Abstract Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.

中文翻译：

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斜长石的冲击变质和选择性非晶化


摘要 斜长石是地球和其他类地行星体表面最常见的造岩矿物之一，它受到普遍存在的超高速撞击过程的影响。然而，斜长石对冲击变质作用的反应仍知之甚少。特别是，限制斜长石中冲击引起的非晶化的开始和进展（即转变为介电玻璃）将提高我们对冲击如何逐渐使斜长石变形的认识。反过来，这些信息将使斜长石能够用于评估陨石和陆地冲击岩的冲击阶段，只要它们缺乏传统上使用的冲击指示矿物，例如橄榄石和石英。在这里，我们报告了对加拿大马尼夸根撞击结构中央隆起的变花岗岩破碎锥中的冲击斜长石颗粒的电子背散射衍射（EBSD）研究。我们的研究表明，在斜长石中，冲击非晶化最初位于预先存在的孪晶内或沿着片层，具有与平面变形特征（PDF）相似的特征，但在周期性上类似于孪晶。这些片层可能代表在冲击条件下优先发生结构破坏的特定晶面。优先非晶化的预先存在的孪生组的取向和非晶片层的取向可能有利于局部冲击波的散射，并且对应于特定冲击压力值的“最弱”取向。这一观察结果支持了硅酸盐矿物中 PDF 的普遍形成机制。