The presence of primary iron sulfides that appear to be aqueously altered in CM and CR carbonaceous chondrites provides the potential to study the effects and, by extension, the conditions of aqueous alteration. In this work, we have used SEM, TEM, and EPMA techniques to characterize primary sulfides that show evidence of secondary alteration. The alteration styles consist of primary pyrrhotite altering to secondary pentlandite (CMs only), magnetite (CMs and CRs), and phyllosilicates (CMs only) in grains that initially formed by crystallization from immiscible sulfide melts in chondrules (pyrrhotite‐pentlandite intergrowth [PPI] grains). Textural, microstructural, and compositional data from altered sulfides in a suite of CM and CR chondrites have been used to constrain the conditions of alteration of these grains and determine their alteration mechanisms. This work shows that the PPI grains exhibit two styles of alteration—one to form porous pyrrhotite‐pentlandite (3P) grains by dissolution of precursor PPI grain pyrrhotite and subsequent secondary pentlandite precipitation (CMs only), and the other to form the altered PPI grains by pseudomorphic replacement of primary pyrrhotite by magnetite (CMs and CRs) or phyllosilicates (CMs only). The range of alteration textures and products is the result of differences in conditions of alteration due to the role of microchemical environments and/or brecciation. Our observations show that primary sulfides are sensitive indicators of aqueous alteration processes in CM and CR chondrites.

中文翻译：

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CM2和CR2碳质球粒陨石中原始铁硫化物的变化：深入了解母体过程

在CM和CR碳质球粒陨石中似乎发生了水相改变的初级硫化铁的存在提供了研究其影响以及扩展水相改变条件的潜力。在这项工作中，我们已使用SEM，TEM和EPMA技术来表征显示出二次变化迹象的主要硫化物。蚀变样式包括初次黄铁矿向次要五芒铁（仅CMs），磁铁矿（CMs和CRs）和层状硅酸盐（仅CMs）的转变，这些晶粒最初是由不溶性硫化物在球粒状结晶中的不溶性硫化物结晶形成的（硫铁矿-戊铁矿共生[PPI]）谷物）。质地，微观结构，一套CM和CR球粒陨石中硫化物的变化组成和组成数据已被用来约束这些晶粒的变化条件并确定其变化机理。这项工作表明，PPI晶粒表现出两种变化形式：一种是通过溶解前体PPI晶粒黄铁矿和随后的二次蒙脱石沉淀（仅CMs）形成多孔的黄铁矿-五氧化铁（3P）晶粒，另一种形成改变的PPI晶粒。通过磁铁矿（CM和CR）或页硅酸盐（仅CM）的假黄铁矿的假晶置换。改变质地和产物的范围是由于微化学环境和/或烧成的作用而导致改变条件不同的结果。