Coesite embedded in silica glass in suevite from the Xiuyan crater has been studied by scanning and transmission electron microscopy to better understand the mechanisms at formation of coesite. Coesite grains in this study mainly occur as vein-like aggregates (10–40 μm in width) and irregular aggregates (IAs; <40 μm in size). Both aggregate types are composed of subhedral to anhedral coesite crystals with random orientations. Most of the crystals are 100–1000 nm in size, and some display twinning. The shape, twinning, and random orientation of coesite crystals suggest rapid crystallization in amorphous silica that became supercooled. The center of vein-like aggregates crystallized from localized silica melt within diaplectic silica glass, whereas the rim of vein-like aggregates and IAs crystallized from diaplectic silica glass. The size and amount of coesite crystals in the vein-like aggregate vary greatly from the rim to the center of such veins. Microstructures suggest that the crystals nucleated heterogeneously at the outer rim of the vein and nucleated homogeneously within the vein. IAs do not show any changes in size and amount of coesite crystals from the rim to core of such aggregates. Coesite crystals in IAs primarily nucleate heterogeneously in diaplectic silica glass. It can be concluded that vein-like coesite aggregates are mainly formed by crystallization from silica melt, and irregular coesite aggregates should be formed by solid-state transformation of diaplectic silica glass.

中文翻译：

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岫岩撞击坑石英玻璃中柯石英的纳米结构及形成过程

已经通过扫描和透射电子显微镜研究了嵌入在岫岩陨石坑中的石英玻璃中的柯石英，以更好地了解柯石英的形成机制。本研究中柯石英颗粒主要以脉状聚集体（宽度为 10-40 μm）和不规则聚集体（IAs；尺寸 <40 μm）的形式出现。两种聚集体类型均由具有随机取向的亚自体到自体柯石英晶体组成。大多数晶体的尺寸为 100-1000 nm，有些显示孪晶。柯石英晶体的形状、孪晶和随机取向表明在变得过冷的无定形二氧化硅中快速结晶。脉状聚集体的中心是从局部二氧化硅熔体中结晶出来的，而脉状聚集体的边缘和 IAs 则是从折射二氧化硅玻璃中结晶出来的。脉状聚集体中柯石英晶体的大小和数量从这种脉的边缘到中心变化很大。微观结构表明晶体在静脉外缘不均匀成核，在静脉内均匀成核。IAs 没有显示柯石英晶体的大小和数量从这种聚集体的边缘到核心的任何变化。IAs 中的柯石英晶体主要在辩证石英玻璃中异相成核。可以得出结论，脉状柯石英聚集体主要是由二氧化硅熔体结晶形成的，不规则柯石英聚集体应该是由硅质硅玻璃的固态转变形成的。微观结构表明晶体在静脉外缘不均匀成核，在静脉内均匀成核。IAs 没有显示柯石英晶体的大小和数量从这种聚集体的边缘到核心的任何变化。IAs 中的柯石英晶体主要在辩证石英玻璃中异相成核。可以得出结论，脉状柯石英聚集体主要是由二氧化硅熔体结晶形成的，不规则柯石英聚集体应该是由硅质硅玻璃的固态转变形成的。微观结构表明晶体在静脉外缘不均匀成核，在静脉内均匀成核。IAs 没有显示柯石英晶体的大小和数量从这种聚集体的边缘到核心的任何变化。IAs 中的柯石英晶体主要在辩证石英玻璃中异相成核。可以得出结论，脉状柯石英聚集体主要是由二氧化硅熔体结晶形成的，不规则柯石英聚集体应该是由硅质硅玻璃的固态转变形成的。