Isomorphous substitution smectites

The Smectite Clays. The smectite-type clays are distinctive in that they expand and cause significant destruction to synthetic (human-made) structures. In this type of 2 1 clay, isomorphous substitution occurs in the aluminum sheet. If there is substitution of lower-oxidation-state metal such as magnesium, there will be an unsatisfied pair of bonding electrons in the interior of the crystal and there will be no noticeable change in the surface. Because the charge is in the interior of the crystal, its attraction for cations is diminished by distance. Thus, smectite crystals are not held together strongly by cations and are able to incorporate more water and ions between sheets when the environment is wet and less when it is dry. 

The material from the Hector area of California is believed to have formed by the action of hot spring waters containing Li and F on clinoptiolite. The Mg was obtained from the alkaline lake waters (Ames and Goldich, 1958). The material from Morocco is associated with marls and is believed to be authigenic. These two types of trioctahedral smectite appear to be the only ones with a relatively pure Si tetrahedral sheet. No analyses were found which indicated tetrahedral Al values between 0.02 and 0.30. Analyses of saponite indicate there is complete isomorphous substitution between the range Si3.70 Al0.3o and Si3.0s Al0.92 (Table XXXIX). Caillere and Henin (1951) reported an analysis of a fibrous expanded clay (diabantite) which had a tetrahedral composition of Si3.i7 Alo.49 Fe3+0.34. There is some question as to whether this should be classified as a smectite regardless, it indicates the possibility of Fe3+ substitution in the tetrahedral sheets of the trioctahedral 2 1 clays. 

As indicated in Table 1, the three 2 1 groups differ from one another in two principal ways. The layer charge decreases in the order illite > vermiculite > smectite, and the vermiculite group is further distinguished from the smectite group by the extent of isomorphic substitution in the tetrahedral sheets. Among the smectites, those in which substitution of Al for Si exceeds that of Fe2+ or Mg for Al are called beidellite, and those in which the reverse is true are called montmorillonite. The sample chemical formula in Table 1 for smectite thus represents montmorillonite. In any of these 2 1 clay... 

Figure 3.8. Schematic of the smectite structure showing one gibbsite sheet between two silicate sheets. The basic unit is repeated many times in the horizontal directions to produce layers. The basic unit with the 9.6 A c-axis spacing expands to 14 A when water enters between layers. The exchangeable cations located between the layers produce the counter charge for the isomorphous substitution. It occurs in the layers marked with an asterisk (from Taylor and Ashcroft, 1972, with permission).

Whereas the surfaces discussed so far have been generated from the bulk by a simple cut, leading to a decrease in the coordination number of the surface atoms, catalytically important acidic surfaces can also be generated in microporous or layered materials by isomorphous substitution of lattice cations. This occurs in zeolites and smectite clays. Zeolites and clays can be considered as aluminosilicates. Their lattice compositions can vary significantly. In zeolites the Al3+ ion can be substituted by many other trivalent cations. Si4+ can be partially substituted by Ti4+ or Ge4+. 

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. 

Finally, clays such as the smectites almost invariably have a net negative structural charge because of isomorphous substitution of cations of lower charge than would be present in a balanced structure. In kaolinite, the amphoteric nature of the hydrated aluminum and silica surface contributes more to surface charge than does substitution. As a result of either substitution or surface dissociation, a region of counter ions (exchangeable and... 

The incorporation of Mg 2+ into OL-1 is consistent with the finding of Mg2+ in some natural birnessite materials. The Mg2+ isomorphous substitution in octahedral sites of OL-1 is also similar to the substitution of Mg2+ ions in octahedral layers of certain smectite clays where Mg2+ substitutes for some A 2+, such as in montmorillonite which has an ideal composition of... 

Hectorite is an aluminum-free mineral of the smectite type. Isomorphous substitution could occur at tetrahedral silicon sites as well as at the octahedral sites originally occupied by lithium and magnesium. Monitoring the x-ray powder diffraction patterns as a frmction of crystallization time, it was found that the hydrothermal crystallization was complete after 12h at 200°C, independent of the alumina content of the reaction mixture. However, NMR spectroscopy proves that some structural change still occurs after this time period. 

Isomorphous substitution in the crystal lattice for example in the clays, AP+ replaces Si in the tetrahedral layer and Mg replaces Al in the octahedral layer. This results in an excess of bonds and is important for illites it is the chief cause of negative surface charge for the smectites and vermiculites. 

The permanent charge of clay minerals is due to lattice imperfections or defects, plus isomorphous substitutions. Sposito (1989) suggests that the permanent negative charge of illites, smectites, and vermiculites in mol sites/kg, ranges from 1.9 to 2.8. 0.7 to 1.7, and 1.6 to 2.5, respectively. 

The cation exchange capacity of clays results from lattice imperfections or defects, isomorphous substitutions, and/or broken bonds on clay particle surfaces. Explain how the CEC s of kaolinite, the smectites, and illite, and their variation with pH, reflect these sources of their surface charge. 

Smectites, which are based on either the trioctahedral 2 1 (talc) or dioctahedral 2 1 (pyrophyllite) structure, differ from these neutral structures by the presence of isomorphous substitution in the octahedral or tetrahedral sheet. For example, the dioctahedral smectite, montmorillonite, has the general formula... 

Montmorillonite is a layered smectite clay. It has a central gibbsitic octahedral layer of alumina sandwiched between two tetrahedral layers of sUica. But usually isomorphous substitution occurs and natural clay has Al substituted by Mg and Fe leading to negative charge on the layers. The interlamellar space between the layers is occupied by hydrated cations usually Na", K Ca" etc. balancing the negative charge on the layCTS. 

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