Post-impact alteration of surficial suevites in Ries crater, Germany: Hydrothermal modification or weathering processes?

Alteration of surficial suevites at Ries crater, Germany was studied by means of X-ray diffraction and scanning electron microscopy. Here, we discuss the origin of hydrous silicate (clay) phases in these suevites that have been previously interpreted as resulting from post-impact hydrothermal processes. The results of this study indicate that the dominant alteration phases are dioctahedral Al-Fe montmorillonite and halloysite, which are typical low temperature clay minerals. We suggest that the surficial suevites are not altered by hydrothermal processes and that alteration occurred by low temperature subsurface weathering processes. If the surficial suevites were indeed hydrothermally modified during the early stages of post-impact cooling, then the alteration was of limited character and is completely masked by later weathering.