The transformations induced on argillaceous materials under thermal stress and the impact on their confining properties is a critical parameter, for example, in the nuclear waste storage. The samples were collected close to a basaltic intrusion in an argillaceous formation (argillites of Laumiere, Aveyron, France), which represents a natural analogue of thermal stress. The clay minerals characterization was conducted by the association of X ray diffractometry (XRD) and far infrared spectrometry (FIR) that allow to show an illitization of the highly illitic I-S minerals at basalt contact. The illitization is a phenomenon primarily dominated by the thermal influence. The weak and nonmonotonous variation of I-S crystallinity indexes corresponds to the influence of the geological context. The illitization is not monotonous because of superimposed processes. Those phenomena that had disturbed the heat transfer and the illitization are related to the geological context. Firstly, hydrothermalism was developing through the network of cracks. The high water/rock ratio had induced the basaltic chemical species transfers. Secondly, the presence of sedimentary heterogeneities as a limestone deposit had constraining the uniform dissipation of the heat. Laumiere study highlighted critical parameters that have influenced the evolution of argillaceous materials in thermal context.