Silicates with layer structures

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

Double layers can be generated by sharing the fourth (apical) 0 atom between pairs of tetrahedra as in Fig. 9.9(a). This would give a stoichiometry SiOa (since each O atom is shared between 2 Si atoms) but if half the Si were replaced by 

Al then the composition would be Al2Si20g as found in Ca2Al2Si20x and Ba2Al2Si20g (Hg. 9.9b). Another way of building up double layers involves the interleaving of layers of the 

The technological importance of the clay minerals is outlined in the Panel on p. 356. 

Micas are formed when one-quarter of the Si in pyrophyllite and talc are replaced by Al and the resulting negative charge is balanced by K  

In a layer formed from Si04 tetrahedra linked to three others by sharing vertices the Si atoms lie at the points of a 3-connected net, which may be plane or puckered. Silicate structures based on four different nets are known (Fig. 23.13), of 

Layers ofSi04 tetrahedra sharing 3 vertices which are found in silicates. In (a) and (b) the small black circles represent Si atoms and the open circles O atoms. Oxygen atoms lying above Si atoms are drawn more heavily. The O atoms are omitted from the more complex nets 

BaFeSi40 o (gillespite), CaCuSi40io (Egyptian blue), Ca4Sig02o. KF. 8 H2O (apophyllite),  

All the layers of Fig. 23.13 are simple layers in the sense that they can be represented on a plane as 3-connected nets without any links overlapping or intersecting. More complex layers can be envisaged which have some extension in a direction perpendicular to the layer, as in the double layer described in (ii). An intermediate possibility is realized in Na2Si307. We may represent the end-on view of an idealized metasilicate chain as in Fig. 23.14(a). In (b) each pair of vertex-sharing tetrahedra represents a chain perpendicular to the plane of the paper. 

(a) MetasiUcate chain ion (SiOs) and end-on view, (b) Layer of composition Si20s built of SiOa chains perpendicular to the plane of the paper, (c) Layer of composition Si307 similarly represented. 

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Clay minerals are alumino silicates with layer structures. The basic constituent elements Si and Al are substituted by other elements by diadochy. Clay minerals swell in water and... 

Clay minerals are hydrous layer silicates of colloidal dimensions, with most if not all of the individual platy particles in the colloidal range of c. 1 nm-1 pm (van Olphen, 1976 Van Damme et al., 1985). The term phyllosilicate (phyllo = leaf like) is applied to the broad group of hydrous silicates with layer structures. The essentia] components of the phyllosilicate structure are two-dimensional tetrahedra and octahedra of oxygen atoms (or ions). The coordinating atoms (or cations) in the centre of the tetrahedra are for the most part Si, but Al3 or Fe3+ may also be present. The coordinating cations in the octahedra are usually Al3, Mg2+, Fe3 or Fe2. Some clay structures (e.g. hectorite) can be synthesized in a reproducible and relatively homogeneous form. 

Silicates with layer structures-clay minerals and micas,... 

For layer silicates with low structural charge (i. . smectites), this expansion is limited to about four molecular layers of water if the exchangeable cation has a charge of +2. Since the silicate platelet is about 0.96 nm thick, the repeat spacing along the c-axis is then approximately 0.96 + (4x.26) = 2.0 nm. 

VUId Silicates with layered anions (the so-called phyllosilicates) Figure 7.11 represents a way to link Si04 tetrahedrons in one layer. The repetitive unit in the structure of figure 7.11 is represented in between the dotted lines and has the formula Si404-. The anion described here for example occurs in the minerals ... 

FIG. 31-4. A portion of an infinite layer of silicate tetrahedra, as present in talc and other minerals with layer structures. 

Minerals with Layer Structures. By a condensation reaction involving three of the four hydroxyl groups of each silicic acid molecule, a condensed silicic acid can be made, rvith composition (H.SisOs), which has the form of an infinite layer, as shown in Figure 31-4. The... 

Acidity of isomorphically substituted crystalline silicic acids with layer Structure. I. H-magadiite... 

Illite A phyllosilicate (layered silicate) with a structure of parallel silicate tetrahedral sheets. Each sheet has the general formula Si20s. 

The clay minerals are subdivided into crystalline silicates with layer lattices or lamelar, and crystalline silicates with fibrous lattice structure, chain lattices. The silicates of lamellar structure can be divided into three groups or families dimorphic (layer 1 1), trimorphic (layers 2 1) and tetramorphic (layers 2 2). The nomenclature 1 1 and 2 2 refers to the number of layers of Si04 tetrahedral and octahedral of hydroxides, resp>ectively, entering the establishment of the unit cell of the crystalline lattice of day minerals. 

Phyllosilicates are a family of silicates with a structure based on the stacking of interconnected Si20s tetrahedral layers (T) with octahedral layers (O) of either Brucite [Mg(OH)3] or Gibbsite [Al(OH)3] type (Figure 8.1). 

Materials of this type with layered structures have various names in the literature (cf. Tables 2, 3, 4). We use the term metal silicate hydrate, abbreviated as M-SH. 

Tetrahedra linked via three vertices correspond to a composition MX1 1X3 2 or MX2 5 = M2X5. Small units consisting of four tetrahedra are known in P4O10, but most important are the layer structures in the numerous sheet silicates and aluminosilicates with anions of the compositions and [AlSiOj-]. Because the terminal vertices of the single... 

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