is infiltrated in a sample containing expansive clay mineral, the (001) basal spacing of the expansive clay mineral increases because the ethylene glycol molecules penetrate into the interlayer spaces of the clay mineral. We applied a modification of this method to assess the degree of dehydration of saponite in the Ryugu grains. Because saponite belongs to the smectite group, its (001) basal spacings will increase after the infiltration of ethylene glycol if the saponite can rehydrate, which means the layer structure of saponite is expandable according to the availability of H₂O molecules. We prepared ultrathin sections of a Ryugu grain at Kyoto University using ethylene glycol as a trough liquid instead of distilled water. If ethylene glycol molecules penetrate the interlayer spaces of saponite, the ethylene glycol molecules do not escape from the interlayer spaces of saponite easily within the high-vacuum (S)TEM column. We compared the (001) basal spacing of saponite prepared by FIB (without ethylene glycol) and that of saponite prepared by this method, by which we can assess whether saponite in Ryugu grains can rehydrate or not.

## (Scanning) Transmission Electron Microscopy

We observed FIB sections or ultrathin sections using (S)TEMs equipped with energy-dispersive spectrometer (EDS) as well as electron energy-loss spectrometer (EELS) on some (S)TEMs at Kyoto University, Kyushu University, Kobe University, the Kochi Institute for Core Sample Research (KOCHI), JAMSTEC, Tohoku University, the University of Hawai'i at Mānoa, the Molecular Foundry, Lawrence Berkeley National Laboratory, the University of Lille, the University of Jena, the Pennsylvania State University, the University of Arizona, the Naval Research Laboratory, the University of Glasgow, and ePSIC. Details of the instruments and analytical conditions were described in Noguchi et al. (2023).

## Transmission Kikuchi Diffraction (Transmission Electron Backscatter Diffraction)

Transmission Kikuchi diffraction (TKD— transmission electron backscatter diffraction) maps of two forsteritic olivine grains in the FIB sections of Ryugu grains A0104-00900802 and A0104-00900501 were collected to assess the degree of shock at the Electron Beam Laboratory of the Structural Engineering Division, NASA Johnson Space Center, using a Supra 55 variable pressure field emission scanning electron microscope (FEG-SEM) and Bruker EBSD system. A column pressure of 9 Pa and no C-coating on the sample is

optimal for the determination of the degrees of shock using this system. We varied camera exposure time and gain to optimize mapping performance—320 × 240 pattern pixilation, frame averaging of 3, 15 kV, and low extractor voltage yield an acceptable balance of hit rate, speed, and TKD map quality using an exposure time of 30 ms.

## X-Ray Computed Nanotomography

X-ray computed nanotomography (SR-nCT) analysis was performed at beamline BL47XU of SPring-8, a synchrotron radiation facility in Japan. Small Ryugu sample grains (5–50 µm) were mounted on Ti-needles using the manipulating system equipped in the Ga FIB-SEM system (Thermo Fisher Helios at Kyoto University). We combined two methods in SR-nCT: dual-energy tomography (DET; Tsuchiyama et al., 2013), and scanning imaging x-ray microscopy (SIXM; Takeuchi et al., 2013). These methods enable us to obtain nondestructive 3-D nanostructures and mineralogy of microsamples (Matsumoto et al., 2019). In DET, absorption images with x-ray linear attenuation coefficients (LACs) were obtained at two energies at 7 and 7.35 keV (~50 nm in voxel size) below and above the K-absorption edge energy of iron (7.11 keV), respectively. The contrasts at 7 keV closely correspond to compositional (Z) contrasts, whereas those at 7.35 keV strongly depend on iron contents, thus emphasizing Fe-rich phases. In SIXM, we obtained x-ray phase-contrast images with refractive index decrements, which is the difference between the refractive index and unity and is roughly proportional to the material density, at 8 keV (~100 nm in voxel size). After registration of the three types of CT images, their RGB color merged images correspond to mineral-phase maps in which we can discriminate most of the mineral phases.

## NanoSIMS Analysis

Carbon- and Si-isotopic compositions of the moissanite (SiC) grain in FIB section A0104-002_006_01 were measured with the Cameca NanoSIMS 50 ion probe (Hoppe et al., 2013) at the Max Planck Institute for Chemistry in Mainz. A focused primary Cs⁺ ion beam (~100 nm, ~1 pA) was rastered over an area with size 2 × 2 µm² (256 × 256 pixels, 10,000 µs/pixel/image plane, 2 image planes) around the SiC grain. Negative secondary ions of ¹²C, ¹³C, ²⁸Si, ²⁹Si, and ³⁰Si were recorded in multicollection with electron multipliers. Carbon- and Si-isotopic compositions of the SiC grain were normalized to the surrounding matrix assuming CI composition for C (δ¹³Cₚ₉₉ = -10‰; Kerridge, 1985) and terrestrial composition for Si.
<figure><img src="image_1.png" /><figcaption>FIGURE 1. A typical small Ryugu grain and a hydrated fine-grained Antarctic micrometeorite. (a, b) A typical small Ryugu grain investigated. Such grains often have large hexagonal platy pyrrhotite, and various morphologies of magnetite (framboidal aggregate in this case). These large minerals are embedded in a phyllosilicate-rich matrix. (c, d) A hydrated fine-grained AMM. It contains framboidal, spherulitic, and plaquette magnetite and pyrrhotite. (a) and (c) are secondary electron images (SEI). (b, d) BSE images. The sample number of the grain shown in (a) and (b) is A0104-01000800. The AMM shown in (c) and (d) was found in surface snow collected near the Dome Fuji Station, Antarctica in 2012.</figcaption></figure>

## RESULTS

### The Size and Morphology of the Fine-Grained Ryugu Samples

Based on the secondary electron images of the allocated grains, the average diameters of the samples collected from TD#1 and stored in Chamber A of the sample catcher A0104 and those collected from TD#2 and stored in Chamber C C0105 are 70 (N = 507) and 54 µm (N = 292), respectively (Noguchi et al., 2023). In addition, the 3-D dimensions of 30 Ryugu grains were obtained by synchrotron radiation nanotomography; their axial ratios b/a and c/b are 0.6910 ± 0.1758 and 0.6389 ± 0.1808 for Chamber A grains (N = 21) and 0.6911 ± 0.1081 and 0.5121 ± 0.1572 for Chamber

C grains (N = 9), respectively, where a, b, and c are longest, middle, and shortest dimensions.

### Morphology of the Most Common Samples

Most grains have relatively flat surfaces with sharp angles when they are observed by FE-SEM (Figure 1a). Grain surfaces often present hexagonal platy pyrrhotite crystals, magnetite in plaquettes as well as framboidal and spherulitic aggregates in backscattered electron (BSE) images (Figure 1b). When we compare with the morphology of hydrated fine-grained (H-fg) Antarctic micrometeorites (AMMs) collected from the uncompacted recently fallen snow, the sharper edges of the fine-grained Ryugu grains are obvious (Figure 1). Because mineralogy of the H-fg AMMs is similar to that of Ryugu grains (e.g.,
FIGURE 2. Phyllosilicates in fine-grained Ryugu grains. (a) HAADF-STEM image showing a typical texture of phyllosilicate-rich matrices. C, coarse; F, fine; phy, phyllosilicates; Sf, sulfides. (b) [Si + Al]-Mg-Fe ternary diagrams (atomic ratios) of phyllosilicates. Compositional fields of phyllosilicates in Orgueil are from Tomeoka and Buseck (1988) and those in Ivuna are from this study. Data were compiled from Ryugu samples A0104-00100301, -00200301, -00200501, -00200702, -00700601, -00200501, -00700501, -00700601, -01500401, -01700301, -01700606, -02203702, -02306901, A0058-C2001-04, and -07 were used. (c) HAADF-STEM image and the corresponding chemical phase map of the phyllosilicate-rich matrix. Coarser phyllosilicates tend to have lower Si content (Low-Si silicate) than finer ones (High-Si silicate). (d) HAADF-STEM image and corresponding composite elemental distribution map of the phyllosilicate-rich matrix. The intimate coexistence of Si-rich material (blue in the element distribution map) with phyllosilicates (green in the map) can be seen. (e, f) High-resolution BF-TEM images of phyllosilicates. (e) Phyllosilicates prepared by FIB without contact with the atmosphere. Both serpentine with 0.73 nm lattice fringes and saponite with 1.05 nm lattice fringes are observed. BF-TEM image. Note that concentric Fresnel fringes in (e) are artifacts formed by diffraction due to a small condenser aperture. (f) Phyllosilicates prepared by ultramicrotomy using ethylene glycol as a trough liquid. The spacing of lattice fringes expanded to >1.3 nm. BF-TEM image. Sample numbers: A0058-C2001-07, A0104-01300103, A0104-00600301, and A0104-01500401. (Color figure can be viewed at wileyonlinelibrary.com)

Noguchi et al., 2002a, 2002b), the sharp edges are preserved better than the fine-grained Ryugu grains.

## Major Minerals in the Fine-Grained Samples

Major minerals identified by (S)TEM are phyllosilicates (saponite and serpentine), Fe-rich sulfides, magnetite, dolomite, and a lesser amount of Mg-Fe-carbonate, breunnerite. These sulfides, magnetite, and carbonates are embedded in the fine-grained phyllosilicate-rich matrix as shown three-dimensionally in Movie S1 generated by SR-NCT (Tsuchiyama et al., 2022).

## Phyllosilicates

Figure 2a shows a HAADF-STEM image of a typical area of the phyllosilicate-rich matrix of Ryugu grains. Phyllosilicates range widely in size. Coarser phyllosilicates often surround the >50 nm across Fe-Ni sulfide grains, appearing as bright spots embedded in finer phyllosilicates. Chemical compositions of the coarser phyllosilicates in 12 Ryugu grains are shown in Figure 2b. They plot along a line with Mg/(Mg + Fe) atomic ratio of 0.86 and between the serpentine and saponite solid solution lines in the (Si + Al)-Mg-Fe ternary diagram, which suggests that the coarser phyllosilicates are a mixture of serpentine and saponite in various proportions. Data were compiled from Ryugu samples A0104-00100301, -00200301, -00200501, -00200702, -00700601, -00200501, -00700501, -00700601, -01500401, -01700301, -01700606, -02203702, -02306901, A0058-C2001-04, and -07. A HAADF-STEM image (upper panel of Figure 2c) shows that a fine-grained Si-rich material intimately coexists with the coarse phyllosilicate. The phase map of the same area of the HAADF-STEM image (lower panel of Figure 2c) shows that coarser phyllosilicates tend to have lower Si contents than finer ones, which suggests that coarser ones contain more serpentine relative to saponite. The finer phyllosilicates contain more saponite embedded with abundant (<50 nm)

Fe-Ni sulfide grains based on the composite elemental distribution map (Figure 2d).

High-resolution bright-field (BF) TEM image of coarser phyllosilicates is shown in Figure 2e. Phyllosilicate lattice spacings ranging from ~1.0 to ~1.2 nm and from 0.72 to 0.75 nm are indicative of the presence of saponite and serpentine, respectively. This image was taken using a thin foil sample prepared by FIB. To assess the degree of dehydration, ultrathin sections of a Ryugu grain were prepared using ethylene glycol as the trough liquid on which ultrathin sections were floated on the ethylene glycol in the air. As explained in the Materials and Methods section, if the rehydration of saponite in Ryugu grains is impossible, the (001) spacing of saponite will not change after ethylene glycol treatment because the ethylene glycol molecules cannot penetrate into the interlayer spaces. If the saponite has the ability to rehydrate (re-absorb H₂O molecules in the interlayer spaces of saponite), then the (001) spacing will expand after the treatment. After ethylene glycol treatment, the (001) spacing expanded to ~1.3 nm (Figure 3f; Table 1), which indicates that saponite on the surface of Ryugu can take up additional water.

## Fe-Ni Sulfides

Fe-Ni sulfides are abundant in all the grains investigated. Pyrrhotite is predominant among large (>0.5 µm across) grains, and hexagonal platy to prismatic Fe-Ni pyrrhotite is very common. On the surfaces of Ryugu grains, these sulfide mineral surfaces often show steps that may be related to their growth and/or dissolution during aqueous alteration (Figure 3a). The most abundant structural type is the monoclinic 4C (Figure 3b) and the other NC-type superstructure diffraction spots are also observed. The 4C superstructure has a period along the *c* crystallographic axis four times longer than the length along the *c*-axis of the basic NiAs-type hexagonal unit (e.g., Yamamoto & Nakazawa, 1982). When the superstructures have *c*-axes
<figure><img src="image_2.png" /></figure>

N times longer than the length of the *c*-axis of the basic unit, they are called NC-type superstructures. Some pyrrhotite grains contain vesicles near their surfaces (Figure 3c). Vesicle-rich areas are evident in places on the

surface of pyrrhotite grains (Figure 3d). Prismatic pyrrhotite crystals elongated along the *c*-axis and containing abundant cracks normal to the *c*-axis are also common (Figure 3e). Pentlandite crystals were identified
<figure><img src="image_3.png" /><figcaption>FIGURE 3. Typical occurrences of Fe-Ni sulfides in Ryugu grains. (a) Large pyrrhotite crystal with a relatively flat surface. There are many steps (some are indicated by arrows) on its surface. SEI. (b) BF-TEM image of such a large pyrrhotite. Its SAED pattern shows it is pyrrhotite-4C. (c, d) Some large pyrrhotite crystals contain tiny (<~100 nm) vesicles near the surface. SEI and HAADF-STEM. (e, f) Some pyrrhotite crystals have many subparallel cracks and slightly Fe-enrich bands subparallel to (001) of pyrrhotite. (e) SEI and (f) HAADF-STEM. (g) Elemental distribution maps of the pyrrhotite in (f). Slightly Fe-enriched bands and pentlandite are observed as shown in the Fe distribution map. Pentlandite (Pn) also exists along some cracks and manganochromite (Mnchr) exists on the surface of this pyrrhotite as shown in the Ni distribution map. Sample numbers: A104-01000800, A104-01700101, A0104-00801200, A0104-01000901, A0104-00801200, A0104-00700602. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

along these cracks as shown in the Ni map in Figure 3g. Darker bands associated with the cracks in the BF-STEM image (Figure 3f) are slightly enriched in Fe as shown in the Fe map in Figure 3e. Although Ni is not enriched in these slightly Fe-enriched bands, pentlandite also exists along some cracks as shown in the Ni distribution map in Figure 3g.

Pyrrhotite and pentlandite are present in almost equal amounts among nanometric Fe-Ni sulfides coexisting with the fine-grained phyllosilicates of the matrix (Figure 2d). On the S-Fe-Ni ternary diagram (Figure 4), most Fe-Ni sulfides plot near pyrrhotite and pentlandite. Highly scattered analytical spots (red in Figure 4) result from a variable amount of Cu. As
<table><caption>TABLE 1. Expansion of (001) of saponite by ethylene glycol treatment.</caption><thead><tr><th></th><th>(001)<br>Serpentine<br>(nm)</th><th>(001) Saponite<br>(nm)</th></tr></thead><tbody><tr><td>FIB section (unexposed to<br>the atmosphere)</td><td>Median 0.73<br>(n = 5)</td><td>Median 1.17 nm<br>(n = 6)</td></tr><tr><td>FIB section (exposed to<br>the atmosphere)</td><td>Median 0.74<br>(n = 9)</td><td>Median 1.15 nm<br>(n = 18)</td></tr><tr><td>Ultrathin section<br>(ethylene-glycol<br>treatment)</td><td>Median 0.75<br>(n = 19)</td><td>Median 1.27 nm<br>(n = 31)</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>FIGURE 4. Chemical compositions of Fe-Ni sulfides. Most sulfides plot between pyrrhotite and pentlandite. Highly scattered analytical spots (red) result from a variable amount of Cu. Sample numbers: A0104-00200801, A0104-00200105, A0058-C2001-07. (Color figure can be viewed at wileyonline library.com)</figcaption></figure>

described in a later section, we identified sphalerite ZnS containing a variable amount of Fe and Cu, and cubanite CuFe₂S₃. Structural variation within a single pyrrhotite crystal was reported by Harries et al. (2022) who suggest that the structural variation may be related to exposure to interplanetary space.

## Magnetite

Magnetite in the Ryugu samples has various morphologies (framboidal, spherulitic, and plaquette), which are common to that in CI chondrites, hydrated interplanetary dust particles (IDPs, e.g., Bradley, 2014), and hydrated fine-grained AMMs (e.g., Dobrică et al., 2019; Noguchi et al., 2002a). A detailed description of spherulitic aggregates was reported by Dobrică et al. (2023), and their formation under nonequilibrium conditions was proposed.

## Minor Minerals in the Most Common Ryugu Grains

Various minor minerals are observed in Ryugu grains (Figures 5–7). Chromite (Mg₀.₁₇Fe₀.₇₉)Cr₂₀₃O₄ (Figure 5a) and manganochromite Mg₀.₀₇Fe₀.₂₃Mn₀.₇₇Cr₁.₉₈O₄ (Figure 5b) occur as separated crystals. The relative height between Cr Kα and Cr Kβ lines is important to distinguish them (Figure 5c). Due to the peak overlap between Cr Kβ and Mn Kα lines, the peak height (indicated by an arrow in Figure 5c) is considerably higher relative to the Cr Kα line in the case of manganochromite. Chromite and manganochromite often coexist with Fe-Ni-Zn sulfides. Figure 5d shows a sulfide aggregate with the manganochromite (Mnchr) crystal shown in Figure 4b,c. Elemental distribution maps of this aggregate show that sulfide minerals consisting of this aggregate are pyrrhotite (Pyh), pentlandite (Pn), and sphalerite (Sp; Figure 4d). Ilmenite Fe₀.₉₂Mn₀.₁₄Cr₀.₀₁Ti₀.₉₇O₃ is minor but ubiquitously distributed in many small Ryugu grains (Figure 4e). Eskolaite Cr₂O₃ is present as another Cr-rich phase (Figure 5f).

Sphalerite is also minor but frequently coexists with Fe-Ni sulfides (Figures 5d and 6a). The sphalerite crystal shown in Figure 6a exhibits twinning with the twin plane on (111) (Figure 6b). An EDS spectrum of the sphalerite in Figure 6a is shown in Figure 6c. This spectrum shows that it contains Fe but it is not clear in this case if the sphalerite contains Cu because this spectrum was obtained using a Cu FIB grid. However, Cu seems to be often associated with Fe-Ni sulfides (Figure 4) although the data shown in Figure 4 were also obtained using a Cu FIB grid. To measure Cu quantitatively, a metallic Si FIB grid was used to measure Cu contents in sulfides in a Ryugu grain. Table 2 clearly shows that Cu is associated with a high abundance of Zn. Copper may be dissolved in sphalerite and/or there may be tiny Cu-bearing inclusions in sphalerite. Aside from these Cu-bearing phase(s), ~500-nm-sized cubanite crystals are sometimes encountered (Figure 6d). Some cubanite crystals are enriched in Co and Ni as shown in Figure 6e. We also found rare polydymite Ni₃S₄ (Figure 6g).

Apatite occurs as a relatively common phase (Figure 7a). Because fluorine (F) is just above the detection limit and because chlorine (Cl) is below the detection limit, the apatite is likely hydroxyapatite (Figure 7b) although some apatite grains contain a detectable amount of Fe. Sodium- and magnesium-bearing phosphate is always observed as amorphous grains (not included in the figures). In addition, not only phosphate but also phosphides are present as phosphorous (P)-bearing phases in Ryugu grains. Transjordanite (Ni, Fe)₂P occurs as aggregates of <100-nm sized grains (Figure 7c,d). The selected area electron
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FIGURE 5. Minor phases (1) chromite, manganese chromite, eskolaite, and ilmenite. (a) Bright-filed transmission electron (BF-TEM) image of chromite with selected area electron diffraction (SAED) pattern. Chromite is often associated with pyrrhotite. (b) BF-TEM image with SAED pattern of manganese chromite (Mnchr). (c) A comparison of energy-dispersive spectrometer (EDS) spectra of chromite and manganese chromite. The height of the peak indicated by an arrow in manganese chromite is considerably higher than the corresponding peak in chromite, which resulted from the overlapping of Mn Kα and Cr Kβ lines. Cu peaks are from the FIB grids. (d) HAADF-STEM image and corresponding elemental distribution maps. The manganese chromite (Mn-chr) in the aggregate is the same as that in (b) and (c). In addition to the Mn-chr, this aggregate contains pentlandite, sphalerite (Sp), and pyrrhotite (Pyh). (e) BF-TEM and SAED of large ilmenite (Ilm). SAED was obtained from the area indicated by a red open circle (f) HAADF-STEM and Fourier transform pattern of eskolaite (Esk). Sample numbers: A0104-01000501, A0104-00500901, A0104-00200105. (Color figure can be viewed at wileyonlinelibrary.com)

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

FIGURE 6. Minor phases (2) sphalerite, cubanite, and polydymite. (a) BF-TEM of sphalerite (Sp) and (b) its SAED pattern. The SAED pattern shows twinning with the twin plane (111). (c) EDS spectrum of the sphalerite. Because the spectrum was obtained using a Cu FIB grid, it is not clear whether the sphalerite contains Cu in this case. (d) HAADF-STEM and SAED of cubanite (Cbn). (e) Elemental distribution maps of cubanite. Elemental maps show that it has a domain containing elevated amounts of Co and Ni. (f) BF-STEM and elemental distribution maps of an aggregate composed of pyrrhotite (Pyh), sphalerite (Sp), pentlandite (Pn), and an unidentified Cu and Fe-rich phase. (g) BF-TEM and SAED of polydymite (Pld). Sample numbers: A0104-00500901, A0104-00200201, A0104-00200802, A0104-00000701. (Color figure can be viewed at wileyonlinelibrary.com)
<figure><img src="image_12.png" /><figcaption>FIGURE 7. Minor phases (3) hydroxyapatite, transjordanite. (a) to (b) BF-TEM, SAED, and EDS spectrum of apatite (Ap). Because chlorine (Cl) is not detected and because only a trace amount of fluorine (F) is detected from the apatite, the apatite is probably hydroxyapatite. (c, d) HAADF-STEM and the elemental distribution maps, EDS spectrum of a part of the transjordanite (Tjrd) in (c). (e) SAED pattern of the transjordanite shows a ring pattern, which suggests that the transjordanite is an aggregate of randomly oriented tiny crystals of transjordanite. (f) The radial intensity profile generated from (e). Asterisks in (f) indicate strong diffraction lines of transjordanite shown in the crystallographic database. Sample numbers: A0104-00200601, A0104-00700801. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

<table><caption>TABLE 2. 100%-normalized atomic ratios of sulfides in 0104-00200101 using a metallic Si FIB grid.</caption><thead><tr><th rowspan="2"></th><th colspan="7">Pyrrhotite</th><th colspan="5">Pentlandite</th><th colspan="5">Zn- and Cu-bearing sulfides</th></tr><tr><th>56.7</th><th>51.6</th><th>50.2</th><th>58.1</th><th>50.7</th><th>53.1</th><th>49.4</th><th>53.0</th><th>47.6</th><th>41.9</th><th>49.4</th><th>47.8</th><th>48.8</th><th>44.3</th><th>41.9</th><th>47.7</th><th>45.5</th><th>37.7</th></tr></thead><tbody><tr><th>S</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th>Fe</th><td>42.5</td><td>47.4</td><td>48.7</td><td>40.6</td><td>47.7</td><td>44.3</td><td>44.6</td><td>39.1</td><td>30.8</td><td>35.3</td><td>27.4</td><td>26.6</td><td>22.3</td><td>25.8</td><td>22.1</td><td>16.5</td><td>22.8</td><td>17.0</td></tr><tr><th>Ni</th><td>0.8</td><td>1.0</td><td>1.1</td><td>1.3</td><td>1.5</td><td>2.6</td><td>6.0</td><td>7.9</td><td>21.6</td><td>22.7</td><td>23.1</td><td>25.6</td><td>28.9</td><td>24.3</td><td>9.4</td><td>0.0</td><td>0.8</td><td>2.9</td></tr><tr><th>Cu</th><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>3.3</td><td>11.3</td><td>4.6</td><td>4.5</td></tr><tr><th>Zn</th><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>b.d.</td><td>5.6</td><td>23.4</td><td>24.5</td><td>26.3</td><td>37.9</td></tr></tbody></table>

diffraction (SAED) pattern of the transjordanite in Figure 7c shows a ring pattern (Figure 7e), which means that it is an aggregate composed of randomly oriented tiny crystals of transjordanite (Tjrd). Transjordanite in Ryugu grains always occurs as such aggregates.

In the radial intensity profile (Figure 7f), the diffraction lines of transjordanite overlap on the intensity curve. Four strong diffraction lines of transjordanite indicated by asterisks coincide with the peaks in the intensity curve.
An ~200-nm-sized moissanite (SiC) crystal was identified in FIB section A0104-00200601 (Figure 8a). Its SAED patterns reveal that its polytype is 3C (cubic) with streaks along g* = 1 1-1, a result of stacking disorder (Figure 8b). The SiC found in Ryugu has a different crystal structure from industrial SiC, which is hexagonal, and is thus not a laboratory contaminant. Elemental distribution maps show that the SiC in a Ryugu grain is enriched in C and Si relative to the surrounding matrix (Figure 8c). The C and Si isotopic compositions of the SiC were measured by NanoSIMS. Figure 8d,e show 20 × 20 µm²-sized overview ion images of ¹²C⁻ and ²⁸Si⁻. The SiC grain is marked by a white circle in Figure 8e. Detailed ion images of a 2 × 2 µm²-sized area in the FIB section are shown in Figure 8f,g, in which the SiC grain is outlined by white curves. Its ¹²C/¹³C ratio is 80.2 ± 4.4‰ or δ¹³C = 121 ± 61‰, relative to CI composition. The slight enrichment in ¹³C has a significance of 2σ compared to CI. Because synthetic SiC has δ¹³C of around -30‰ relative to Pee Dee Belemnite (PDB) (or -20‰ relative to CI), the significance relative to synthetic SiC would be 2.3σ. δ²⁹Si is -3 ± 40‰, and δ³⁰Si is 25 ± 51‰, that is, Si is normal within the uncertainties of 4%–5%. Figure 8h shows a histogram of ¹²C/¹³C ratios of presolar SiC grains; the ratio for the SiC from Ryugu is plotted by a pink bar. The width of the bar represents the ±1σ error bar. In Figure 8i, the Si-isotopic composition of the SiC grain from Ryugu is plotted together with mainstream and Type X presolar SiC grains. Overall, the isotope data of the SiC grain from Ryugu are compatible with presolar mainstream SiC grains.

## Olivine- and Low-Ca Pyroxene-Bearing (OP-Bearing) Grains

In the previous sections, the mineralogy of the majority of Ryugu grains was described. In this section, we describe Ryugu grains showing mineralogy and petrology that are distinct from the majority of Ryugu grains. These grains have spongy morphology and they can be distinguished easily from the other grains only using FE-SEM (Figure 9a,b). They often contain anhydrous minerals such as Mg-rich olivine (Fo₉₇.₈–₉₉.₇) crystals (Figure 9c) and low-Ca pyroxene (Figure 9d). Therefore, we call them olivine- and low-Ca pyroxene-bearing (OP-bearing) grains. Anhydrous minerals are embedded in a highly porous hydrated matrix (Figure 9b) composed of curved fibrous phyllosilicates (Extended Data fig. 7a in Noguchi et al., 2023). Measurements of ~1.0–1.1 nm lattice fringes suggest that the phyllosilicates are saponite. Their shorter (001) spacing also indicates the dehydration of interlayer H₂O molecules.

Olivine crystals have fragmental but subrounded shapes (Figure 9c). Olivine in the Ryugu grains is an

important recorder of shock and space weathering. Because space weathering of olivine in this type of Ryugu grains has been reported by Noguchi et al. (2023), we focus on the degree of deformation of olivine here. Two olivine grains from an OP-bearing grain were investigated. The local misorientation angles within two olivine grains were measured by the TKD method (Figure 9f–k). Figure 9f,i are BSE images of the two FIB sections. Two olivine crystals enclosed by white open rectangles in Figure 9f,i were measured using TKD. Figure 9g,j show the average local misorientation angles of both olivine grains are 0.49 ± 0.21 (1σ) and 0.41 ± 0.38 (1σ) degrees, respectively. Local misorientation (kernel average misorientation, KAM) maps show that the misorientation angles do not vary significantly within each (sub)grain as suggested by their homogeneous blue color (Figure 9h,k). These results are similar to those from the EBSD study of ordinary chondrites where hundreds of olivine grains of known shock state were mapped under identical conditions (Zolensky et al., 2022). We also observed these two olivine grains using low-angle annular dark field (LAADF) imaging (Figure 9l,m). No free dislocation and subgrain boundaries were observed in the olivine grain shown in Figure 9f–h,l. We found a possible free dislocation and subgrain boundaries in the olivine grain shown in Figure 9i–k,h. A dislocation array is found at a subgrain boundary although we did not determine their Burgers vectors. There is an insufficient number of dislocations in these olivine grains by which the number density of free dislocations can be estimated. We realize that two individual grains are not a sufficient population size to generalize to an entire asteroid, nor even a single meteorite. However, these are all the data we could gather during the brief period of the Hayabusa2 preliminary examination activities. We expect that future more comprehensive EBSD and TEM studies on a much larger suite of the Ryugu samples will yield a more confident result.

An Mg-rich low-Ca pigeonite (Wo₂En₉₇Fs₁-Wo₁En₉₈Fs₁) (Figure 9d) shows a euhedral-shaped zoning pattern indicated by arrows in an Al map in Figure 9e. The other anhydrous minerals found in the OP-bearing grains are spinel (Sp), transjordanite, and schreibersite (Fe, Ni)₃P (Scb) (Figure 10a–c). In addition to these anhydrous minerals, calcite is one of the characteristic minerals found in the OP-bearing grains (Figure 10d–f). The calcite crystal shows multiple twin lamellae with {1 0-1 8} twin planes, suggestive of deformation (Figure 10e,f).

In OP-bearing grains, rare poorly crystalline phyllosilicates occur as aggregates (<2–3 µm across; Figure 11). These aggregates are embedded in coarser phyllosilicates (Figure 11a,b). The aggregates seldom contain other minerals such as Fe-Ni sulfides. Radial intensity profiles were calculated from SAED patterns of both coarser surrounding phyllosilicates and the poorly
<figure><img src="image_13.png" /><figcaption>FIGURE 8. Minor phases (4) moissanite. (a, b) BF-TEM image and SAED pattern of moissanite (SiC). The moissanite is enclosed by a red circle in (a). Its SAED shows that the crystal is moissanite-3C. (c) Elemental distribution maps of moissanite, which show enrichments in C and Si above the surrounding matrix. (d) and (e) Overview ion images of ¹²C⁻ and ²⁸Si⁻ in the FIB section recorded with the NanoSIMS. The moissanite grain is located within the white circle. The field of view (FOV) is 20 × 20 µm². (f, g) Detailed ion images of the moissanite grain, which is outlined by white curves in each image. The grain has ¹²C/¹³C = 80.2 ± 4.4 or δ¹³C = 121 ± 61‰ relative to CI, δ²⁹Si = -3 ± 40‰, and δ³⁰Si = 25 ± 51‰. δ²⁹,³⁰Si = [(²⁹,³⁰Si/²⁸Si)grain/(²⁹,³⁰Si/²⁸Si)terrestrial-1]] × 1000. FOV: 2 × 2 µm². (h) Histogram of ¹²C/¹³C ratios of presolar SiC grains; a few grains are off-scale to the right. The ratio for the moissanite grain from Ryugu is shown in pink; the width of the bar represents the ±1 sigma error bar. (i) Si-isotopic composition of the moissanite from Ryugu along with those of mainstream and Type X presolar SiC grains for comparison. Carbon- and Si-isotopic data for presolar SiC grains in panels H and I are taken from the Washington University Presolar Grain Database (Stephan et al., 2021). Sample number: A0104-00200601. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>
crystalline phyllosilicates (Figure 11c). The poorly crystalline phyllosilicates show peaks with d-spacings close to ~0.44, ~0.25, and ~0.15 nm that are broader and weaker than those in the coarser surrounding phyllosilicates. Poorly crystalline silicates in the matrices of primitive carbonaceous chondrites such as Acfer 094, Paris, and A-12164 have broad and weak peaks with d-spacing close to ~0.25 and ~0.15 nm (e.g., Leroux et al., 2015; Matsumoto et al., 2019; Noguchi et al., 2021). A broad and weak peak at ~0.45 nm was also reported in Acfer 094 (Matsumoto et al., 2019). These similarities suggest that the crystallinity of the poorly crystalline phyllosilicates shown in Figure 11a,b is comparable with that of these (nominally) amorphous silicates in the carbonaceous chondrites. By contrast, the coarser surrounding phyllosilicates have a higher crystallinity than the poorly crystalline phyllosilicates (Figure 11c). They show a wider Mg/(Mg + Fe) variation and lower Mg/(Mg + Fe) ratios than the coarse phyllosilicates (Figure 11d).

## Space-Weathered Small Ryugu Grains

Over 1000 grains were observed by FE-SEM and ~6% of small Ryugu grains in both A0104 and C0105 were found to have detectable surface modification related to space weathering (Noguchi et al., 2023). Noguchi et al. (2023) reported three major types of surface modifications: smooth layers, frothy layers, and melt splashes. They considered that the smooth layers were formed by solar irradiation by comparison with the textures of 4 keV He⁺ ion irradiated surface of a non-space-weathered Ryugu grain. They concluded that the frothy layers were formed by meteoroid impact heating and melting by comparison with the pulse laser heating experiments on the Murchison meteorite. In addition, they suggested that the texture of the frothy layer suggests in situ melting, probably related to the frictional heating among loosely packed regolith grains on Ryugu caused by meteoroid impact. Comparisons with the space weathering of Itokawa grains were also reported by Noguchi et al. (2023).

Chemical compositional maps of a smooth layer are shown in Figure 12a. There is no detectable compositional difference between the smooth layer and the phyllosilicate-rich matrix beneath the smooth layer (Figure 12a). This result is consistent with Noguchi et al. (2023). Chemical compositional maps of two frothy layers on small Ryugu grains and a frothy layer on a large Ryugu grain are shown in Figure 12b-d. It is clear that Fe-Ni sulfide grains with <50 nm across are densely distributed in the frothy layers. The frothy layers contain minerals that were not melted within the duration of micrometeoroid impact heating. For example, a porous apatite is embedded in a

frothy layer (Figure 13a,b). Its SAED pattern clearly shows it is crystalline although it contains many pores, which may be related to a short heating duration. Phyllosilicates below the frothy layer locally show lattice fringes of ~1.0 nm thick (Figure 14). However, they easily lost their lattice fringes during analysis, and the oxygen/cations ratios in these phyllosilicates obtained by EDS analysis were almost anhydrous, which suggests that they were more sensitive to electron beam exposure than phyllosilicates in non-space-weathered grains (Noguchi et al., 2023).

## Severely Melted Ryugu Grains

Some Ryugu grains from TD#2 site showed the most severe surface melting observed during the initial analysis of our team (Figure 15). The small Ryugu grain shown in Figure 15a has a unique, complex, rounded surface covered with abundant bright spots, probably sulfides, which can be seen from the center to the bottom right of Figure 15a. The interior of the grain is shown in the center left of Figure 15a, indicated by a rectangle. Figure 15b shows that the interior has an extremely high vesicularity (abundance and density of vesicles), indicative of the violent emission of volatiles from the grain when it was heated. Bubbles are interconnected and solidified melt appears as a network. Elongated bubbles indicated by arrows in Figure 15b suggest that they may have been slightly compressed or drawn out by flow during formation. Another example of the most severely modified grain is shown in Figure 15c. Its cross section is shown in Figure 15d. A frothy layer completely covers this grain as indicated by arrows.

# DISCUSSION

## Comparison of Mineralogy with those of mm-Sized Ryugu Grains and CI Chondrites

Major minerals identified by (S)TEM in this study are phyllosilicates (saponite and serpentine), Fe-bearing sulfides, magnetite, dolomite, and a lesser amount of Mg-Fe-carbonate, breunnerite. These minerals are consistent with the major phases identified using SEM-EDS and electron microprobe in previous works on large (mm-sized) Ryugu grains (Table 3; Ito et al., 2022; Nakamura et al., 2022, 2023; Nakato et al., 2022; Yamaguchi et al., 2023; Yokoyama et al., 2023).

Minor to rare minerals, hydroxyapatite, Na-Mg phosphate, Na-rich phyllosilicate, poorly crystalline phyllosilicate, olivine, low-Ca pyroxene, high-Ca pyroxene, Mg-Al spinel, hibonite, perovskite, Mn-bearing ilmenite, manganese chromite, eskolaite, sphalerite, cubanite, daubréelite FeCr₂S₄, zolenskyite (high-pressure
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<figure><img src="image_15.png" /><figcaption>(c)</figcaption></figure>

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FIGURE 9. Olivine- and low-Ca pyroxene-bearing (OP-bearing) grains. (a, b) SEIs of a porous Ryugu grain characteristically containing almost-Fe-free olivine and low-Ca pyroxene and its enlarged image. The OP-bearing grains are present as one out of every ~100 small Ryugu grains. They also experienced aqueous alteration as suggested by spherulitic magnetite. (c) BF-TEM image and SAED pattern of olivine. (d) BF-TEM image and SAED pattern of low-Ca pyroxene. (e) The elemental distribution maps of the low-Ca pyroxene crystal in (c) and (d). The aluminum distribution map shows that this crystal probably grew in a melt. The zoning pattern indicated by arrows suggests their facets during crystallization. Local misorientation (kernel average misorientation: KAM) maps were obtained from two olivine crystals in A0104-0090080 using transmission Kikuchi diffraction (TKD). (f) A backscattered electron (BSE) image of the section A0104-00900802. An olivine crystal enclosed by a white open rectangle in (f) was measured using TKD. (g) A local misorientation diagram of the olivine in (f). (h) A KAM map of the olivine crystal shown in (f). (i) A BSE image of the section A0104-00900501. An olivine crystal enclosed by a white open rectangle in (i) was measured using TKD. (j) A local misorientation diagram of the olivine in (i). (k) A KAM map of the olivine crystal shown in (i). This grain is composed of three subgrains with slightly different orientations. However, the misorientation angle within each subgrain is very small, which is suggested by the homogeneity of color within each subgrain. (l) Low-angle annular dark field (LAADF)-STEM image of the olivine grain shown in (f) to (h). No free dislocation was observed. (m) LAADF-STEM image of the olivine grain shown in (i) to (k). A possible free dislocation (Dsl?) was observed. As shown in the TKD analysis, this grain is composed of three subgrains. A dislocation array (Dsl array) is present at a subgrain boundary. Sample numbers: A0104-02902500, A0104-00900802, A0104-00900501. (Color figure can be viewed at wileyonlinelibrary.com)

<figure><img src="image_27.png" /></figure>

FIGURE 10. Spinel (SpI), schreibersite (Scb), and calcite (CaI) in the OP-bearing grains in Figure 9. (a) BF-TEM and SAED of spinel. (b) BF-TEM and SAED of schreibersite. (c) Elemental distribution maps of the schreibersite in (b). (d) to (f) BF-TEM and SAED patterns of calcite. SAED patterns show that it has {1 0-1 8} twinning, suggestive of mechanical twinning. Sample numbers: A0104-00801206, A0104-00000501, A0104-02403104.

phase of daubréelite), schreibersite, allabogdanite (Fe, Ni)₂P, calcite, aragonite, tochilinite, digenite, PGE sulfide, wüstite, metallic iron, and graphite were also reported in mm-sized Ryugu grains (Table 2; Ito et al., 2022; Kawasaki

et al., 2022; Kimura et al., 2022; Liu et al., 2022; McCain
et al., 2023; Mikouchi et al., 2022; Matsuno et al., 2022;
Nakamura et al., 2023; Tomioka et al., 2023; Yamaguchi
et al., 2023). In addition, pyrrhotite with various polytypes,
<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

FIGURE 11. Poorly crystalline phyllosilicate in an OP-bearing clast. Comparison of textures of (a) coarser and (b) poorly crystalline phyllosilicates. SAED patterns were obtained from Areas I and II in these BF-TEM images. Insets of (a) and (b) show that the crystallinity of phyllosilicates is obviously different between the two areas. (c) Radial intensity profiles of the two SAED patterns obtained from Areas I and II. The integrated peaks in Area II are lower and broader than those in Area I, which indicates lower crystallinity in poorly crystalline phyllosilicates than in coarser phyllosilicates. (d) The chemical compositions of the poorly crystalline phyllosilicates plot between the saponite and serpentine solid solution lines and some clasts have higher Fe relative to [Si + Al] and Mg. Sample numbers: A104-000700801, A0104-00000701, A0104-00000501. (Color figure can be viewed at wileyonlinelibrary.com)

FIGURE 12. Elemental distribution maps of a smooth layer and frothy layers on Ryugu grains. (a) A smooth layer. (b, c) Frothy layers on small Ryugu grains. (d) Frothy layer on a mm-sized grain A0058. The boundary between the smooth layer and the interior and the boundaries between the frothy layers and the interiors are indicated by yellow broken curves. In the case of (d), the frothy layer covers all the field of view, and the lower half of each panel shows the cross section of the frothy layer. All the frothy layers have more elevated Fe, S, and Ni than the interior phyllosilicate-rich matrix. Not only the tiny Fe-Ni sulfide particles but also host melt are enriched in these elements. In addition, Ca is also slightly enriched in (c). Sample numbers from left to right: A0104-02306901, C0105-03003701, A0104-02203703, A0058-C2001-02. (Color figure can be viewed at wileyonlinelibrary.com)
<figure><img src="image_32.png" /></figure>
<figure><img src="image_33.png" /><figcaption>FIGURE 13. Relict apatite in the frothy layer on a mm-sized Ryugu grain A0058. (a, b) HAADF-STEM and elemental distribution maps of the apatite-rich area of the frothy layer, and SAED pattern of the porous apatite. The porous morphology of apatite (Ap) suggests that it was probably partially decomposed during melt formation although it keeps high crystallinity. The abbreviation V is a vesicle. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

polydymite Ni₃S₄, and transjordanite (Ni, Fe)₂P, moissanite, and periclase were reported in fine-grained Ryugu grains (Table 3; Dobrică et al., 2023; Harries et al., 2022; Matsumoto et al., 2022; Nakato et al., 2022; Noguchi et al., 2023; Thompson et al., 2022; this study).

These minerals with the exception of perovskite and polydymite as well as space weathering-related phases such as wüstite, metallic iron, and periclase have not been reported in CI chondrites yet (e.g., Brearley & Jones, 1998; Endreß & Bischoff, 1996; Frank et al., 2011; Leshin
<figure><img src="image_34.png" /><figcaption>FIGURE 14. BF-TEM images of the boundary between a frothy layer and the phyllosilicate-rich matrix. (a) Boundary between the bottom of the frothy layer and the phyllosilicate-rich matrix. (b) An enlarged image of the boxed area in (a). Lattice fringes of saponite are observed near the boundary between the frothy layer and the matrix. The concentric Fresnel fringes in (b) are artifacts formed by diffraction due to a small condenser aperture. BF-TEM. Sample number: C0105-03003701.</figcaption></figure>

et al., 1997; Rubin & Ma, 2021 and references therein; Mikouchi et al., 2022). Although schreibersite, zolenskyite or daubréelite (FeCr₂S₄), and hibonite have not been reported in CI chondrites until recently, they have recently been reported in CI chondrites (Table 3; Ando et al., 2022). Moissanite (SiC) has been known as one of the presolar grains in the Orgueil CI chondrite (Huss et al., 1997).

Although polydymite Ni₃S₄ and transjordanite (Ni, Fe)₂P have not been found among mm-sized Ryugu grains and CI chondrites, the majority of minerals found among the fine-grained Ryugu grains, mm-sized Ryugu grains, and CI chondrites are common. The exception is the absence of ferrihydrite and sulfates in Ryugu samples. Ferrihydrite and several sulfates such as gypsum, bassanite, and bloedenite are common among CI chondrites (e.g., Brearley & Jones, 1998). However, none have been identified yet among the Ryugu grains investigated. It has been noted that sulfates in CI chondrites are of terrestrial origin (Gounelle & Zolensky, 2014), and the absence of sulfates in the investigated Ryugu grains strongly supports this interpretation of their origin. It is also known that fine-grained phyllosilicates in Orgueil CI chondrite are intimately intergrown with ferrihydrite Fe³⁺₁₀O₁₄(OH)₂ (Tomeoka & Buseck, 1988). In Figure 2b, a ternary diagram shows the chemical compositions of coarse-grained and fine-grained phyllosilicates in Orgueil

(Tomeoka & Buseck, 1988) and those in Ivuna CI chondrite analyzed in this study. In both cases, the fine-grained phyllosilicates plot with Fe-enriched chemical compositions compared to the coarse-grained ones because of their intimate mixing with ferrihydrite although ferrihydrite is also distributed throughout these CI chondrites. By contrast, the fine-grained phyllosilicates in the Ryugu samples intimately coexist with tiny Fe-Ni sulfide grains (Figure 2a,d) instead of ferrihydrite. These observations suggest that ferrihydrite in CI chondrites may have been an alteration product of tiny Fe-Ni sulfide grains by terrestrial weathering. The most likely hypothesis is that CI chondrites share the same mineralogy as Ryugu before the CI chondrites were affected by terrestrial weathering. This interpretation is consistent with all the previous studies of the mm-sized Ryugu grains (Ito et al., 2022; Nakamura et al., 2022, 2023; Yamaguchi et al., 2023; Yokoyama et al., 2023).

## Comparison of Mineralogy with that of CI Chondrite-Like Clasts in Meteorites

CI chondrite-like clasts are present in the Nilpena polymict ureilite find (Brearley & Prinz, 1992) and the Tsukuba H chondrite regolith breccia fall (Nakashima et al., 2003). Both clasts contain saponite, serpentine, magnetite, pentlandite, and pyrrhotite (Brearley & Prinz,
<figure><img src="image_35.png" /><figcaption>FIGURE 15. Severely melted Ryugu grains. (a) A partially melted Ryugu grain. SEI. (b) An enlarged image of the open boxed area indicated by a rectangle in (a). The interior is filled with densely packed bubbles and melt exists as a network. Elongated bubbles are indicated by arrows. SEI. (c) A frothy layer on a Ryugu grain. (d) A cross section of the grain shown in (c). This grain is completely enclosed by a frothy layer as indicated by arrows. Sample numbers: C0105-03020500 and C010509000.</figcaption></figure>

1992; Nakashima et al., 2003). Like Ryugu samples, neither CI-like clasts contain sulfates, supporting the likely terrestrial origin of these minerals. Ferrihydrite, however, is only present in the clast in the Nilpena find (Brearley & Prinz, 1992). Terrestrial weathering is likely more limited in ferrihydrite-free Tsukuba, which was recovered immediately after its fall and analyzed 6 years later. In contrast, the Nilpena find was probably exposed to the terrestrial environment for a longer time, suggesting that the ferrihydrite in these CI-like clasts formed on Earth.

## Comparison of Mineralogy with that of Hydrated IDPs and AMMs

Interplanetary dust particles that experienced severe aqueous alteration are known as chondritic smooth (CS) IDPs (e.g., Bradley, 2014 and references therein). There

are also many H-FgAMMs (e.g., Dobrică et al., 2019; Klöck & Stadermann, 1994; Noguchi et al., 2002a), and most of them contain only saponite as phyllosilicates. However, there are IDPs with CI-like mineralogy (Keller et al., 1992), and ~20% of H-fg AMMs contain both saponite and serpentine as phyllosilicates (Noguchi et al., 2002b). Except for the presence or absence of serpentine, the other minerals in these hydrated IDPs and AMMs are very similar to those of CI chondrites and Ryugu grains. They contain framboidal, spherulitic, and plaquette magnetite, pyrrhotite, dolomite, and Mg- and Fe-carbonate as major minerals but lack ferrihydrite and sulfates. The observations provide further support for the terrestrial origin of most ferrihydrite and sulfates in CI chondrites. Of course, due to the small sizes of the hydrated IDPs and most of the hydrated AMMs, it is not unexpected that rare minerals such as ilmenite and sphalerite found in CI chondrites and Ryugu grains have
<table><caption>TABLE 3. Identified phases and objects by different teams of the analysis of Ryugu samples performed from June 2021 to May 2022 and comparison with those in CI chondrites.</caption><thead><tr><th>Team name</th><th>Identified minerals</th><th>Essential minerals</th><th>Accessory minerals</th><th>Identified objects</th><th>References</th></tr></thead><tbody><tr><td>Chemistry</td><td>Saponite, serpentine, magnetite (framboids, plaquettes, spherulitic), pyrrhotite, pentlandite, dolomite, breunnerite</td><td></td><td>Hydroxyapatite, Mg-Na phosphate, Na-rich phyllosilicate, olivine, low-Ca pyroxene, Mg-Al spinel, chromite, ilmenite, cubanite</td><td>Olivine- and low-Ca pyroxene-bearing clasts</td><td>Yokoyama et al. (2023), Kawasaki et al. (2022)</td></tr><tr><td>Stone (Min-Pet Coarse)</td><td>Ditto [pyrrhotite has 4C polytype]</td><td></td><td>Hydroxyapatite, Mg-Na phosphate, Na-rich phyllosilicate, olivine, low-Ca pyroxene, high-Ca pyroxene, Mg-Al spinel, hibonite, perovskite, Mn- bearing ilmenite, manganochromite, sphalerite, cubanite, daubréelite, schreibersite, allabogdanite, calcite, aragonite, tochilinite, digenite, PGE sulfides, wüstite, metallic iron, gypsum<sup>a</sup></td><td>Chondrule-like objects, CAIs, less altered clasts, GEMS-like objects, fluid inclusion in pyrrhotite</td><td>Nakamura et al. (2022), Mikouchi et al. (2022), Matsuno et al. (2022), Kimura et al. (2022), Nakashima et al. (2023), Nakato et al. (2022)</td></tr><tr><td>Sand (Min-Pet Fine)</td><td>Ditto (pyrrhotite has 4C, 5C, and 6C polytypes, and some areas of pyrrhotite are troilite)</td><td></td><td>Hydroxyapatite, Mg-Na phosphate, Na-rich phyllosilicate, olivine, low-Ca pyroxene, Mg-Al spinel, chromite, manganochromite, eskolaite, ilmenite, cubanite, mackinawite, polydymite, transjordanite, schreibersite, calcite, moissanite, poorly crystalline phyllosilicate, metallic iron, periclase</td><td>Olivine- and low-Ca pyroxene-bearing grains</td><td>Noguchi et al. (2023), Harries et al. (2022), Matsumoto et al. (2022), Thompson et al. (2022), Dobricā et al. (2023), Nakato et al. (2022), this study</td></tr><tr><td>P2K</td><td>Ditto</td><td></td><td>Hydroxyapatite, Mg-Na phosphate, Na-rich phyllosilicate, olivine, low-Ca pyroxene, spinel, eskolaite, ilmenite, cubanite, zolenskyite, graphite, aragonite, poorly crystalline phyllosilicate</td><td>Chondrule remnant, AOA remnant</td><td>Ito et al. (2022), Liu et al. (2022), Tomioka et al. (2023), McCain et al. (2023), Yamaguchi et al. (2023)</td></tr><tr><td>P2M</td><td>Ditto</td><td></td><td>Hydroxyapatite, mejillonesite, olivine, low-Ca pyroxene</td><td>Chondrule remnant, AOA remnant</td><td>Nakamura et al. (2022)</td></tr><tr><td>CI chondrites (for comparison)</td><td>Saponite, serpentine, magnetite (framboids, plaquettes, spherulitic), pyrrhotite, pentlandite, dolomite, Mg-Fe carbonate, ferrihydrite, sulfates</td><td></td><td>Hydroxyapatite, olivine, low-Ca pyroxene, ilmenite, calcite, hibonite, Mg-Al spinel, cubanite, merrillite, periclase, brucite, Ti<sub>3</sub>O<sub>5</sub>, magnesiochromite, eskolaite, Zn- bearing FeCr<sub>2</sub>S<sub>4</sub>, (Fe, Ni)<sub>2</sub>P, PGE metals</td><td>Chondrule, CAI, olivine- and low-Ca pyroxene clasts, ilmenite-rich clasts</td><td>Tomeoka and Buseck (1988), Brearley and Prinz (1992), Endreß and Bischoff (1996), Leshin et al. (1997), Brearley and Jones (1998) and references therein, Frank et al. (2011), Rubin and Ma (2021) and references therein; Mikouchi et al. (2022), Ando et al. (2022)</td></tr></tbody></table>

Abbreviation: PGE, platinum group elements.

<sup>a</sup>Formed after exposure to the Earth's atmosphere.
not been identified in them yet. It is expected that CI
chondrites, Ryugu grains, and hydrated IDPs and
AMMs experienced similar aqueous alteration in their
parent bodies. Thermodynamic calculation of CI
chondritic material shows that if the aqueous alteration
temperature increases at elevated pressures, the saponite/
serpentine ratio increases (Zolotov, 2012). The parent
bodies of the hydrated IDPs and most of the hydrated
AMMs might have experienced higher alteration
temperatures than CI chondrites and Ryugu materials
although more analyses are needed for these IDPs
and AMMs.

Degree of Dehydration of Phyllosilicates

The mineralogy of small Ryugu grains indicates that
the asteroid Ryugu experienced severe aqueous alteration
comparable to CI chondrites and did not experience
post-aqueous alteration heating that was sufficient
(>500°C) to generate secondary anhydrous minerals such
as olivine, pyroxene, and Fe metal, which are common in
severely heated carbonaceous chondrites (e.g., Endreß &
Bischoff, 1996). It is well known from studies of
terrestrial saponite that water (H₂O) molecules can easily
enter or leave the interlayer spaces parallel to (001) in
saponite, which results in a change of (001)-spacing
in response to the availability of H₂O, for example, the
amount of atmospheric moisture (e.g., Grim &
Bradley, 1948). The (001)-spacing of saponite containing
interlayer H₂O is ~1.6 nm in the atmosphere when
interlayer H₂O are fully introduced in saponite:
R₀.₆₆⁺(Mg₅.₃₄R₀.₆₆³⁺)(Si₆.₆₈Al₁.₃₂)O₂₀(OH)₄₈H₂O (e.g.,
Moore & Reynolds Jr., 1997). In this structure, saponite
has a positive charge in the tetrahedral sheets and a
negative charge in the octahedral sheets. Interlayer
cations compensate for the total charge valance of
saponite.

If complete loss of the interlayer H₂O molecules
and the migration of interlayer cations into the 2:1 (a
combination of tetrahedral–octahedral–tetrahedral
sheets) layers occur, the (001) spacing of expansive clay
minerals, including saponite, would not expand even after
the exposure to water or the ethylene glycol treatment.
This is known as irreversible collapse among smectite
studies, which means that each 2:1 layer binds strongly to
each other due to the electrostatic forces between
neighboring 2:1 layers (e.g., Bish, 1988; Grim &
Bradley, 1948). It is clear that saponite in Ryugu surface
samples did not experience the irreversible collapse of its
layer structure on Ryugu. When saponite was heated at
700°C at 10⁻³ to 10⁻⁴ total pressure, the (001)-spacing of
saponite shrunk to ≤1.0 nm and was not recovered after
exposure to the air (Nozaki et al., 2006). Therefore, it is
expected that the Ryugu grains were not heated to such

temperatures on Ryugu or in the ancestor(s) of Ryugu.
This estimation is consistent with all the results obtained
by the other groups (e.g., Ito et al., 2022; Nakamura
et al., 2022, 2023; Naraoka et al., 2023; Yabuta et al.,
2023; Yamaguchi et al., 2023; Yokoyama et al., 2023).
For example, Naraoka et al. (2023) estimated the
effective heating temperature of mm-sized Ryugu grains
was ≤150°C based on soluble organic matter in Ryugu
samples.

Yokoyama et al. (2023) reported that interlayer H₂O
molecules in saponite had been lost on Ryugu based on
the thermogravimetric analysis. This means that saponite
in Ryugu samples did not experience the irreversible
collapse of the layer structure of saponite even though
most of the interlayer H₂O molecules were lost from
saponite. As described above, the (001) spacing of
irreversibly collapsed saponite is ≤1.0 nm. By contrast,
TEM observations of saponite in Ryugu grains show that
the (001)-spacing of saponite ranges from ~1.0 to
~1.2 nm, which may support that the migration of
interlayer cations into the 2:1 layers did not occur in
Ryugu saponite, that is, the irreversible collapse did not
occur in Ryugu saponite. The reversible dehydration of
saponite might occur at temperatures low enough not to
preclude reversible rehydration in the ancestor(s) of
Ryugu or the reversible dehydration might be caused by
vacuum drying during exposure to the ultrahigh vacuum
(10⁻¹⁴ Pa) of interplanetary space (Horneck, 2015) on the
surface of Ryugu. Because the returned samples existed
on the surface of Ryugu, it is expected that TEM
observation performed at ~10⁻⁵ Pa did not affect the
(001) spacing of saponite in the samples. Yabuta et al.
(2023) and Viennet et al. (2023) reported the presence of
NH-bearing organics trapped between the interlayer
spaces of saponite in Ryugu samples. This could be
another factor in maintaining the (001)-spacing of
saponite of ~1.0 to ~1.2 nm and the organic molecules
might contribute to inhibiting the irreversible collapse of
Ryugu saponite.

Comparison of OP-Bearing Ryugu Grains with OP-
Bearing Clasts in mm-Sized Ryugu Grains

It is well documented that calcite is extremely rare in
CI chondrites (e.g., Endreß & Bischoff, 1996) and it is the
major carbonate in CM and CR chondrites (e.g., Brearley
& Jones, 1998). Calcite in CI chondrites was reported
from specific clasts containing minor olivine and low-Ca
pyroxene (e.g., Lee & Nicholson, 2009). The occurrence
of calcite in OP-bearing clasts in Ryugu is similar to the
case of CI chondrites (Kawasaki et al., 2022; Mikouchi
et al., 2022; Nakamura et al., 2023). Nakamura et al.
(2023) pointed out that both calcite and aragonite were
found in the OP-bearing clasts and that schreibersite and
allabogdanite also selectively exist in the clasts. In our case, OP-bearing grains always appear as porous grains (Figure 9a,b). Mineralogy of the other minor minerals: calcite, aluminous spinel, and schreibersite were also very similar to the OP-bearing clasts. Although a few differences exist between them; for example, the presence of transjordanite (Ni, Fe)₂P and the absence of allabogdanite (Fe, Ni)₂P in the OP-bearing grains, the majority of minerals in the grains and the clasts are common, which suggests that the OP-bearing grains might be the OP-bearing clasts that detached from the host matrix during fragmentation during regolith formation and gardening. As suggested by Nakamura et al. (2023) and Kawasaki et al. (2022), a slightly weaker aqueous alteration was experienced by these OP-bearing grains and clasts.

Interestingly, there is a morphological difference between the OP-bearing grains and the OP-bearing clasts in mm-sized grains. The OP-bearing grains (Figure 9a,b) are more porous than the OP-bearing clasts based on their BSE images (e.g., fig. 2a in Kawasaki et al., 2022). Their porous morphology is diagnostic to distinguish these grains from the other small Ryugu grains using SEM. Because the OP-bearing grains contain many cracks, it is impossible to make FIB sections of these grains without embedding them in epoxy resin. The abundant cracks obviously contribute to the high porosity of the OP-bearing grains. They may have been formed during the detachment of the OP-bearing clasts from the host phyllosilicate-rich matrix during regolith formation and gardening. The OP-bearing clasts may be more fragile than the host. Nakamura et al. (2023) showed that OP-bearing clasts that had experienced lower degrees of aqueous alteration are more porous than the other clasts and the host that had experienced higher degrees of aqueous alteration. One possibility of the origins of the porous nature of the OP-bearing grains might be related to a slightly lower degree of aqueous alteration experienced by the OP-bearing grains than that experienced by the OP-bearing clasts. The OP-bearing grains contain poorly crystalline phyllosilicate rounded aggregates with ~2 µm in diameter (Figure 11b), which were not reported in the OP-bearing clasts. Their presence may support the slightly lower degree of aqueous alteration. Another is that the phyllosilicates in the OP-bearing grains might be more susceptible to dehydration when exposed to the ultrahigh vacuum of the interplanetary space 10⁻¹⁴ Pa (Horneck, 2015), which might result in higher porosity by dehydration.

We found OP-bearing grains from the fine-grained samples collected at both TD#1 and TD#2 sites. However, Nakamura et al. (2023) and Kawasaki et al. (2022) found OP-bearing clasts in the samples collected only at the TD#2 site. These differences may reflect the

heterogeneous distribution of the rare OP-bearing clasts in mm-sized Ryugu grains. However, it is difficult to distinguish if the rare OP-bearing clasts distribute randomly or heterogeneously in Ryugu material.

In addition to the OP-bearing clasts, isolated olivine crystals, small ferromagnesian chondrule-like objects, and calcium- and aluminum-rich inclusions (CAIs) were reported from the mm-sized grains (e.g., Ito et al., 2022; Kawasaki et al., 2022; Liu et al., 2022; Nakamura et al., 2022, 2023; Nakashima et al., 2023; Yamaguchi et al., 2023; Yokoyama et al., 2023). However, such objects were not found in the samples allocated to our team, which is probably related to even lower abundances of these objects than the OP-bearing clasts (e.g., the abundance of olivine is ~0.5 area%; e.g., Kawasaki et al., 2022; Nakamura et al., 2023).

## Shock Effects and Shock Pressure and Temperature

Because Ryugu is a rubble pile asteroid (Watanabe et al., 2019) whose surface is exposed to interplanetary space, constituents of Ryugu must have experienced shock events due to impactors with various sizes from sub-µm to asteroid-sized bodies. The average impact velocity of meteoroids at 1 astronomical unit (au) was estimated to be ~20 to ~24 km s⁻¹ (Grün et al., 1985; McNamara et al., 2004). Because the average orbital radius of Ryugu is 1.2 au (Watanabe et al., 2019), the average impact velocity is probably lower than ~20 km s⁻¹. If meteoroids impact Ryugu at such high velocities, it is likely to cause melting and degassing from the outermost surface of Ryugu grains (Noguchi et al., 2023).

When larger meteoroids impact on Ryugu, they would also cause melting and degassing from the Ryugu material at and near-by the impact points (Nakamura et al., 2023; Noguchi et al., 2023; Tomioka et al., 2023). Because the Ryugu grains are fragile (a Ryugu grain showed 4.9-MPa bending strength was reported by Nakamura et al., 2023), it is inferred that a large amount of ejecta, which do not experience severe shock, would be released from the areas distant from the impact points (e.g., Stöffler et al., 2018). Therefore, it is likely that most Ryugu grains investigated should not show strong shock features. Actuality, olivine in the Ryugu grains shows small crystallographic misorientations (their averages are smaller than 0.5°) and rare free dislocations (Figure 9f–m). Olivine with such small crystallographic misorientations is comparable with olivine in ordinary chondrites belonging to shock stage S2 (Stöffler et al., 2018; Zolensky et al., 2022). The other weak shock effects found in Ryugu grains are subparallel dislocations found in a large pyrrhotite crystal (Langenhorst et al., 2022) and compaction and deformation of phyllosilicates around
large rigid minerals such as pyrrhotite and dolomite (Lee et al., 2022). There is a similar issue in H-Fg AMMs. Although it is expected that the H-FgAMMs were released from asteroids by impacts, almost all of them show no evidence of significant shock and are classified ≤S2 (Dobrică et al., 2021).

Zolenskyite FeCr₂S₄, a high-pressure phase of daubréelite (Ma & Rubin, 2022), was found in a mm-sized Ryugu grain A0002 recovered from TD#1 site (Tomioka et al., 2023). They suggested that the grain experienced at least near 2 GPa considering the maximum shock heating temperature of ~500°C that was inferred from the preservation of phyllosilicates in the grain. By contrast, daubréelite, a low-pressure polymorph of zolenskyite, was found in mm-sized grains recovered from the TD#2 site (Nakamura et al., 2023). These differences might be related to the difference in the maximum pressure during shock events at the two sampling sites, or the two polymorphs might have experienced temperature and pressure conditions near the boundary between the stability fields of these phases.

Allabogdanite (Fe, Ni)₂P, a low-temperature phase of barringerite was found in the grains recovered at the TD#2 site (Nakamura et al., 2023). At ~2 GPa and ~500°C, allabogdanite is the stable phase based on ab initio calculation (Litasov et al., 2020). At this PT condition, both zolenskyite and daubréelite are stable. In addition, we found transjordanite (Ni, Fe)₂P in the OP-bearing grains collected at both sites. Transjordanite is a low-pressure polymorph of barringerite (Britvin et al., 2020). It occurs in an iron meteorite Cambria and probably Boriskino and ALH 83100 CM chondrites (Britvin et al., 2020). Buseck and Goldstein (1969) implied that barringerite could be formed by terrestrial weathering of schreibersite in Ollague pallasite. It could be possible that both allabogdanite and transjordanite in Ryugu grains were also formed by aqueous alteration of schreibersite at relatively low temperature and pressure conditions although it cannot be estimated at present. This hypothesis may be supported by the occurrence of transjordanite in CM chondrites. These data are also consistent with the shock classification S2.

## C-Type Asteroids as a Possible Source of Reactive Phosphorous to the Primordial Earth

Ryugu grains contain two different types of P-bearing minerals: phosphates such as hydroxyapatite Ca₅(PO₄)₃OH and phosphides. As described previously, Fe- and Ni-phosphides —transjordanite, allabogdanite, and schreibersite—were identified in Ryugu grains. Phosphides are likely more resistant to aqueous alteration than we expected. It has been proposed that schreibersite, which is ubiquitous in iron meteorites and enstatite chondrites, could

have been a possible source of active phosphorus on the early Earth (Kitadai & Maruyama, 2018; Pasek & Lauretta, 2005) although some irons such as Tishomingo lack phosphides (Rubin & Ma, 2021 and the references therein). Unlike phosphates such as hydroxyapatite, phosphides are chemically reactive in the Earth's surface environment and form various kinds of phosphorous species that lead to the formation of prebiotic phosphorylated compounds (e.g., Pasek et al., 2007; Pasek & Lauretta, 2005; Riston et al., 2020). The minor but ubiquitous presence of various phosphides in Ryugu grains suggests that C-type asteroids could be one of the possible sources of reactive phosphorous to the Earth.

Because oxygen isotopic ratios of Ryugu grains plot almost on the terrestrial fractionation line, like CI chondrites (Yokoyama et al., 2023), the oxygen isotopic composition of the Earth would have been scarcely affected by the addition of Ryugu-like C-type asteroids to the primordial Earth. By contrast, bulk H and N isotope compositions of Ryugu are not exactly the same as the terrestrial ocean and air; δD = +252 ± 13‰ relative to the Vienna Standard Mean Ocean Water (VSMOW) isotope reference, δ¹⁵N = +43.0 ± 9.0‰ relative to Earth atmospheric nitrogen (Naraoka et al., 2023). These values are similar to the previous studies of the Ivuna and Orgueil CI chondrites (Kerridge, 1985). Therefore, the contribution of C-type asteroids to the primordial Earth was probably limited, which is consistent in terms of the nonchondritic–chondritic dichotomy or trichotomy (Hopp et al., 2022; Warren, 2011).

## Severe Shock Heating and Melting

Only 0.3% of Itokawa grains show surface melted structures that resemble the frothy layers (Figures 12 and 13) found on Ryugu grains (Noguchi et al., 2023). By contrast, we found a partially melted grain showing a violent evolution of volatiles among fine-grained Ryugu samples (Figure 15a). Its internal structure is similar to terrestrial pumice grains (e.g., Klug et al., 2002). Vesicles are interconnected and melt exists as a three-dimensional network (Figure 15b). As indicated by arrows in Figure 15b, there are elongated vesicles, which were probably formed by coalescence. Because the directions of elongation of these vesicles are almost oriented, the partial melt might have flowed before solidification. The morphology and texture of this grain are similar to those of “scoriaceous” micrometeorites that experience partial melting during atmospheric entry heating (e.g., Genge et al., 2008). It may be worth considering if some partially melted micrometeorites may have experienced melting on their parent bodies.

Noguchi et al. (2023) proposed that one of the major formation mechanisms of the frothy layers might be frictional heating among loose regolith grains by
meteoroid impact. Figure 15c,d shows that a small Ryugu grain is enclosed by a frothy layer. Its internal structure supports this formation mechanism.

## CONCLUDING REMARKS

We studied the fine-grained Ryugu grains using FE-SEM, (S)TEM, synchrotron nanotomography, and NanoSIMS. The mineralogy of the fine-grained Ryugu grains is consistent with that of the mm-sized Ryugu grains investigated by other researchers. Although the presence or absence of minor minerals did not necessarily coincide among results obtained from mm-sized grains, these differences are not statistically significant. By considering several sulfide and phosphide minerals, it was inferred that most Ryugu grains experienced weak shock event(s). Space weathering induced by micrometeoroid impact and heating is more remarkable among Ryugu grains than Itokawa grains. We found a Ryugu grain exhibiting partial melting with a violent release of volatiles during melting. The minor but ubiquitous presence of phosphide minerals in Ryugu samples suggests that C-type asteroids might be one of the possible sources of active phosphorus, a critical ingredient for the origin of life, on the Earth.

*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 of the members of the Hayabusa2 project for their technical and scientific contributions. TEM works at Kyushu University was supported by the Advanced Characterization Platform of the Nanotechnology Platform Japan (grant number JPMXP09A21KU0393) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The SR-CT at SPring-8 was conducted under the proposals 2021A0166, 2021A0602, and 2021B0188. SR experiments at SPring-8 are supported by RIKEN. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. PB, HVP, and RR acknowledge the Centre National d'Etude Spatiale (CNES). MZ was supported by NASA's Hayabusa2 Mission Participating Scientist Program. HAI and JPB were supported by NASA's Laboratory Analysis of Returned Samples Program. HL, CLG, DJ, FDLP, and MM acknowledge the advanced characterization platform, University of Lille-CNRS. JSPS KAKENHI Grant Numbers related to this study are JP19H00725, JP19KK0094, JP21H05424, JP20H01965, JP20H00198, JP20H00205, JP15H05695, 16H06348, and 20H00188. UK Science and Technology Facilities

Council (STFC) grant numbers related to this study are ST/T002328/1 and ST/S000429/1. Start-up funds from Purdue University, University of Glasgow COVID-19 Researcher Support Scheme (LD) ISITE ULNE, Métropole Européenne de Lille, the region "Hauts-de-France" and the ERDF program of the European Union, European Research Council (ERC) under grant agreement number 771691 (Solarys) are also used for this study. AT was supported by the Chinese Academy of Sciences' President's International Fellowship Initiative, grant no. 2019VCA0004.

*Conflict of Interest Statement*—The authors declare that there is no conflict of interest.

*Data Availability Statement*—The data that support the findings of this study are available from the corresponding author upon reasonable request.

*Editorial Handling*—Dr. Akira Yamaguchi

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## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article.

**Figure S1.** Optical images of the fine-grained samples allocated. (a) Grains collected at touchdown #1 site, sample number A0104. (b) Grains collected at touchdown #2 site, sample number C0105.

**Movie S1.** Movie of successive slice images of C0105-04300400. This was prepared by color merging three types of SR-nCT images: red for absorption images at 7.35 keV (LAC = 0–950 cm⁻¹), green for phase-shift

images at 8 keV (RID = 0–20 × 10⁻⁶), and blue for absorption images at 7 keV (LAC = 0–450 cm⁻¹). Green: phyllosilicates (saponite or serpentine) composed of the matrix. Pink: Fe-Ni sulfides (mostly pyrrhotite) as hexagonal plates and very fine grains in the matrix. Orange to pinkish orange: magnetite, as framboids, spherulites and their aggregates, and elongated crystals. Dark blue: dolomite mostly as euhedral crystals. Bright blue: apatite as smaller crystals. Dark green: insoluble organic matter (IOM) in the matrix. Binning images by 2 × 2 × 2 (94.4 nm per voxel).