Vermiculite interlayer cations

Water on Smectites. Compared to vermiculites, smectites present a more difficult experimental system because of the lack of stacking order of the layers. For these materials, the traditional technique of X-ray diffraction, either using the Bragg or non-Bragg intensities, is of little use. Spectroscopic techniques, especially nuclear magnetic resonance and infrared, as well as neutron and X-ray scattering have provided detailed information about the position of the water molecules, the dynamics of the water molecule motions, and the coordination about the interlayer cations. 

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. 

Foster (1963) established empirically that the cation exchange capacity of vermiculites could be calculated by multiplying the positive charges carried by the interlayer cations by 200. The calculated range of cation exchange capacities of the macroscopic vermiculite she studied ranged from approximately 80 to 200 mequiv/ 100 g. Nearly half the samples have a C.E.C. less than 140 and more than a third have values less than 120. Because it was not known what proportions of the Mg to assign to the interlayer position and what proportion to the octahedral sheet, these calculated values can only be considered minimum values. Macroscopic vermiculites most com-... 

A large attraction force between interlayer cations and adjacent siloxane cavities allows some cations with certain hydration energy to dehydrate. If the dehydrated cation radius is smaller than the inside diameter of the siloxane cavity, the mineral could collapse and an inner-sphere complex would form (e.g., K-vermiculite) (Fig, 4.3). When vermiculite contains a relatively strongly hydrated cation such as Ca2+ or... 

An important feature of the smectites, vermiculites and other 2 1 layer silicates is that isomorphous substitutions can occur in both the tetrahedral and octahedral sheets. Thus, substitution of Si by A1 occurs in the tetrahedral sheet, together with replacement of A1 by Mg, Fe, Li or other small atoms in the octahedral sheet. The substitutions lead to a deficit of positive charge, which is compensated by the presence of exchangeable, interlayer cations. 

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.)... 

But it was not to be. Try as we might, the difference in scattering lengths between the 6Li and 7Li isotopes was too small to permit us to measure the lithium ion distribution in the swollen state. We had to content ourselves with the results for the crystalline phase, where the behavior of the lithium ions is different from that of the larger alkali metal cations [27], Potassium and cesium ions bind directly to vermiculite clay surfaces rather than hydrating fully. Because only lithium-substituted vermiculites of the alkali metal series will swell macroscopically when soaked in water, it seems that interlayer cations must form fully hydrated ion-water complexes if the particles are to expand colloidally. This conclusion has since been supported... 

Note Cations listed first in curved brackets for the smectites and vermiculites (Na, Ca, K, and Mg) arc present as exchangeable interlayer ions. All the smectites and vermiculites (and thus interlayer iilite-smectites) have important amounts of interlayer water, the amount of which depends upon the clay and the nature of interlayer cations (cf. Brindley and Brown 1980). As is customary, these waters are left out of the mineral formulae. 

Another member of the illite group is hydrous mica, in which the principal interlayer cation is K. A smectite you might encounter is montmorillonite smectites can expand by incorporating water or organics between the structural layers. Vermiculite is derived from the Latin vermiculare, which means to breed worms, and describes what appears to happen when the material is heated rapidly. Otherwise it is very similar to phlogopite mica. As you would guess, most of these minerals have complex chemical compositions. 

Vermiculite is a hydrous, silicate mineral, which exfoliates greatly when heated sufficiently. The structure of vermiculite consists of 2 tetrahedral sheets for every one octahedral sheet. It has medium shrink-swell capacity with limited expansion. The cation exchange capacity is high in the range of 100-150 meq/100 g. The structure of typical vermiculite contains a central octahedrally-coordinated layer of Mg ions, which lies between two inwardly pointing sheets of silicate tetrahedra. These silicate layers are normally separated by two sheets of interlayer water molecules. Complete removal of water molecules leads to 9.02 A lattice. These layers are electrically neutral and interlayer cations occupy only about one-third of the available sites. The cohesion between the layers is typically weak [10]. 

The basal spacing of illite is 10 A. The K, Ca or Mg interlayer cations prevent the entrance of H2O into the structure. For this reason the basal spacing in illite is constant. Thus, the illite group minerals cannot expand by absorption of water like halloysite, vermiculite and smectite. 

Substitution of Si by Al in the tetrahedral site causes excess negative charge, which is balanced by interlayer cations. The most common interlayer cation in vermiculites is Mg others are Ca and Na. ... 

Smectite clay minerals, such as montmorillonite, saponite, nontronite, hectorite, stevensite, vermiculite and haloysite have been known to act as hosts of intercalated compounds [7] and to function as catalysts for various organic reactions [8]. The properties of smectites depend upon the interlayer cations [9], which are easily exchangable for inorganic or organic cations. 

The foregoing discussion indicated that several phyllosihcate minerals, either naturally or as the result of chemical treatment, have molecular species inserted between the siUcate layers. Water is the most common interlayer sp>ecies in nature, and water is normally found in smectites, vermiculites and hydrated halloysites. The quantity of interlayer water is a function of relative humidity and the type of interlayer cation, in the case of smectites and vermiculites. 

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