An extended revision of the interlayer structures of one- and two- layer hydrates of Na- vermiculite

Abstract

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).

Single crystal structure refinement of one- and two-layer hydrates of sodium fluorohectorite

“…Contrary to 1WL, for 2WL the interlayer cations reside at the central plane of the interlayer space as has been suggested by others, 23,31 and this is also supported by the Fo-map of the interlayer space (see Fig. S4, ESI{).…”

“…The H 2 O/Na molar ratio as determined gravimetrically was 3.2 and 5.6 for the 1WL and 2WL, respectively. The value for the 2WL was significantly higher than what has been reported by Beyer and von Reichenbach, 31 who have used a phlogopite with a layer charge per formula unit (p.f.u.) of x = 0.98.…”

“…2). As pointed out by Beyer and von Reichenbach, 31 with x = 1.0 pf.u., two densely packed planes of interlayer water result (see Fig. S6, ESI{).…”

“…Despite these difficulties, for 2WL a stable refinement was possible without restraining or even constraining atomic distances and bond angles between interlayer constituents of the 2WL structure, as done by Beyer and von Reichenbach. 31 For 1WL, all water molecules coordinating to sodium could be identified in the difference Fourier map, however, for stable refinement the distances between water molecules and between water and sodium had to be restrained by applying DFIX. Please note that the refinements were not restricted to selected classes of reflections that are less affected by stacking faults, instead all reflections have been used.…”

Impact of Intrinsic Structural Properties on the Hydration of 2:1 Layer Silicates

“…They are among the most dominant minerals in many soils and clay deposits. These types of clay minerals impart unique properties as a result of their intrinsic shrink–swell characteristics. − At low moisture content, crystalline swelling of 2:1 layer silicates proceeds in a stepwise expansion of the layer-to-layer distance. − The swelling shows hysteresis and desorption, and adsorption of water proceeds differently. − In this paper, we studied the hysteresis in clay swelling as a function of relative humidity (rh) using powder X-ray diffraction (XRD), gas adsorption analysis, and infrared (IR) spectroscopy and related these experimental results to the intrinsic properties (e.g., layer charge, charge location, octahedral composition, and particle size) of the clay minerals being studied. Besides their widespread importance in soils, clay minerals are also used in many different applications, such as construction materials or barrier materials in waste repositories .…”