Transmission electron microscope studies of fine‐grained rims in three CM2 carbonaceous chondrites, Y‐791198, Murchison, and ALH 81002, have revealed the presence of widespread nanoparticles with a distinctive core–shell structure, invariably associated with carbonaceous material. These nanoparticles vary in size from ~20 nm up to 50 nm in diameter and consist of a core of Fe,Ni carbide surrounded by a continuous layer of polycrystalline magnetite. These magnetite shells are 5–7 nm in thickness irrespective of the diameter of the core Fe,Ni carbide grains. A narrow layer of amorphous carbon a few nanometers in thickness is present separating the carbide core from the magnetite shell in all the nanoparticles observed. The Fe,Ni carbide phases that constitute the core are consistent with both haxonite and cohenite, based on electron diffraction data, energy dispersive X‐ray analysis, and electron energy loss spectroscopy. Z‐contrast scanning transmission electron microscopy shows that these core–shell magnetite‐carbide nanoparticles can occur as individual isolated grains, but more commonly occur in clusters of multiple particles. In addition, energy‐filtered transmission electron microscopy (EFTEM) images show that in all cases, the nanoparticles are embedded within regions of carbonaceous material or are coated with carbonaceous material. The observed nanostructures of the carbides and their association with carbonaceous material can be interpreted as being indicative of Fischer–Tropsch‐type (FTT) reactions catalyzed by nanophase Fe,Ni metal grains that were carburized during the catalysis reaction. The most likely environment for these FTT reactions appears to be the solar nebula consistent with the high thermal stability of haxonite and cohenite, compared with other carbides and the evidence of localized catalytic graphitization of the carbonaceous material. However, the possibility that such reactions occurred within the CM parent body cannot be excluded, although this scenario seems unlikely, because the kinetics of the reaction would be extremely slow at the temperatures inferred for CM asteroidal parent bodies. In addition, carbides are unlikely to be stable under the oxidizing conditions of alteration experienced by CM chondrites. Instead, it is most probable that the magnetite rims on all the carbide particles are the product of parent body oxidation of Fe,Ni carbides, but this oxidation was incomplete, because of the buildup of an impermeable layer of amorphous carbon at the interface between the magnetite and the carbide phase that arrested the reaction before it went to completion. These observations suggest that although FTT catalysis reactions may not have been the major mechanism of organic material formation within the solar nebula, they nevertheless contributed to the inventory of complex insoluble organic matter that is present in carbonaceous chondrites.

中文翻译：

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CM2碳质球粒陨石细晶粒边缘中的纳米碳化铁：太阳星云中费-托催化的有机物质形成

透射电子显微镜对三种CM2碳质球粒陨石Y-791198，Murchison和ALH 81002中的细粒边缘进行的研究表明，存在着广泛的具有独特核壳结构的纳米粒子，这些粒子总是与碳质材料相关。这些纳米粒子的大小从直径约20 nm到最大50 nm不等，由Fe，Ni碳化物的核组成，并被连续的多晶磁铁矿层包围。这些磁铁矿壳的厚度为5–7 nm，与铁，镍，碳化物芯的直径无关。在观察到的所有纳米颗粒中，存在厚度为几纳米的无定形碳的窄层，将碳化物核与磁铁矿壳隔开。根据电子衍射数据，构成核的Fe，Ni碳化物相与六方晶石和方钴矿都一致，能量色散X射线分析和电子能量损失谱。Z对比扫描透射电子显微镜显示，这些核-壳磁铁矿-碳化物纳米粒子可以作为单个孤立的晶粒出现，但更常见于多个粒子的簇中。此外，能量过滤的透射电子显微镜（EFTEM）图像显示，在所有情况下，纳米颗粒均嵌入碳质材料区域内或涂有碳质材料。所观察到的碳化物的纳米结构及其与含碳材料的缔合可以解释为指示了在催化反应过程中被渗碳的纳米相Fe，Ni金属晶粒催化的费托反应（FTT）反应。与其他碳化物以及碳质材料局部催化石墨化的证据相比，这些FTT反应最可能的环境似乎是太阳星云，其与on石和Cohenite的高热稳定性相符。但是，尽管这种情况似乎不太可能，但不能排除在CM母体中发生此类反应的可能性，因为在推断为CM小行星母体的温度下，反应动力学极慢。此外，碳化物不太可能在CM球粒陨石经历的氧化氧化条件下稳定。取而代之的是，所有碳化物颗粒上的磁铁矿边缘最有可能是Fe，Ni碳化物母体氧化的产物，但这种氧化是不完全的，因为在磁铁矿和碳化物相之间的界面上形成了无定形碳的不可渗透层，从而在反应完成之前就阻止了该反应。这些观察结果表明，尽管FTT催化反应可能不是太阳星云内有机物质形成的主要机理，但它们仍然有助于碳质球粒陨石中存在的复杂的不溶性有机物。