The interlayer structures in one- and two-layer hydrates of Na-vermiculite are described by presenting the positional parameters of their constituent atoms, including protons.

The revision of these structures has been accomplished by: (1) determining accurate water contents of the two hydrates by thermoanalysis (TG, DSC); (2) using earlier X-ray diffraction data published by Slade et al.(1985) and de la Calle et al. (1984) as a reference; and (3) applying least-squares refinements when considering the constraints for atomic distances and bond angles between interlayer constituents.

In the 1.485 nm hydrate of Na-vermiculite (nH2O/nNa ≈ 4), sharing edges of Na(OH2)6 octahedra cause their chainlike arrangement. The chains are aligned along [100] and are stabilized by H-bonds between interlayer water molecules and oxygens of the silicate layer. The partial loss of these bonds during dehydration forces the stacking order to change from V3 to Vc in the resulting 1.185 nm (nH2O/nNa ≈ 2) hydrate.

This new understanding may help to explain differences in the rotational correlation time of water molecules between one- and two-layer hydrates of vermiculite as observed by quasielastic neutron scattering (Swenson et al., 2000).

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