CM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM‐like clasts in three achondrites by chemical analysis of the tochilinite‐cronstedtite‐intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2‐2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0‐CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4‐CM2.6) or less altered fragments (CM2.7‐CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO₂ ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO₂ ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0‐CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS‐REx and Hayabusa 2 missions in the future.

中文翻译：

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CM球粒陨石角砾岩的分类-对OSIRIS-REx和Hayabusa 2任务样品评估的意义

CM球粒陨石是复杂的撞击角砾岩（多为白垩纪）角砾岩，其中石屑岩屑显示出不同程度的水蚀蚀变。在这里，我们通过化学分析了斜方晶石-钙钛矿-共生体（TCI；以前被称为“特征欠佳的相”），研究了19种不同CM球粒陨石中的单个碎屑的变化程度以及三个长粒陨石中CM状碎屑的变化程度。为了识别各种软骨状岩相中的TCI，我们使用了抛光薄片的反向散射电子（BSE）概览图像，然后对适当的样品进行了电子微探针测量。因此，对75块岩屑进行了分类。通常，鲁宾等人的出色工作和特定标准。（2007）被用来并以类似于普通球粒陨石角砾岩（例如CM2.2-2.7）的方式对CM角砾岩进行分类。在BSE图片中，CM球粒陨石（CM2.0-CM2.2）中发生强烈变化的碎片中的TCI呈深灰色，与周围物质（通常为碎屑基质）的对比度较低，可以与中等（TC2.4-CM2）中的TCI区别开来。 6）或更少的变化片段（CM2.7-CM2.9）; 后者明亮，与周围环境形成鲜明对比。我们发现，仅考虑“ FeO” / SiO即可获得准确的分类。TCI化学比为₂。人们还可以考虑TCI的S / SiO ₂比率和金属丰度，但由于一些缺点而未用于分类。大多数CM球粒陨石都是在炎热和寒冷的沙漠中经历过陆地风化的发现。因此，观察到的金属丰度易受风化影响，可能不是亚型分类的可靠指标。这项研究提出了基于Rubin量表的扩展分类方案，该分类方案是从CM2.0-CM2.9子类型中考虑到角砾石化的，并且包括了单个碎屑的最小到最大变化程度。CM球粒陨石中水相变化的范围以及具有不同变化历史的碎屑混合的小空间尺度对于将来解释OSIRIS-REx和Hayabusa 2任务返回的样本非常重要。