Fundamental aspects of smectites

Natural clays are the products of weathering of rocks [26]. From the structural point of view, all clays are layered silicates, also described as phyllosilicates (phyllo = leaf-like).

Table 1.1. shows the classification of natural and synthetic smectites. Dioctahedral aluminous smectites are represented by the montmorillonite-beidellite series according to the structural formula

(Al

2-y

Mg

y2+

)(Si

4-x

Al

x

)O

10

(OH)

2

E

⁺x+y

.nH

2

O .

Where the amount of E⁺ represents the interlayer cation,

x

and

y

the octahedral and tetrahedral substitutions, respectively. The smectites with

y>x

are called montmorillonite (Figure 1.2.), and those with

y<x

are called beidellites. Appreciable amounts of trivalent Fe often occur in octahedra. Montmorillonites are commonly the main constituents of the rocks known as bentonites, whereas beidellites are frequently found in soils as weathering products of detrital micas.

The basic structural components of the smectites are the octahedral (consists of two planes of spherical anions (O, OH)) and tetrahedral sheets (are composed of six-fold hexagonal rings) and interlayer configurations.

The final structure of a clay sheet is the result of a condensation of the tetrahedral silica sheets with the octahedral sheets. This happens by sharing the apical oxygens of the silica layer with the free oxygens of the octahedral layer (Figure 1.3.).

Table 1.1. Classification of natural and synthetic smectites [26].

DIOCTAHEDRAL SMECTITES TRIOCTAHEDRAL

SMECTITES Ratio between Predominant Smectite Predominant Smectite Tetrahedral (xt)

and

Octahedral Species Octahedral Species

Octahedral (x0) Cation (s) Cation (s)

(tetrahedral Fe³⁺ Nontronite Fe²⁺ Iron saponite

Charges Cr³⁺ Volkonskoite Zn Sauconite

Predominant) V³⁺ Vanadium Co Cobalt smectite

Smectite Mn Manganese

Both octahedral and tetrahedral cations might be substituted by other elements, as long as these new cations have the appropriate size to fit in the structure (e.g.

Si

⁴⁺o

Al

³⁺,

Al

³⁺o

Mg

²⁺,

Mg

²⁺o

Li

⁺). The phenomenon is called ’’isomorphic substitution’’ and is responsible for some very important properties of the clay minerals [27]. Since the substituting ions might have another charge (mostly a lower valence), the initially neutral clay sheet will now carry a net negative charge. This excess of negative layer charge is compensated by the adsorption on the layer surfaces of cations, which are too large to be accommodated in the interior of the crystal.

In the presence of water, the compensating cations on the layer surfaces may be easily exchanged by other cations when available in solution; hence they are called

“exchangeable cations”. The total amount of these cations may be determined analytically. This amount, expressed in milliequivalents per 100g (or per gram) of dry clay, is called the cation exchange capacity (CEC) of the clay. One equivalent of Ca²⁺

would be equal to one mole divided by the oxidation state of calcium or could be a combining weight of 40 divided by the oxidation state. The number of interlayer cations is assumed to be stoichiometric with respect to the net negative layer charge. Although cations with higher valencies may substitute in the interlayer, the univalent and divalent cations are more common.

Figure 1.2. Diagrammatic sketch of the structure of the montmorillonite after Grim [28].

The property of ion exchange is great fundamental and practical importance in the investigation of the clay minerals. In the application of clay mineralogy it is important because the nature of the exchangeable ion may influence substantially the physical properties of the material. Thus, a clay material carrying sodium frequently has very different plastic properties than material the same in every way except that calcium is the exchangeable cation. There are various methods to determine the CEC [29-39], however, they can give mutual differences. This is because the charge on the sheets does not only arise from the isomorphic lattice substitution, there is also a contribution from broken bonds at the edges of the layers, and from protonated or de-protonated hydroxy-groups.

Figure 1.3. The condensation of a tetrahedral and octahedral layer results in the formation of so-called TO-clay sheet after Nemecz [27].

As a result, there is a pH-independent part of the CEC, which arises only from the isomorphic substitution, and a pH-dependent part. The pH-dependent CEC might be up to 10% of the total CEC and depends strongly on the crystal size and shape, pH, type of exchangeable cation and also the method that was used to determine the CEC.

It has generally considered that the CEC of montmorillonite does not change substantially. The change in CEC is reduced on heating to various temperatures but the effect is not uniform and varies with the cation present.

Another noticeable property is the mobility of the interlayer cations in the clay structure.

Upon heating the clay, these cations might irreversibly migrate into the octahedral layer of the clay. The effect is called the Hofmann-Klemen effect [40] and four conditions have to be fulfilled in order to see this migration (Figure 1.4):

• The clay layer must have a negative charge, from the octahedral layer.

• The octahedral layer must have vacant sites (as in dioctahedral clays).

• The cations should be small enough to migrate through the Si-O 6-rings of the tetrahedral layer into the octahedral part of the clay sheet.

• The effect occurs mainly at increased temperatures.

Figure 1.4. The Hofmann-Klemen effect. Here the migrations of Li⁺ ions through the ditrigonal Si-O rings of the T layer into the vacant cation sites of the octahedral layer.

The result of the Hofmann-Klemen effect is that the interlamellar cations are trapped in the octahedral layer and cannot be exchanged for other cations any more. Not only looses the clay its CEC, also its ability to swell in polar solvents is lost.

The HK-effect might be used however in a positive way, namely to reduce the charge or CEC of clays in a controlled way or to distinguish dioctahedral clays with isomorphic substitution in the octahedral layer from those with substitution in the tetrahedral layer.

If the Li⁺-form of the first type is heated, then the clay will not re-expand any more due to the HK-migration of the lithium ions. In the latter case, the negative charge is located in the tetrahedral part, and Li⁺ can not migrate into the dense tetrahedral structure.

Since the cation remains in the interlamellar space, even after heating, this type of clay will still be able to expand in polar solvents. For some applications however, it might be useful to select other clay minerals. If, for some reason, the HK-effect has to be

avoided, one has to choose a trioctahedral smectite, saponite [41]. Rectorites are used if high thermal stabilities are required [42] and for the investigation of the alumina pillars with ²⁷Al-MAS-NMR, it is better to choose an Al-deficient clay, hectorite [43].

In document Pillérezett rétegszilikátok előállítása és vizsgálata (Pldal 12-17)