A baseline study of mineralogical and morphological properties of different size fractions of illite du Puy

Abstract

The objective of this study is to revisit the mineralogical and physico-chemical properties of illite du Puy material (Puy-en-Velay, France), which is envisaged as a model clay system in the European Joint Programme on Radioactive Waste Management (EURAD) project. This illite material is compared to the raw illite material commonly extracted from the Velay ore site. Both materials were collected and separated into different subfractions (i.e., <0.05, 0.05–0.1, 0.1–0.2, and 0.2–2 μm) to examine the mineralogical heterogeneity. The X-ray diffraction bulk analyses indicated that the observed variability in the literature, in terms of the nature of accessory minerals and the presence of kaolinite, was directly related to the locations of geological sampling. Moreover, the respective mass contributions of the different subfractions were similar for the two materials.Further analyses were performed on the subfractions of the illite used for the EURAD project by utilizing a suite of different techniques. The clay mineralogy was refined as an illite–smectite mixed-layer mineral with a small amount (<5%) of smectite layers. No significant changes in chemical properties or particle lamellarity were observed between the different subfractions. However, the different subfractions displayed a wide range of specific surface areas (i.e., N2 BET surface) and cation exchange capacity values, the latter of which increased by factors of ~3 and ~ 5 from the 0.2–2 μm to the <0.05 μm size fractions, respectively. The EURAD illite and associated subfractions can be considered appropriate material models to be used for the refinement of our understanding of the adsorption and diffusion processes of solutes in porous media made from charged clay particles.