Renazzo‐type (CR) carbonaceous chondrites belong to one of the most pristine meteorite groups containing various early solar system components such as matrix and fine‐grained rims (FGRs), whose formation mechanisms are still debated. Here, we have investigated FGRs of three Antarctic CR chondrites (GRA 95229, MIL 07525, and EET 92161) by electron microscopy techniques. We specifically focused on the abundances and chemical compositions of the amorphous silicates within the rims and matrix by analytical transmission electron microscopy. Comparison of the amorphous silicate composition to a matrix area of GRA 95229 clearly shows a compositional relationship between the matrix and the fine‐grained rim, such as similar Mg/Si and Fe/Si ratios. This relationship and the abundance of the amorphous silicates in the rims strengthen a solar nebular origin and rule out a primary formation mechanism by parent body processes such as chondrule erosion. Moreover, our chemical analyses of the amorphous silicates and their abundance indicate that the CR rims experienced progressive alteration stages. According to our analyses, the GRA 95229 sample is the least altered one based on its high modal abundance of amorphous silicates (31%) and close‐to‐chondritic Fe/Si ratios, followed by MIL 07525 and finally EET 92161 with lesser amounts of amorphous silicates (12% and 5%, respectively) and higher Fe/Si ratios. Abundances and chemical compositions of amorphous silicates within matrix and rims are therefore suitable recorders to track different alteration stages on a submicron scale within variably altered CR chondrites.

中文翻译：

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无定形硅酸盐记录了太阳状星云和母体过程—透射电镜研究了三种南极CR球粒陨石的细粒边缘和基质

雷纳佐型（CR）碳质球粒陨石属于最原始的陨石群之一，含有各种早期太阳系成分，例如基质和细粒轮缘（FGR），其形成机理尚有争议。在这里，我们通过电子显微镜技术研究了三种南极CR球粒陨石（GRA 95229，MIL 07525和EET 92161）的FGR。通过分析透射电子显微镜，我们专门研究了边缘和基体中非晶态硅酸盐的含量和化学组成。将无定形硅酸盐成分与GRA 95229的基体区域进行比较，可以清楚地看出基体与细晶粒边缘之间的成分关系，例如相似的Mg / Si和Fe / Si比。这种关系和轮辋中无定形硅酸盐的丰度加强了太阳星云的起源，并排除了由母体过程（如软骨侵蚀）引起的初步形成机制。此外，我们对无定形硅酸盐及其丰度的化学分析表明，CR轮辋经历了渐进的蚀变阶段。根据我们的分析，基于无定形硅酸盐的高模态丰度（31％）和接近于软骨的铁/硅比，GRA 95229样品变化最少，其次是MIL 07525，最后是EET 92161，其中非晶硅酸盐（分别为12％和5％）和较高的Fe / Si比。