Weathering features in shocked quartz from the Ries impact crater, Germany

Abstract

Shocked quartz from the ejecta of the Ries impact structure has been investigated by analytical transmission electron microscopy (ATEM). Quartz grains display numerous planar fractures (PFs) and planar deformation features (PDFs). Both are partly or fully replaced by a mineral of the kaolinite group (likely halloysite). Its formation involves fluid circulation into the dense fracture networks, dissolution and removal of the amorphous phase initially present in PDFs, and finally, precipitation and crystallization of the kaolinite group mineral from solutions resulting from the chemical alteration of adjacent minerals (feldspars and biotite). Kaolinite group minerals are typical of hydrothermal alteration at low temperature, in humid climate, and under moderately acid conditions and, thus, this alteration may not be directly related to the impact event itself. However, the weathering features were strongly enhanced by the shock-generated microstructure, in particular by fractures that provided pathways for fluid circulation.