On the Occurrence of Anhydrite in the Sulphatic Claystones of the Gypsum Keuper

We investigate why the sulphatic claystones of the Gypsum Keuper contain anhydrite rather than gypsum even at small depths of cover. This question is relevant due to the phenomenon of swelling of anhydritic claystones, which is attributed to the transformation of anhydrite into gypsum and has caused serious damage to a number of tunnels. In tunnelling, the Gypsum Keuper formation is crossed at rather small depths, where simplified thermodynamic considerations indicate that the calcium sulphate should be encountered in its hydrated form, i.e. as gypsum rather than as anhydrite. Understanding why anhydrite can be found at small depths is not only interesting from a fundamental point of view, but also necessary in order to formulate adequate initial conditions for the continuum-mechanical models that simulate the chemo-mechanical and transport processes in swelling anhydritic claystones. The paper quantitatively examines three reasons which, alone or in combination, might explain the occurrence of anhydrite: the small size of the pores in argillaceous rocks; locally high stresses in the vicinity of the sulphate crystals; and the thermodynamic state of the pore water. The computations of the paper take account of the results of porosimetry experiments on samples from two Swiss tunnels in Gypsum Keuper and show that the most probable reason is the thermodynamic state of the pore water, i.e. its ability to participate in chemical reactions. More specifically, the clay minerals reduce the chemical potential of the pore water, thus increasing the solubility of the gypsum and shifting the thermodynamic equilibrium in favour of anhydrite.