The surface chemistry of pyrrhotites from intact particles directly collected from asteroid (162173) Ryugu was investigated by micro-Raman spectroscopy. The Raman peak characteristic to pyrrhotite was observed at around 115 cm in Ryugu pyrrhotites, similar to freshly cleaved surfaces of terrestrial pyrrhotites. Additional Raman bands centered at around 220, 275, and 313 cm with broadened features were also detected from the Ryugu pyrrhotites. The set of Raman bands at 220 and 275 cm was assigned to typical Fe-S stretching vibrations of ν (225 cm) and ν (275 cm). These bands are not clearly observed in bulk crystals of pyrrhotite but appear in its nanoparticulate phase. These bands are ordinarily seen in amorphous monosulfides that formed under low oxygen fugacity (O) conditions in nature, indicating that the structural alteration of pyrrhotite surfaces occurred heterogeneously on the nanoscale under low O conditions. Further, the Raman band at 313 cm was attributed to a characteristic tetrahedral bonding of Fe(III) in the lattice of FeFeS, followed by the local breakdown of the crystal lattice structures from planar bonding with Fe(II). In addition, some areas of the Ryugu pyrrhotite grains showed corroded structures with iridescence. Furthermore, assemblages of magnetite particles were also preferentially observed on small areas of the likely-dissolved pyrrhotite crystals in phyllosilicate matrices. These characteristic features in the Raman spectra and in corroded structures of Ryugu pyrrhotites record changes in the local environmental conditions via aqueous alteration. The corrosion of pyrrhotite crystals followed by the preferential formation of magnetite particles by asteroidal water is the likely product of dissolution of Fe(II) from the pyrrhotite surface and its oxidative precipitation in microchemical environments on the Ryugu parent body.

中文翻译：

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小行星 162173 Ryugu 中的磁黄铁矿：记录了其表面水蚀变的初始变化


通过显微拉曼光谱研究了直接从小行星 (162173) Ryugu 收集的完整颗粒的磁黄铁矿的表面化学性质。在 Ryugu 磁黄铁矿中，在 115 cm 左右观察到磁黄铁矿特有的拉曼峰，类似于陆地磁黄铁矿的新劈裂表面。在 Ryugu 磁黄铁矿中还检测到了中心位于 220、275 和 313 cm 左右且具有加宽特征的其他拉曼谱带。 220 和 275 cm 处的一组拉曼谱带被分配给典型的 Fe-S 伸缩振动 ν (225 cm) 和 ν (275 cm)。这些带在磁黄铁矿块状晶体中并未清晰观察到，但出现在其纳米颗粒相中。这些带通常出现在自然界低氧逸度 (O) 条件下形成的无定形单硫化物中，表明磁黄铁矿表面的结构变化在低 O 条件下在纳米尺度上发生不均匀。此外，313 cm 处的拉曼带归因于 FeFeS 晶格中 Fe(III) 的特征四面体键合，随后晶格结构因与 Fe(II) 的平面键合而局部分解。此外，龙宫磁黄铁矿颗粒的某些区域显示出带有虹彩的腐蚀结构。此外，在页硅酸盐基质中可能溶解的磁黄铁矿晶体的小区域上也优先观察到磁铁矿颗粒的聚集。拉曼光谱和龙宫磁黄铁矿腐蚀结构中的这些特征记录了当地环境条件通过水蚀变而发生的变化。 磁黄铁矿晶体的腐蚀以及小行星水优先形成磁铁矿颗粒可能是磁黄铁矿表面溶解 Fe(II) 及其在 Ryugu 母体微化学环境中氧化沉淀的产物。