Coesite and quartz characteristic of crystallization from shock-produced silica melt in the Xiuyan crater

Abundant coesite and quartz coexist with the shock-produced silica glasses that retain the morphology of primary quartz in the gneiss clasts of polymict breccia from the Xiuyan crater. Quartz occurs as idiomorphic, needle-like and spherulitic crystals, and polycrystalline aggregates. Coesite occurs as granular or idiomorphic, dendritic and needle-like crystals, and polycrystalline aggregates of stringers. The occurrence and morphology of coesite and quartz characterize crystallization from an undercooled dense silica melt. During decompression, coesite firstly crystallizes from shock-produced dense silica melt at high pressure. As pressure decreases to the stability field of quartz, quartz crystallizes from the melt, whereas coesite remains metastable state. A growth rate of 10−3 m/s is evaluated for the crystallization of both coesite and quartz based on an estimation of the pressure duration of 10 ms in the coesite stability field for the crater. Higher pressure in silica melt could play a key role for promoting the rapid growth of coesite and quartz by reducing the viscosity of silica melt and the glass transition temperatures substantially. The occurrence of both coesite and quartz embedded in silica glass provides unambiguous evidence for an impact origin of the crater.