Defect microstructure of pyrrhotite in regolith material returned from C-type asteroid 162173
Ryugu – evidence for shock metamorphism
Falko Langenhorst1, Dennis Harries2, Takaaki Noguchi3, Toru Matsumoto3,4, Akira Miyake3, Yohei Igami3, the Min-Pet“Sand” Team, and the Hayabusa2 Initial Analysis Core.
1
Institute of Geoscience, Friedrich Schiller University Jena; Carl-Zeiss-Promenade 10, D-07745 Jena, Germany;
2
Luxembourg Institute of Science and Technology, European Space Resources Innovation Centre; Belvaux, Luxembourg;
3
Division of Earth and Planetary Sciences, Kyoto University; Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan;
4The Hakubi Center for Advanced Research, Kyoto University; Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
Introduction. The Hayabusa2 mission successfully returned regolith materials from two sampling sites of the C-type NearEarth Object (NEO) 162173 Ryugu [1]. Mineralogical characterization of samples revealed that aqueous alteration was anomnipresent process, which resulted mainly in the formation of phyllosilicates, carbonates, phosphates, and other phases
forming under hydrous conditions [2]. Since Ryugu is an airless body whose surface is directly exposed to interplanetary
space, it was also expected that regolith samples contain traces of space weathering (effects due to solar wind particle andmicrometeoroid bombardments). Effects of solar wind bombardment were discovered in several samples [3,4], while traces ofshock metamorphism are scarce [5], which is unusual considering the rubble pile nature of asteroid Ryugu.
Samples and methods. Ryugu samples from the two sampling sites were collected in separate chambers A and C of thesample catcher [2]. Within the activities of the Min-Pet “Sand” team, we have examined three samples from chamber A(A0058-T4, AP002-g03, AP007-g06), which probed the uppermost surface material of Ryugu. The three samples were firstinvestigated by field emission scanning electron microscope (FE-SEM: JSM-7001F at Kyoto University). We then extracted
electron-transparent sections of regions of interest on the Ryugu grains, using a focused ion beam (FIB) system (HeliosNanoLab G3 CX at Kyoto University). The FIB sections were finally observed using analytical transmission electronmicroscopy (TEM, Tecnai G2 FEG at Univ. of Jena). Bright-field and dark-field TEM imaging techniques combined withselected area electron diffraction (SAED) were employed to characterize the defect microstructures.
Results. Among all the mineral grains in the three FIB sections, only a relatively large (9 x 4 µm dimensions) pyrrhotite grainfrom sample AP007-g06 turned out to exhibit a rich defect microstructure. This grain is only partially surrounded byphyllosilicates, where it displays well-developed faces parallel to (001), (010), and (011). At the free surface, the grain is
however irregularly shaped and pitted. Our previous investigation revealed a systematic variation in the superstructure typetowards the free surface with N values ranging from 4.0 (4C-pyrrhotite, x ≈ 0.125) in the grain interior to 5.9 (close to 6Cpyrrhotite, x=0.083) at the surface [4].
Additionally, we observe a high density of dislocations in various geometric configurations pervading the entire grain.
Straight, up to 1 µm long dislocations with dislocation lines parallel to [001] are most abundant (density up to 2 x 1013 m-2),
while dislocations reorganized in sub-grain boundaries and dislocation loops occur subordinately. The straight elongateddislocations are apparently glissile with 1/3[110](001) acting as slip system. Bright-field TEM images and SAED patterns
indicate also the presence of multiple microtwins on the (001) plane throughout the entire grain. Sub-grain boundaries anddislocation loops occur in the vicinity of internal voids that might have been once filled with fluid.
Discussion. This overall rich defect microstructure of pyrrhotite implies that the grain was affected by various processes. Thevariation in the superstructure type below the irregular pitted surface was attributed to incipient space weathering, causing aloss of sulfur on the free surface and an inward diffusion of Fe into the grain interior [4]. Internal voids and dislocation loops
are, however, likely defects produced during aqueous alteration. The activation of a large number of perfect dislocations andmicrotwins on the basal plane are a typical response of monosulfides to dynamic loading [7]. Although no crater is observedon the surface, we interpret these defects as clue to shock metamorphism affecting the entire grain.
References[1] Watanabe S. et al. 2019. Science 364, 268-272. [2] Nakamura T. et al. 2022. Science 377,
https://doi.org/10.1126/science.abn8671. [3] Noguchi T. et al. 2022. Abstract #1747, 53rd LPSC, [4] Harries et al. 2022.
Meteoritics & Planetary Science 57: 6073. [5] Kikuiri M. et al. 2022. Meteoritics & Planetary Science 57: 6199. [6] Yada T. etal. 2021. Abstract #2008, 52nd LPSC. [7] Langenhorst F. et al. 2014, Earth, Planets and Space 66: 118.