Hydrothermally altered rhyolite rocks in the Permian Donnersberg Formation of a geothermal borehole in the Northern Upper Rhine Graben (Germany) were investigated to find out answers for the low hydraulic conductivity of the rocks. The composition of clay minerals and the temperature of smectite–illite transformation were carried out using X-ray diffraction, X-ray fluorescence, transmission electron microscopy, Fourier transform infrared spectroscopy, and polarized-light microscopy analyses. Clay mineral (CM) composition includes illite/muscovite (1M and 2M₁ polytypes), illite–smectite interstratifications (IS-ml), smectite, and chlorite; and non-clay minerals such as quartz, feldspars, epidote, calcite, dolomite, and hematite were detected. The 2M₁-polytype mica might be the only primary sheet silicates from the parent rocks, while the others occur as authigenic neo-formed CMs under heat flow and geothermal gradient. The development of CMs indicates different mechanisms of illitization and smectitization. Based on the texture, morphology, structure/polytype, and chemistry of rocks and minerals, in particular CMs, the study grouped the CM formation into three transformation processes: smectitization during magma cooling and possible contact metamorphisms with decreasing and low temperature, smectite illitization controlled by burial diagenesis and hydrothermal alteration, and illite smectitization followed exhumation and Cenozoic subsidence with decreasing temperature. The rhyolites were altered to all of the orders IS-R0, IS-R1, and IS-R3 by the dissolution-precipitation and layer-to-layer mechanisms. The first one supported small xenomorphic plates and flakes of 1M_d, elongated particles of 1M, and pseudo-hexagonal forms of 2M₁. The second one could lead to the platy particles of 1M_d and 2M₁ polytypes. The dominant temperature range for the transformation in the area has been 140–170 °C– ~ 230 °C.

对北莱茵上格拉宾（德国）北部一个地热钻孔的二叠纪唐纳斯贝格组中的热液蚀变流纹岩进行了研究，以找出岩石的低导水率的答案。使用X射线衍射，X射线荧光，透射电子显微镜，傅立叶变换红外光谱和偏光显微镜分析进行了粘土矿物的组成和蒙脱石-伊利石相变的温度。粘土矿物（CM）的组成包括伊利石/白云母（1 M和2 M ₁多型），伊利石-蒙脱石层间化（IS-ml），蒙脱石和绿泥石。并检测到了非粘土矿物，例如石英，长石，埃奇狄德，方解石，白云石和赤铁矿。2 M_1个-多型云母可能是母岩中唯一的初级片状硅酸盐，而其他云母则是在热流和地热梯度下以自生的新形成的CM形式出现的。CMs的发展表明了非法化和绿色化的不同机制。根据岩石和矿物（特别是CM）的质地，形态，结构/多型性和化学性质，研究将CM形成分为三个转变过程：岩浆冷却过程中的蒙脱石作用以及低温和低温时可能发生的接触变质作用，蒙脱石的不受控化作用通过埋藏成岩作用和热液蚀变，伊利石变绿化伴随着发掘和新生代沉陷随温度降低而发生。流纹岩已更改为所有IS-R0，IS-R1，和IS-R3通过溶解沉淀和层到层机制。第一个支持1的小异形板和薄片M _d，1 M的细长颗粒和2 M _1的准六边形形式。第二个可能导致1 M _d和2 M ₁多型的板状颗粒。该区域转换的主要温度范围为140–170°C–〜230°C。