Impact of interlayer cation composition and strongly bound water on smectite ẟ²H, as determined by a modified TCEA method

Nadine J. Kanik, The University of Western Ontario

Abstract

Interlayer cation composition and cation hydration enthalpy can potentially affect the excess hydrogen-yield and δ²H of smectitic clays. Complexities arise from condensed atmospheric water adsorbed on the clay, as it constitutes a potential additional source of hydrogen when attempting to measure the δ²H of structural OH groups. To approach this problem, the δ²H of different cation-saturated, dried forms of six Clay Minerals Society Source Clay smectites were measured. A modified sample drying and on-line High-Temperature-Conversion-Elemental-Analysis (TCEA) Continuous-Flow-Isotope-Ratio-Mass-Spectrometry protocol, facilitated isotopic measurements. A stronger interlayer cation hydration enthalpy (Ca²⁺>Na⁺>K⁺), caused higher residual (post-degassing) adsorbed water contents, which produced poorer smectite δ²H reproducibility. Drying K-saturated smectites, 4 hours at 220˚C, produced the lowest adsorbed water contribution and the most reproducible and possibly ‘accurate’ δ²H for smectite hydroxyl hydrogen. Notwithstanding limited rehydration potential, the TCEA method provided the lowest measurement error for hydroxyl δ²H and facilitated greater sample throughput than classical methods.