Abstract
Hydration phase transitions in Na vermiculite, a specific example of expandable 2:1 layered silicates, were examined as a function of temperature at a constant water-vapor pressure by x-ray diffraction. The samples exhibited well-defined 0-, 1-, and 2-water-layer hydration states (WLHS's) with the basal spacings of 9.81, 11.84, and 14.89 \AA{}, respectively. The transitions between the hydration states were found to be first order, evidenced by a two-phase coexistence and large hysteresis. Substantial elastic deformation, which involves rotations of the tetragonal bases in the host silicate layers, was observed during the transitions. It was concluded that elastic distortion in the host layers may be responsible for the hydration phase transitions. In addition, formation of a (2a\ifmmode\times\else\texttimes\fi{}2b) superlattice specific to the 1-WLHS was found. A new interpretation of the previous Raman data from 1-WLHS Na vermiculite is also presented.
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