記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

Abstract

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

DOI： 10.1038/s41550-023-02137-z

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その他リンク： https://www.nature.com/articles/s41550-023-02137-z

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley  

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, manganochromite, 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.

DOI： 10.1111/maps.14093

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley  

Abstract

Samples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small‐sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine‐grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S‐ and Ni‐rich ferrihydrite is abundant. The coarse‐grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x‐ray microscopy at the Fe‐L₃ edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe³⁺) than in Ryugu (<50% Fe³⁺). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine‐grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe‐based nanophases. On the other hand, the coarse‐grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long‐term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.

DOI： 10.1111/maps.14101

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV  

DOI： 10.1016/j.gca.2023.02.003

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Mineralogical Association of Canada  

Abstract

The basic crystallographic properties of polygonal serpentine (PS) have been revealed in previous studies, while their variation, occurrence, and formation conditions are poorly understood. This study defines four categories of chrysotile and PS: clinochrysotile (chrysotile 2Mc1), orthochrysotile (chrysotile 2Orc1), clino-type PS, and ortho-type PS. When chrysotile and PS coexist or are indistinguishable, we will call them fibrous serpentine (FS), regardless of their ratios. By researching the occurrence, distribution, microtexture, crystal structure, and chemical composition of PS and chrysotile based on these categories, we reveal the features of each type and discuss their relationships and formation processes. The chrysotile and PS of the Kurosegawa belt in Kyushu are enriched in ortho-type FS (O-FS) compared with other localities. O-FS-enriched samples (group-O) show a linear, wide vein occurrence and splintery texture or irregular veins with space-filling textures, while clino-type FS (C-FS)-enriched samples occur as thin veins in serpentinites or as massive-foliated aggregates. Polygonal serpentine is more frequently seen in group-O compared with group-C. In our observation, the crystal structure of chrysotile and PS is continuous, making it difficult to define a clear border between them. Ortho-type PS is frequently seen, whereas clino-type PS is absent in most localities. Orthochrysotile tends to grow into 15-sectored ortho-type PS, whereas clinochrysotile grows into 30-sectored clino-type PS. These features of the crystal structure and coexistence and proportion of the fibrous serpentines in each group were mostly common in all studied localities in the Kurosegawa belt in Kyushu, indicating similar formation conditions within group-O or -C. Based on the textures and occurrences, group-O formed by crystallization from the fluid in the fracture made by brittle deformation, while group-C formed in the last stage of alteration mainly by replacement by alteration. The serpentines in the Kurosegawa belt in Kyushu seem to have followed a similar process, at least at the late stage of serpentinization where FS forms and the difference in the ratio of C-FS to O-FS may have been derived from the difference in the timing of formation during the emplacement process. A formation model for chrysotile and PS is proposed based on these observations. In this model, C-FS and O-FS form under different conditions, such as differences in pressure. This model still needs verification by further observations at different localities and by synthetic experiments.

DOI： 10.3749/2200017

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

Abstract

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe³⁺ to Fe²⁺ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

DOI： 10.1038/s41550-022-01841-6

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その他リンク： https://www.nature.com/articles/s41550-022-01841-6

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

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The hyperenrichment of gold is critical for the formation of high-grade gold deposits in orogenic systems, where gold enrichment can reach several percent. However, many details on the nature of the hyperenrichment mechanism remain unclear. Here we show a “self-purification” model that could explain the hyperenrichment of gold from nanoparticles to veinlets in deformed pyrite. The three-dimensional spatial relationship and coexistence of gold nanoparticles and veinlets indicate a direct transformation from the former to the latter in solid-state. Theoretical evaluation reveals that this transformation involves a thermodynamically favorable self-purification process. We thus propose that deformation/thermo drives the transport of gold nanoparticles towards veinlets in pyrite through atomic diffusion. This deformation/thermo-driven model is applicable to any deformed nano-gold-bearing pyrite and explains how solid-state transport can effectively concentrate gold to a very high grade. Similar mechanisms may have broad implications in both ore-forming and beneficiation processes.

DOI： 10.1038/s43247-022-00628-x

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その他リンク： https://www.nature.com/articles/s43247-022-00628-x

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO ₂ -bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation.

DOI： 10.1126/science.abn8671

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：EDP Sciences  

Glass with embedded metal and sulfides (GEMS), the major components of chondritic-porous interplanetary dust particles (CP-IDPs), is one of the most primitive materials in the Solar System and may be analogous to the amorphous silicate dust observed in various astronomical environments. Mineralogical characteristics of GEMS should reflect their formation process and condition. In this study, synthetic experiments in the sulfur-bearing system of Fe–Mg–Si–O–S were performed with a systematic change in redox conditions using thermal plasma systems to reproduce the mineralogy and textures of GEMS. The resulting condensates were composed of amorphous silicates with Fe-bearing nano-inclusions. The Fe content and texture in the amorphous silicates as well as the mineral phases of the nanoparticles correlate with redox conditions. Fe dissolved in the amorphous silicate as FeO in oxidizing conditions formed Fe-metal nanoparticles in intermediate redox conditions, and gupeiite (Fe₃ Si) nanoparticles in reducing conditions. In intermediate to reducing redox conditions, Fe-poor amorphous silicate formed a biphasic texture with Mg- and Si-rich regions, indicating liquid immiscibility during the melt phase. Most Fe-metal particles were surrounded by FeS and formed on the surface of amorphous silicate grains. Condensates produced in intermediate to slightly reducing redox conditions resemble GEMS in that they have similar mineral assemblages and chemical compositions to amorphous silicate, except that the Fe-metal grains are absent from the interior of the amorphous silicate grains. This textural difference can be explained by the sulfidation at high temperatures in this study, in contrast to sulfidation occurring at low temperatures in the presence of H₂ in natural GEMS formation. Based on the two-liquid structures observed in the experimental products and in GEMS, also recognized in infrared spectra, we propose that GEMS condensed as silicate melt under limited redox conditions followed by incorporation of multiple metal grains into the silicate melt or by aggregation of coreshell structured grains before sulfidation of the metallic iron. Condensates produced in oxidizing conditions are similar to GEMS-like material in the matrices of primitive carbonaceous chondrite meteorites, indicating the possibility that they form by direct condensation from nebula gas in relatively oxidizing conditions compared to GEMS.

DOI： 10.1051/0004-6361/202142620

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：{EDP} Sciences  

Glass with embedded metal and sulfides (GEMS) is a major component of chondritic porous interplanetary dust particles. Although GEMS is one of the most primitive components in the Solar System, its formation process and conditions have not been constrained. We performed condensation experiments of gases in the system of Mg–Si–O (MgSiO₃ composition) and of the S-free CI chondritic composition (Si–Mg–Fe–Na–Al–Ca–Ni–O system) in induction thermal plasma equipment. Amorphous Mg-silicate particles condensed in the experiments of the Mg–Si–O system, and their grain size distribution depended on the experimental conditions (mainly partial pressure of SiO). In the CI chondritic composition experiments, irregularly shaped amorphous silicate particles of less than a few hundred nanometers embedded with multiple Fe–Ni nanoparticles of ≤20 nm were successfully synthesized. These characteristics are very similar to those of GEMS, except for the presence of FeSi instead of sulfide grains. We propose that the condensation of amorphous silicate grains smaller than a few tens of nanometers and with metallic cores, followed by coagulation, could be the precursor material that forms GEMS prior to sulfidation.

DOI： 10.1051/0004-6361/202141216

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

DOI： 10.3847/1538-4357/abe5a0

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/abe5a0/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japan Association of Mineralogical Sciences  

DOI： 10.2465/jmps.201130d

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記述言語：日本語  

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japan Association of Mineralogical Sciences  

DOI： 10.2465/jmps.170131

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Mineralogical Society  

DOI： 10.1180/minmag.2015.079.1.05

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：日本鉱物科学会  

Yukonite and wallkilldellite–(Fe) were discovered at the Uriya deposit, Kiura mine, Ume, Saiki City, Oita, Japan, as first occurrences in Japan. Both minerals partly cover the surface of quartz and arsenopyrite masses and are closely associated, forming layered radiating aggregates. The layers are divided roughly into yellowish–brown and reddish–brown layers, where the former is yukonite and the latter is wallkilldellite–(Fe). The X–ray diffraction (XRD) pattern of Kiura yukonite has sharp peaks compared with that from the type locality, indicating that it is well crystalline. The empirical formula of the yukonite from electron probe microanalyzer (EPMA) analysis on the basis of As = 3 is Ca_2.31Fe³⁺_4.20 (AsO₄)_3.00(OH)_8.22·12.07H₂O. The empirical formula of the wallkilldellite–(Fe) on the basis of O = 20 is Ca_3.51Fe²⁺_5.13(AsO₄)_4.55(OH)_3.64·12.46H₂O. The wallkilldellite–(Fe) from the Kiura mine is very weak against heat, and the XRD pattern changes at 50 °C. On the other hand, the XRD pattern of the yukonite changes at 150 °C, a temperature higher than those of yukonite at other localities, indicating that it is well crystalline. We also found the signature of a structural relationship between the yukonite and wallkilldellite–(Fe) based on XRD and transmission electron microscope (TEM) observations.

DOI： 10.2465/jmps.141022b

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会議種別：ポスター発表  

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会議種別：ポスター発表  

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会議種別：口頭発表（一般）  

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会議種別：ポスター発表  

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会議種別：口頭発表（一般）  

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会議種別：ポスター発表  

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会議種別：口頭発表（一般）  

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会議種別：ポスター発表  

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会議種別：口頭発表（一般）  

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記述言語：日本語   会議種別：口頭発表（一般）  

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会議種別：口頭発表（一般）  

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記述言語：英語   会議種別：口頭発表（一般）  

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記述言語：日本語   会議種別：口頭発表（一般）  

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記述言語：日本語   会議種別：ポスター発表  

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記述言語：日本語   会議種別：ポスター発表  

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記述言語：日本語   会議種別：ポスター発表  

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記述言語：日本語   会議種別：口頭発表（一般）  

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記述言語：日本語   会議種別：口頭発表（一般）  

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会議種別：ポスター発表  

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