Magnetite and non-magnetite framboids in Ryugu sample
Yuki Kimura1, Takeharu Kato2, Satoshi Anada2, Ryuji Yoshida2, Kazuo Yamamoto2, Toshiaki Tanigaki3, Tetsuya Akashi3,
Hiroto Kasai3, Tomoki Nakamura4, Toru Matsumoto5, Takaaki Noguchi5, Masahiko Sato6, Tomoyo Morita4, Mizuha Kikuiri4,
Kana Amano4, Eiichi Kagawa4, Hisayoshi Yurimoto7, Ryuji Okazaki8, Hikaru Yabuta9, Hiroshi Naraoka8, Kanako Sakamoto10,Sei-ichiro Watanabe11, Yuichi Tsuda10, and Shogo Tachibana5,101
Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
2
Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, 456-8587, Japan
3
Research & Development Group, Hitachi, Ltd., Hatoyama, Saitama, 350-0395, Japan
4
Department of Earth Sciences, Tohoku University, Sendai 980-8578, Japan
5
Division of Earth and Planetary Sciences, Kyoto University; Kyoto 606-8502, Japan
6
Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan
7
Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
8
Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 819-0395, Japan9Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-Hiroshima 739-8526, Japan10
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
11
Department of Earth and Environmental Sciences, Nagoya University, Nagoya 464-8601, Japan

There are many magnetite (Fe3O4) particles found in chondrites together with clay minerals [1,2] suggesting thatthey were formed by aqueous alteration [3]. Magnetite can record magnetic field environment at the time of its
formation as natural remanent magnetization [4]. Therefore, it is important to know when and how magnetite particles
formed in order to estimate the magnetic field environment of the early solar system and to understand the physicalevolutionary processes that have a relationship with the magnetic filed. A large amount of magnetite is also found in
Ryugu samples [5] and it has been confirmend that submicron-scale magnetite is a main carrier of the natural remanent
magnetization [6]. We conducted electron holography to visualize the magnetic domain structure of the flamboidal
magnetite and found that the magnetite particles have a concentric circular magnetic field (vortex structure) and anexternal leakage of the magnetic field that could be a carrier of remanent magnetization [5,6]. In addition to the typicalframboidal magnetite, here we report the discovery of a flamboidal magnetite-like mineral that does not exhibit amagnetic field in a sample.
Figure 1 shows a part of a typical flamboidal magnetite in an ultrathin section extracted from a Ryugu sample A0064FO007 using a cryo-focused ion beam (FIB; NB5000, Hitachi High-Tech) under -90oC at Japan Fine Ceramics Center (JFCC).
The bright-field transmission electron microscope (TEM) image shows at least eight magnetite particles of ~1 μm in diametersurrounded by phyllosilicates at the bottom part. The upper side of the image corresponds to the surface of the parent sample
before they were extracted. The strong contrast area marked with W on the upper side is tungsten that was deposited before
processing to prevent damage by FIB machining. Figures 1 i and ii show magnetic flux distribution images corresponding totwo boxes observed by electron holography using a specially designed TEM (HF-3300, Hitachi High-Tech) at JFCC. The
concentric stripes inside the particles represent the phase change due to magnetization, because the internal potential has been
subtracted by taking images from the back and front. Namely, magnetite particles have a vortex magnetic domain structure.
In addition, an external magnetic field can be seen on the outside of the particle. Magnetite particles are thought to haveformed by aqueous alteration between 3.1-6.8 million years after the formation of first mineral, calcium–aluminium-richinclusion, at the beginning of the Solar system [7]. The external magnetic field is a result of the magnetic field environmentthat existed when magnetite formed by aqueous alteration in the parent body of Ryugu [6].
Figure 2 shows a high-angle annular dark field scanning TEM (STEM-HAADF) image of an ultrathin section of a portionof a flamboid from a different fragment in the Ryugu sample A0064-FO007, which was broken into several pieces when fixed
to the indium plate for easier handling, extracted using the cryo-FIB. Similar to the Fig. 1, several spherical particles of ~1μm can be seen. Corresponding magnetic flux distribution image has been shown in Fig. 2B. Unlike i and ii in Fig. 1, thecontrast within the particles is flat and there is no magnetic domain structure characteristic of magnetite. Elementalcomposition analysis using an energy dispersive X-ray spectroscopy (EDS) confirms that the particles are iron oxide (Fig. 2C).
Although it looks like typical framboidal magnetitee, electron energy-loss spectroscopy analysis indicated that the bonding
states of oxygen and iron were clearly different from that of magnetite. The observed bonding state was consistent with

wüstite (Fe1-y O), which is antiferromagnetic and does not acquire remanent magnetization.. Although particles appeared tobe framboidal magnetite based on a micrograph, these particles were actually wüstite. We refer to this particle as framboidal
pseudo-magnetite.
The framboidal pseudo-magnetite is present on the top surface of the host sample, and the bottom is a matrix composed
mainly of phyllosilicates. TEM-EDS mapping showed iron-rich region that may be leached from the pseudo-magnetite into
the matrix. Magnetite with porous surface texture and metallic iron has been found in the analysis by Sand team [8].
Sample analysis in the first Hayabusa mission has reported the formation of iron particles on a silicate grain by space
weathering [9]. In case of our pseudo-magnetite, although the presence of metallic iron particulates was also confirmed, they
appear to be coarser than those found in previous studies and are We expect that framboidal pseudo-magnetite may also haveformed by space weathering of magnetite, but different mechanisms from other two samples. Similar pseudo-magnetite
particles have also been found in other FIB thin sections. We will discuss the formation process of framboidal pseudomagnetite at the symposium.

Figure 1.

TEM image of a thin section of a typical magnetic framboid extracted from the sample A0064-FO007 and its characteristic

magnetic structure observed by electron holography.

Figure 2.

Framboidal pseudo-magnetite.

prepared by FIB.

Arrows show the direction of the magnetic flux.

A. STEM-HAADF image of a thin section of a fragment from the sample A0064-FO007

B. Magnetic flux distribution image obtained from the reconstruction of remanent magnetic state of magnetites observed

by electron holography.

Uniform contrast indicate that the particles are not magnetic minerals.

C. Corresponding elemental mapping ofiron.

The authors acknowledge the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (grants
JPMXS0450200421, JPMXS0450200521).
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