Iron speciation in Opalinus clay minerals

Abstract

Clay formations are under considerations in various countries to host a deep geological repository for high-level nuclear waste owing to beneficial physicochemical properties. These properties can, however, be influenced by the chemical form of structural Fe. The goal of this study was to obtain information on speciation of Fe associated with the clay fraction separated from the Opalinus clay rock, one of the clay formations at an advanced stage of repository planning. The clay rock was purified and the clay fraction was separated. Analyses by X-ray powder diffraction evidenced the presence of 1:1 (kaolinite) and 2:1 layer minerals (e.g., illite and possibly illite/smectite mixed layers), no other phase in significant amounts could be detected. Infrared spectroscopy corroborated the presence of both 1:1 and 2:1 layer minerals, and the absence of Fe3+2-OH bending in the spectrum ruled out extensive Fe clustering and hinted at a rather homogeneous distribution within the structure. Information on Fe speciation was obtained by X-ray absorption spectroscopy (XAS) at the K-edge. To reduce uncertainties on structural parameters, overlapping contributions from in-plane and out-of-plane shells were filtered by recording polarized XAS data. The position of the pre-edge features indicated the prevalence of Fe3+, while the intensity and splitting were typical of Fe located at octahedral site. The pre-edge of the reference kaolinite differed from that of the Opalinus clay sample, excluding the prevalence of Fe within that 1:1 layer mineral. Data thus suggested the prevalence of Fe3+ within the octahedral sheet of 2:1 layers. Polarized XANES and EXAFS exhibited significant angular dependencies hinting at the presence of various neighboring shells with specific orientations. The first O shell located at 2.03 Å contains about six O atoms oriented in-plane. At higher distances, three in-plane Al/Mg and Fe and about four out-of-plane Si atoms were detected at ~3.05 Å and 3.23 Å, respectively, suggesting that Fe is located in a dioctahedral octahedral sheet of 2:1 layers. The presence of significant amounts of Fe in the tetrahedral sheet was excluded. Combining results from XAS with that from chemical analysis enabled to estimate first the structural formula of the 2:1 layer fraction and subsequently the proportions of 1:1 and 2:1 layer minerals within the purified clay fraction of the Opalinus rock. The estimated proportions of 1:1 and 2:1 layer minerals were in reasonable agreement with that of the whole Opalinus rock.