Smectites structural formulas

Table III. Structural Formulae for Smectites Used in Wetting and Drying Experiments, Based on 0io(OH)2 [Formulae Calculated from Data in Table II, Unless Noted Otherwise]... 

Van der Kaaden and Quakernaat (1968) reported a manganiferous smectite in which Mn3 + occupies an octahedral position. They gave the following structural formula ... 

Christidis, G. E. 2008. Validity of the structural formula method for layer charge determination of smectites A re-evaluation of published data. Appl. Clay Sci. 42 1-7. 

Table I. Idealized Structural Formulae for Principal Smectite Clays...

Table I. Idealized structural formulae for principal smectite clays.

The importance of chemical analyses of smectites is easily demonstrated. It leads to the establishment of a structural formula, and each time a formula for a smectite is established, it provides additional proof of the H-M-H model. On the other hand, the structural formula is a fundamental element of the definition and description of a particular smectite. 

Smectites are fairly widespread in soils. All smectites can be synthetically produced by hydrothermal processes. The structural formula and consequently the charge are inherited from the initial composition of the gel. 

These results are very important The formation conditions of natural smectites are quite variable and are certainly different from those of hydrothermal synthesis. Koizumi and Roy s data show that the structural formulas and the charges on the natural minerals determine their stability. 

The smectite group [see Table 9.1 and Fig. 9.3(a)] includes any clay whose interlayer repeat distance (thickness of individual T 0 T layers plus interlayer spacing) expands to 17 A on treatment with ethylene glycol (cf. Drever 1988). This is indicative of a structure in which the number of interlayer cations is smaller than 0.65 (Greenland and Hayes 1978), and is usually about 0.2 to 0.5 per formula unit Ojo(OH)2 (Drever 1988). The interlayer cations are adsorbed, and are necessary to balance the unsatisfied net charge (usually negative) of the clay crystal lattice caused by structural substitutions or vacancies in the octahedral and/or tetrahedral layers. For example Mg or another divalent cation may substitute for Al in the octahedral layer, or Al or Fe may replace Si" " in the tetrahedral layer. 

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... 

Mostly focused on cationic clays, and particularly on montmorillonite and hectorite, smectite-type layered silicates and clay-based nanofillers have recently been extended to the family of LDH. Hydrotalcite-like LDH materials are described according to the ideal formula, [M1/ xM"l(OH)2]frl(lra [A H20]inter, where Mn and Mm are metallic cations, A the anions, and intra and inter denote the intralayer and interlayer domain, respectively. The structure consists of brucite-like layers constituted of edge-sharing octahedra. The presence of trivalent cations induces positive charges in the layers that are counterbalanced by interlamellar anions (Scheme 15.16). 

The 2 1 layer type has two tetrahedral sheets sandwiching an octahedral sheet. The three clay groups with this structure are (illitic) mica, vermicu-lite, and smectite (montmorillonite), each with the general unit cell chemical formula ... 

These figures establish the validity of the H-M-H model. At the same time, they show that the dioctahedral and trioctahedral smectites form two distinct series there are no intermediate forms, at least in the principal types outlined above. Each time that an analysis places, for example, 4,5 atoms in the octahedral layer, it can be assumed that the formula is incorrect. The reason is that either there is too great an amount of impurities or there is an error (for example, confusion between the exchangeable ions and the atoms integrated in the structure). 

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