Nanotextures and formation process of coesite in silica glass from the Xiuyan impact crater

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.