The mineralogical composition of the host rock and the confining units is directly correlated to several safety-relevant aspects and serves as a good proxy for many relevant barrier properties. It provides direct input for estimating the sorption of radionuclides and for defining the porewater composition. In this context, it affects the speciation of solutes and therefore the mobility of radionuclides and the stability of the engineered barriers. Mineralogy, notably the clay-mineral content, correlates with porosity and geomechanical properties and thus plays a key role for self-sealing behaviour, permeability, and diffusion coefficients.