Structural beidellite

Clays have a loose layer structure (Figure 4.6). Characteristic minerals are montmoril-lonite and beidellite. Aluminosilicates such as montmorillonite, kaolinite, and feldspar can act as cation and anion exchangers. 

As we have seen in the previous section, the bulk chemical compositions of montmorillonites taken from the literature are dispersed over the field of fully expandable, mixed layered and even extreme illite compositions. Just what the limits of true montmorillonite composition are cannot be established at present. We can, nevertheless, as a basis for discussion, assume that the ideal composition of beidellite with 0.25 charge per 10 oxygens and of montmorillonite with the same structural charge do exist in nature and that they form the end-members of montmorillonite solid solutions. Using this assumption one can suppose either solid solution between these two points or intimate mixtures of these two theoretical end-member fully expandable minerals. In either case the observable phase relations will be similar, since it is very difficult if not impossible to distinguish between the two species by physical or chemical methods should they be mixed together. As the bulk chemistry of the expandable phases suggests a mixture of two phases, we will use this hypothesis and it will be assumed here that the two montmorillonite... 

When pillared smectites without tetrahedral substitution are calcined, there is no reaction between the pillars and the smectite layers. By contrast, a considerable structural transformation occurs when pillared beidellite is calcined, which has been interpreted as the growth of a three-dimensional quasi-zeolitic framework between the two-dimensional clay layers. The acidic properties of the product are comparable with those of zeolite Y and much more pronounced than those of calcined pillared smectites without tetrahedral substitution. 

TABLE XXVI Statistical data on structural fomulas of 101 montmorillonites-beidellites ... 

Fig. 14. Histograms showing the distribution of the cations of 101 montmorillonite-beidellite structural formulas.

There is considerable overlap between minerals named montmorillonite and nontronite. Many of the montmorillonite samples that overlap the nontronite field are relatively high in iron but so are a number of samples that lie in the restricted montmorillonite zone. The structural formulas indicate that there is a complete gradation between montmorillonite-beidellite and nontronite so that any boundary is arbitrary. 

Exploration of the pillar-clay sheet reactivity and connectivity also indicate the important role of the specific clay type. 27 1 and 29si-MASNMR experiments have shown distinctive differences between pillaring mechanisms in trioctahedral hectorite and dioctahedral montmorillonite. Whereas Plee et al. (22) concluded that chemical crosslinking may occur between the pillar and tetrahedral layer in a beidellite montmorillonite, Pinnavaia et al. (23) showed that it did not occur in a hectorite. These are the first observations of a complex process that may depend upon several structural and chemical factors, such as substitution of Al in the tetrahedral layer, or the need for vacancies in the octahedral layer to allow rotation of structural units or migration of reactant species to facilitate crosslinking. Ongoing research should further elucidate refinements on these mechanisms, and direct the technology towards more optimized catalysts - presumably those which form chemical bonds between the pillar and clay layer. 

Figure 5 Expanded view of calcined pillared beidellite. Note the modified pillar structure that shares three oxygen atoms with inverted AIO4 tetrahedra of the tetrahedral layers. (From Ref. 30.)...

The smectite clays, for example, montmorillonite, hectorite, beidellite, and the mica minerals, for example, talc and pyrophillite possess a structure which is exemplified in Figure 67. The host lattice is formally composed of three sublayers two tetrahedral Si/O and one octahedral M/O, OH (M = Al, Mg) central layer. These layers bear an excess negative charge and compensation of these charges is achieved by the presence of interlayer cations. 

We here use a beidellite system as the smectite mineral instead of montmorillonite because in our target bentonite both ate included and have similar properties. The molecular formulation with n-mole water is denoted as NamAl2[Si i,3Alif3]0 o(OH) . /1H2O (n is called the hydration number). For this hydrated beidellite we investigate the structure of interlayer water molecules and exchangeable... 

Figure 3. Structural srmp shots of hydrated beidellite for various hydration numbers n at 293K and O.IMPa. Circles are hydrogen (dark blue ), silicon (yellow green ), aluminium (pink o ), sodium (sky blue o), and oxygen (yellow O) atoms.

Other studies in this field for which the SIESTA approach has been used include the study of cation distributions in phyllosilicates like pyrophyllite, beidellite and several smectites and ilhtes [149], a study of the (001) surface of galena [150] and the combined theoretical-experimental study of the structure oh the high-pressure monoclinic phase II of cristobalite [151]. 

It has been previously reported that the structure of interstratified clays (such as rectorite) consists of a combination of mica-(nonexpandable) and smectite- (expandable) type layers. The mica layers are more probably Na-paragonite while the smectite layers are of the montmorillonite or beidellite type (3, 11-13). For mica-like layers, the most typical interlayer spacing is equal to 9.6A while it is of 9.5A for the smectite-like layers, see Figure 6-2. However, these d-values may change depending on the nature of the interlayer cations... 

Schutz et al. 1987. Preparation and characterization of bidimensional zeolitic structures obtained from synthetic beidellite and hydroxy-aluminium solutions. Clays and Clay Minerals 35 251-61. 

The structure is based on groups of three layers Single sheet of (Al, Mg) (O, OH)g octahedra are sandwiched between two sheets of SiO tetrahedra. Montmorillonite is a member of the smectite group and forms solid solution with beidellite. 

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