Vermiculite water structure

Water on Vermiculite. For low water contents (that is, one or two water layers), the evidence for highly structured water in the interlayer spaces of smectites and vermiculites is most easily seen in X-ray diffraction structure determinations of ordered hydrate structures such as the two-water layer hydrate of Ca-vermiculite (14. 15) and Na-vermiculite (15., 16). 

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

Our model for the adsorption of water on silicates was developed for a system with few if any interlayer cations. However, it strongly resembles the model proposed by Mamy (12.) for smectites with monovalent interlayer cations. The presence of divalent interlayer cations, as shown by studies of smectites and vermiculites, should result in a strong structuring of their primary hydration sphere and probably the next nearest neighbor water molecules as well. If the concentration of the divalent cations is low, then the water in interlayer space between the divalent cations will correspond to the present model. On the other hand, if the concentration of divalent cations approaches the number of ditrigonal sites, this model will not be applicable. Such a situation would only be found in concentrated electrolyte solutions. 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

Vermiculites have a 2 1 layer structure similar to smectites, but expand less freely in water, presumably because of the higher layer charge in the former minerals. Most of this structural charge resides in the tetrahedral layers of the vermiculite platelets. Even when fully wetted, vermiculites do not expand beyond the two water-layer stage ( " 1.5 nm c-spacing). 

Fig. 2. 15 Schematic representation of the magnesian-vermiculite structure. (A) The structure projected on (010) showing the layering of T and O sheets, 2 1, with the additional molecular water and ion sheet. (See Fig. 2.13 for comparison of...

Vermiculites exist in various stages of dehydration. Because of the similar dimensions of the water-cation layer in vermiculite and the brucitelike layer in chlorite, vermiculites can be confused with the chlorites. The common substitutions of Fe" or Fe for Mg (in either the water or octahedral sheet of vermiculites), and AF for Si (in the tetrahedral sheets), as well as the hydration variations, present enormous potential for structural distortion in these types of minerals. Fibrous vermiculite was described by Weiss and Hofmann (1952). 

There are more complicated structures intermediate between pyrophyllite and talc with variable substitution of A1J and Mg2. Electroneulrality is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual days forming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip puints. They are derived from the formulation AU(OH)jSi40 ,-x-H2o with variable amounts of water, Mg3+ (in place of some Al5 ), and compensaUng cations. M"+ (M = Ca in fuller s earth, which is converted to bentonite, M = Na). Vermiculite likewise has variable amounts of water and cations, (t dehydrates to a talc-like structure with much expansion when heated (see page 750). 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

Considerable advances in the understanding of the structure of interlayer water in both smectites and vermiculites have been also achieved by means of computational simulations [61]. 

Recently, it was noted that the addition of poly(vinyl methyl ether) to a clay-salt-water system induced the contraction of the interlayer separation between the clay platelets [18], The K-butylammonium vermiculite used in those experiments provided an ideal structure of regularly spaced and parallel charged colloidal platelets in a salt solution [19,20], Swenson et al. [21-23] studied the effect of the addition of poly(ethylene oxide) (PEO) on the distance between the vermiculite layers and observed that, with increasing PEO concentration, the distance between the vermiculite layers decreased. They estimated that the bridging force per bridge was 1.4 pN. 

FIGURE 1.1 Schematic illustration of the structure of a crystalline vermiculite. Within the clay plates, the open circles represent oxygen, the closed circles silicon, and the shaded circles magnesium. The oxygen-oxygen separation of the surface layers is about 7 A. In the interlayer region, the open circles represent water molecules and the shaded circles univalent cations. The dotted lines show the unit cell. 

FIGURE 1.2 Structure of a hydrated sodium Llano vermiculite determined by X-ray diffraction [5]. The experimental structure amplitudes were assigned phases calculated on the basis of scattering by the atoms of the silicate layers only, and the resulting observed structure factors (Fo values) were used, in conjunction with the calculated structure factors (Fc values), to compute Fo-Fc projections of the electron densities onto the 010 and the 100 faces of the unit cell, shown in the parts (a) and (b), respectively. That the interlayer cations and water molecules are in octahedral coordination accords with these Fourier projections. (Reproduced with kind permission of the Clay Minerals Society, from Slade, P.G., Stone, P.A., and Radoslovich, E.W., Clays Clay Min., 33, 51, 1985.)... 

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