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A B S T R A C T : 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 (n H 2 O /n Na & 4), sharing edges of Na(OH 2 ) 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 (n H 2 O /n Na & 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).

Dehydration and rehydration of a phlogopitic Mg-vermiculite were studied by thermo-analysis (TG, DSC) and in situ X-ray powder diffractometry. A detailed analysis of the relation between temperature and state of hydration of the vermiculite was rendered possible by use of slow heating rates in combination with a fast-recording position-sensitive detector. The results confirm the existence of a number of definite states of hydration characterized by basal spacings of 1.441,1.429,1.376, 1.165, 1.151,1.002 and 0.926 nm, all of which have already been described in the literature. In addition, so far undetected regular 1 : 1 interstratifications have been found. They formed during those transformations which lead to a change in the number of sheets of interlayer water molecules. These structures exhibit integral series of 00l reflections and d(001) spacings of 2.54 and 2.15 nm for the 2 : 1 and 1 : 0 transitions in the ratio of molecular layers of water, respectively.

Dehydration and rehydration of a phlogopitic Ca-vermiculite were studied by thermoanalysis (TG, DSC) and by in situ X-ray powder diffractometry. A detailed analysis of the relationship between temperature and state of hydration of the vermiculite close to equilibrium at atmospheric pressure was rendered possible by use of low heating rates in combination with a fast recording position-sensitive detector. The results reveal the existence of a number of distinct states of hydration characterized by basal spacings of 1.515, 1.493, 1.473, 1.188, 1.176 and 0.95 nm. In confirmation of results obtained earlier with Mg-vermiculite, regular interstratification occurred at dehydration during the transition between two-to one-and between one-to zero-layer hydrates showing integral series of 001 reflections with basal spacings of 2.67 and 2.11 nm, respectively.

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