Clay mineral formation in Permian rocks of a geothermal borehole at Northern Upper Rhine Graben, Germany

Abstract Hydrothermally altered rhyolite rocks in the Permian Donnersberg Formation of a geothermal borehole Northern Upper Rhine Graben (Germany) were investigated to find out answers for low hydraulic conductivity rocks. The composition clay minerals and temperature smectite–illite transformation carried using X-ray diffraction, fluorescence, transmission electron microscopy, Fourier transform infrared spectroscopy, polarized-light microscopy analyses. Clay mineral (CM) includes illite/muscovite (1 M 2 1 polytypes), illite–smectite interstratifications (IS-ml), smectite, chlorite; non-clay such as quartz, feldspars, epidote, calcite, dolomite, hematite detected. -polytype mica might be only primary sheet silicates from parent rocks, while others occur authigenic neo-formed CMs under heat flow gradient. development indicates different mechanisms illitization smectitization. Based on texture, morphology, structure/polytype, chemistry minerals, particular CMs, study grouped CM formation into three processes: smectitization during magma cooling possible contact metamorphisms with decreasing temperature, smectite controlled by burial diagenesis hydrothermal alteration, illite followed exhumation Cenozoic subsidence temperature. rhyolites all orders IS-R0, IS-R1, IS-R3 dissolution-precipitation layer-to-layer mechanisms. first one supported small xenomorphic plates flakes d , elongated particles pseudo-hexagonal forms . second could lead platy polytypes. dominant range area has been 140–170 °C– ~ 230 °C.