Isomorphous substitution vermiculites

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... 

Variations on this overview conceptualization occur because of differing isomorphic substitution patterns in 2 1 clay minerals. For example, both Li- and Na-vermiculite can form mono- and bilayer hydrates, whereas K-, Rb-, and Cs-vermiculite cannot. For the latter, inner-sphere surface complexes are stable against solvation of the cations because of the softer Lewis acid character of the latter and a favorable siloxane-surface stereochemistry [23]. 

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. 

Vermiculite, a 2 1 clay mineral, is also a hydrous mica, with isomorphous substitution in the tetrahedral sheet, resulting in a charge of between 1.2 and 1.9 mol negative charge per unit cell. In this case the... 

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. 

Aluminosilicates such as montmorillonite, kaolinite, illite, and vermiculite are solids that have structures readily accessible to counter ions. The excess negative charge resulting from isomorphic substitution of Al for Si is primarily distributed over the three adjacent surface O atoms of the layer, where it is electrically balanced by mobile, exchangeable cations- Thermodynamically, ion exchange can be interpreted in terms of the interlayer electrostatic interaction between surface charges and hydrated cations in accordance with the classical Eisenman theory (Eisenman, 1983). A comprehensive description has recently been given by Maes and Cremers (1986). 

The primary mineral, chlorite, which occurs in rocks as large crystals, possesses an interlayer sheet composed largely of Mg(OH)2. Since the mineral brucite is composed of magnesia sheets with the same basic structure, the single interlayer sheet in chlorite is termed the brucite layer. Isomorphous substitution of part of the Mg by produces a positively chained hydroxide sheet (see formula above) that props the 2 1 layers apart at a c-spacing of 14 A. This rigid interlayer contrasts with the hydrated interlayer of vermiculite, and even though chlorite and vermiculite have similar c-... 

The configuration of water molecules between the basal planes in vermiculite depends sensitively on the nature of the cation complexed on these surfaces to balance the negative charge produced by isomorphic substitution of for in the tetrahedral sheet. Because of this substitution, the siloxane ditrigonal cavities exhibit a relatively ItKulized... 

The tetrahedral sheet of chlorites contains isomorphic substitution by Al(III), and sometimes Fe(III) or Cr(III) is found (Kohut and Warren 2002). The octahedral sheet normally contains Mg(II), Al(III), Fe(II), and Fe(III), but some substitution by Cr(III), Ni(II), Mn(II), or other metals occurs as well. Chlorites, in contrast with vermiculites and smectites, are hardly expandable, because the hydroxide cations are not easily exchangeable, nor is water easily incorporated to the intermediate octahedral sheet. 

This particle is naturally occurring and found around the world. It is easily mined and purified. The reactor for the particle was a volcano. The ash from many volcanoes was spread around the earth during an intense period of activity many millions of years ago. This ash was transformed into clay (montmorillonoids or smectites) by natural processes, into uncharged species (talc and pyrophyllite) and charged species through isomorphic substitution of the crystal structure (hectorite, montmorillon-ite, saponite, suconite, volchonskoite, vermiculite, and nontronite). 

The other key difference is the level and type of isomorphous substitution in each of these minerals. Among the trioctahedral clays, talc has no isomorphous substitution, hectorite has moderate substitution of Li for Mg and vermiculite and mica have high levels of substitution. 

SMECTITE GROUP MINERALS. The structuTe of Water adsorbed by smectite group minerals has been studied extensively in both its static (D structure) and dynamic aspects. As with water molecules on vermiculite, the behavior of water on smectite surfaces is conditioned sensitively on the type of exchangeable cation and on the location of isomorphic cation substitutions in the layer structure. In many respects, a discussion of the configuration of water molecules hydrating smectites is parallel to that for vermiculite. 

These experiments do not prove, but suggest the existence of a qualitative boundary separating saponites from vermiculites. This boundary cannot be found in the structural formula. If it exists, it can only be found in the distribution (ordered and disordered) of isomorphous replacements. The exact structure of saponites is still poorly known. Vermiculites are most often derived from biotite and phlogopite and could inherit the structure of the tetrahedral layers of micas. The A1 atoms in the tetrahedral layers of micas form unidimensional, ordered ensembles (linear chains of substitution Gatineau [1964] and Gatineau and Merino [1966]). 

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