Hydration of Na-saturated Synthetic Stevensite, a Peculiar Trioctahedral Smectite
Résumé
Smectite interlayer water plays a key role on the mobility of elements and molecules, but also in a variety of geological processes. In contrast to saponite and hectorite, whose layer charge originates from isomorphic substitutions, stevensite layer charge originates from the presence of octahedral vacancies. Despite its common occurrence in lacustrine environments, hectorite hydration has received little attention, compared to saponite and hectorite. Early reports mention a specific hydration behavior, however, with the systematic presence of a low-angle reflection attributed to the regular interstratification of different hydration states. The present study aims at revisiting this specific hydration behavior in more depth. Within this scope, the hydration behavior of the above three smectite varieties are compared using synthetic trioctahedral smectites of similar layer charge, and different compositions of their octahedral sheets. The chemical composition of the octahedral sheet does not appear to influence significantly smectite hydration for saponite and hectorite. Compared to its saponite and hectorite equivalents, H2O content in stevensite is lower by ~2.0 mmol H2O per g of dry clay. Consistent with this lower H2O content, Zn-stevensite lacks a stable monohydrated state, dehydrated layers prevailing from 60 to 0% RH. The presence of the regular interstratification of 0W and 1W layers is responsible for the low-angle reflection commonly observed for stevensite under air-dried conditions. Finally, stevensite identification method based on X-ray diffraction of heated and EG-solvated samples is challenged by the strong influence of octahedral sheet chemical composition (Zn or Mg in the present study) on hectorite swelling behavior. The origin of this effect remains undetermined and further work is needed to propose a more general identification method.
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