Structural vermiculite

A different clay derives from the layered mineral talc, Mg3(Si40io)(01T)2. If iron(II) and aluminum replace magnesium and silicon in varying proportions and water molecules are allowed to take up positions between the layers, the swelling clay vermiculite results. When heated, vermiculite pops like popcorn, as the steam generated by the vaporization of water between the layers puffs the flakes up into a light, fluffy material with air inclusions. Because of its porous structure, vermiculite is used for thermal insulation or as an additive to loosen soils. 

Silicates with layer. structures include some of the most familiar and important minerals known to man, partieularly the clay minerals [such as kaolinite (china clay), montmorillonite (bentonite, fuller s earth), and vermiculite], the micas (e.g. muscovite, phlogopite, and biotite), and others such as chrysotile (white asbestos). 

Vermiculite is a naturally occurring group of hydrated aluminum-iron-magnesium silicates having a laminate structure. When subjected to direct heat in a furnace, the pulverized material exfoliates or expands in size, and then consists of a series of parallel plates with air spaces between. 

The two types of clay mineral structures which are of interest in the present discussion are the expanding 2 1 structures (the smectites and vermiculites) and the 1 1 structures (the kaolins). 

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). 

Roth CB, Jackson ML, Syers JK. 1969. Deferration effect on structural ferrous-ferric iron ration and CEC of vermiculites and soils. Clays and Clay Minerals 17 253-264. 

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). 

Chlorite and Vermiculite. Chlorite is a 1,4-nm (14 A) clay mineral that cannot be expanded or collapsed by traditional laboratory procedures. Structurally, the unit layer of chloride is composed of a 2 I layer combined with a ().4-nin Mg or Al interlayer or hydroxide sheet. 

Figure 7.37 Comparison of the X-ray crystal structure of (a) / -sulfonatocalix[4]arene with (b) the naturally occurring clay mineral sodium vermiculite. (Reproduced with permission from [49]).

---