Nonequilibrium spherulitic magnetite in the Ryugu samples
Nonequilibrium spherulitic magnetite in the Ryugu samples
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DOI：
10.1016/j.gca.2023.02.003
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发表时间：
2023-02-13
影响因子：
5
通讯作者：
Tsuda，Yuichi
中科院分区：
文献类型：
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作者：
Dobrica，Elena;Ishii，Hope A.;Tsuda，Yuichi
我们使用各种电子显微镜技术研究了隼鸟二号航天器在 C 型小行星 162173 Ryugu 表面两次着陆期间收集的一些粒子。我们详细的透射电子显微镜研究表明，各种形态的磁铁矿紧密共存。这是 CI 球粒陨石类物质的特征，与 Ryugu 母体中的矿物组合和成分一致。我们描述了具有不同形态的磁铁矿的微观结构特征，这些特征可能是由其形成过程中的化学条件（生长与扩散速率）造成的。此外，我们描述了具有由单个辐射纤维组成的球晶结构的磁铁矿的存在，其特征是普遍的、均匀分布的自形到亚形孔隙，这在以前的球粒陨石研究中尚未描述。这种特殊的球晶结构与非平衡条件下的结晶一致。此外，磁铁矿纤维内存在的高密度缺陷、这种形态的高表面/体积比以及针状纤维的多个孔中和边缘处的无定形材料的存在进一步支持了它们在非平衡条件下的形成。我们认为，导致这种结构的生长过程是由于溶液达到过饱和状态，从而通过成核和快速生长调整到较低的自由能条件。
We have investigated several particles collected during each of two touchdowns of the Hayabusa2 spacecraft at the surface of the C-type asteroid 162173 Ryugu using various electron microscope techniques. Our detailed transmission electron microscopy study shows the presence of magnetite with various morphologies coexisting in close proximity. This is characteristic of CI chondrite-like materials and consistent with the mineral assemblages and compositions in the Ryugu parent body. We describe the microstructural characteristics of magnetite with different morphologies, which could have resulted from the chemical conditions (growth vs. diffusion rate) during their formation. Furthermore, we describe the presence of magnetites with a spherulitic structure composed of individual radiating fibers that are characterized by pervasive, homogeneously distributed euhedral to subhedral pores that have not been described in previous chondrite studies. This particular spherulitic structure is consistent with crystallization under nonequilibrium conditions. Additionally, the presence of a high density of defects within the magnetite fibers, the high surface/volume ratio of this morphology, and the presence of amorphous materials in several pores and at the edges of the acicular fibers further support their formation under nonequilibrium conditions. We suggest that the growth processes that lead to this structure result from the solution reaching a supersaturated state, resulting in an adjustment to a lower free energy condition via nucleation and rapid growth.