Clay Minerals 1 Silicate Mineral Structures

Clays are silicate minerals that may be platy or fibrous and are usually of exceedingly fine grain size, ranging from colloidal, a few nanometers in diameter, to a maximum of about a tenth of a micrometer. Similarly to other aluminosilicates, clays show considerable range in chemical composition with concomitant structural modifications (Fig. 2.13). A few of the clay minerals described as fibrous are briefly mentioned here. 

IHite/Smectite. Another common intergrowth of sheet silicates is the mixed-layering of illite and smectite. As discussed above, illite and smectite are clay minerals whose basic structures resemble the mica muscovite. Their compositions may differ significantly from muscovite, but they generally have a lower occupancy of the interlayer sites than mica. Numerous other compositional differences are possible for smectite however, this discussion will be restricted to a dioctahedral illite and a dioctahedral smectite containing potassium and vacancies in the interlayer sites as given above. 

Illite as a non-swelling clay mineral of layer structure plays a very important role in our studies. This special role is due to the fact that both sides of the surface of the silicate lamellae are made up of Si04-tetrahedron planar lattices, and this structure - even when hydrophobized - is identical with the surface structure of montmorillonite and vermiculite, both of which are of the swelling type. 

Laird, D.A. 1994. Evaluation of the structural formula and alkylammonium methods of determining layer charge, p. 79-103. In A.R. Mermut (ed.) Layer charge characteristics of 2 1 silicate clay minerals. Clay Miner. Soc., Boulder, CO. 

The extender should be acid-resistant (e.g., barytes, talc, mica, china clay, silica, silicates). Minerals with a platelet (lamellar) structure (e.g., talc, mica, and china clay) are particularly suitable for corrosion protection systems since they cover the surface better than particles of other shapes. Better covering makes the paint film less... 

Talc, mica, and clays are silicate minerals with structures of this type. Talc has the chemical structure Mg3H2Si40i2 = MgO Si40jo H2O = MgO Si40g (OH)2, while mica has roughly the structure IG l2H2Si30j2 = KAl2Si30jo(OH)2- There are many different variations of minerals called mica as we shall presently describe. 

Silicates with layer. structures include some of the most familiar and important minerals known to man, partieularly the clay minerals [such as kaolinite (china clay), montmorillonite (bentonite, fuller s earth), and vermiculite], the micas (e.g. muscovite, phlogopite, and biotite), and others such as chrysotile (white asbestos). 

Many studies have been made of the rates of water evolution from layer-type silicate minerals which contain structural hydroxyl groups (clays and micas). Variations in composition of mineral specimens from different sources hinders comparison of the results of different workers. Furthermore, the small crystallite sizes and poor crystallinity that are features of clays limit and sometimes prevent the collection of ancillary observations (e.g. microscopic examination and diffraction measurements). 

Layer-silicate structure, as in other silicate minerals, is dominated by the strong Si-O bond, which accounts for the relative insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH. The spatial arrangement of Si and these metals with O and OH results in the formation of tetrahedral and octahedral sheets (see Fig. 8-2). The combination of the tetrahedral and octahedral sheets in different groupings, and in conjunction with different metal oxide sheets, generates a number of different layer silicate clays (see Table 8-1). 

Fig. 8-2 Structure of a 1 1 (kaolinite) and a (montmorillonite) layer-silicate clay mineral.

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

Clay minerals are structure of aluminum- and silicate-oxides. Such structures have free bond and vacancies on selective site of the mineral structure allowing them to participate in both physical adsorption and chemical reactions modifying the available concentration of the chemicals in the environment. 

In order to study in more detail the clay minerals, it is first helpful to review briefly the basic structural classification of the silicates in general. Although ultimately complicated, the general progression is logical, and is based on the degree of polymerization of the basic structural unit which is the Si04 tetrahedron (see below). The sequence runs as follows ... 

The clay minerals can now be discussed in terms of their relationship with the phyllosilicates (sheet silicates). It is important to keep clearly in mind here the difference between clay - the material which is dug out of the ground, and which may be a mixture of different clay minerals, together with various nonclay minerals (such as quartz, pyrite, etc), as well as unaltered rock fragments and incorporated organic material (Grim, 1968) - and the clay minerals themselves, which are crystalline compounds of specified stoichiometry and structure. At this stage, we are only considering the structure of the clay minerals. 

Section 4.3 sets out the principles underlying the structure of the silicate mineral family. Natural clay deposits are formed by the chemical weathering of rocks -largely as a result of the attack by slightly acidic surface waters. Rainwater,... 

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

As a function of their structural properties, clays interact differently with organic and inorganic contaminants. Two major groups of clay minerals are selected for discussion here (a) kaolinite, with a 1 1 layered structured aluminosilicate and a surface area ranging from 6 to 39 m g" (Schofield and Samson 1954) and (b) smectites with a 2 1 silicate layer and a total surface area of about 800m g" (Borchardt 1989). 

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

Bailey, S. W. (1980). Structures of layer silicates, pp. 1-124. In G. W. Brindley and G. Brown, Eds. Crystal Structures of Clay Minerals and Their X-ray Identification. Min. Soc. Gt. Brit./Ireland, Mono. 5, London. 

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