Silicon-oxygen Sheets

Silicon-Oxygen Sheets.—The chain structure of Fig. XXVI-3 can be extended to form a two-dimensional sheet, as shown in Fig. XXVI-4. A unit of this structure has the composition (Si206), but as with the 

Another very important group of materials, the clays, are formed from silicon-oxygen sheets. In this case, however, the sheet is not the simple one shown in Fig. XXVI-4, but each sheet is bound by homopolar valences to another sheet of different composition. For instance, in kaolinite, one form of clay, one starts with a silicon-oxygen sheet. Above that, there is an aluminum layer, with as many aluminums as silicons, bonded to the oxygens. The other bonds of the aluminums extend to a 

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Because the silicon/oxygen (Si O) ratio is 4 10, or 1 l, this mineral should have an infinite sheet structure with the repeating unit Si205. The oxidation states of Si and O are -1-4 and —2, as usual, and that of calcium (Ca) is -1-2. For the total oxidation number per formula unit to sum to 0, the oxidation state of copper (Cu) must be +2. 

Although the unit cell of talc was initially reported to be monoclinic [18], further studies have concluded that the unit cell is triclinic [19-21]. The material is composed of a brucite (MgO) sheet between silicon-oxygen layers, as shown in Figure 1. Each layer is electrically neutral and adjacent layers are held together by weak van der Waals forces. The slippery property of talc is the result of these layers sliding over one another. 

Fig. 11.06. Some possible open silicon-oxygen groupings in silicates, (a) Tetrahedra sharing two oxygen atoms to form open chains, [Si03]n2n , e.g. pyroxenes. (b) Tetrahedra sharing alternately two and three oxygen atoms to form open chains, [Si40n]n6n , e.g. amphiboles. (c) Tetrahedra sharing three oxygen atoms to form open sheets, [Si401()]n4n-, e.g. talc.

Most silicates consist of networks of silicon-oxygen tetrahedra linked together in ways that range from chains, rings, and sheets to three-dimensional networks. 

The dimension of cross-linking (e.g., 1-, 2-, or 3-dimensional), the extension (as applies to, e.g., the inosilicates), and the number of cross-linked elements (e.g., 2 for double layers or double chains) are identified by the anion complex notation as proposed by Liebau [16]. The symbol 2/< of the anion complex of the metal silicate hydrates describes the two-dimensional cross-linking of the silicon oxygen tetrahedra to tetrahedral sheets. Following an older model, one, two, three, and five tetrahedral sheets are connected to form the bulk layer of kanemite or makatite, ilerite, magadiite, and kenyaite. 

A classification of silicates with predominantly two-dimensional cross-linking of [SiOJ tetrahedra leads to the trichotomy presented in Figures 2 and 3. Layered silicate hydrates, disilicates, and clay minerals differ in the repetitive characteristic binding element within the layers. The silicon oxygen tetrahedra within a sheet are connected by three bridging oxygen atoms. In the case of clay minerals, the fourth valency is saturated by foreign metal ions (e.g., Mg +)... 

Fig. 1.12 Coordination model of a tetrahedrally polymerized sheet. Perfect cleavage parallel to the silicon-oxygen tetrahedral layers totally avoids the bridging oxygens. These planar structural units characterize the micas, chlorites and clay minerals. Weak bonding across the layers accounts for their softness and causes many sheet silicates to decompose readily at elevated temperatures. A angstroms (Ernst 1969)...

Silicates are covalent atomic sohds (see Section 11.12) that contain silicon, oxygen, and various metal atoms. Rocks, clays, and soils contain silicates. Their great diversity illustrates a theme that we have encountered since Chapter 1 of this book the properties of substances are determined by their atomic and molecular structures. The structures of silicates determine their properties—and since these structures are varied, their properties are also varied. Some silicates form strong three-dimensional materials, while others break into sheets, and still others are highly fibrous. Let s examine more closely several of these structures. 

The layered structures of clays (Figure 17.4) consist of sheets of silicon oxide alternating with sheets of aluminum oxide. The silicon oxide sheets are made up of tetrahedra in which each silicon atom is surrounded by fom o g gen atoms. Of the four oxygen atoms in each tetrahedron, three are shared with other silicon atoms that are components of other tetrahedra. This sheet is called the tetrahedral sheet The alumimun oxide is contained in an octahedral sheet, so named because each alum-imun atom is surroimded by 6 oxygen atoms in an octahedral configuration. The structure is such that some of the oxygen atoms are shared between aluminum atoms and some are shared with the tetrahedral sheet. 

Silicon never occurs free it invariably occurs combined with oxygen and, with trivial exceptions, is always 4-coordinate in nature. The Si04 unit may occur as an individual group or be linked into chains, ribbons, rings, sheets or three-dimensional frameworks (pp. 347-59). 

Silicates also exist in which each silicon atom bonds to one outer oxygen and to three inner oxygen atoms. The result is a linked network in which every silicon atom forms three Si—O—Si links, giving a planar, sheet-like structure. The empirical formula of this silicate is S12 O5. In many minerals, aluminum atoms replace some of the silicon atoms to give aluminosilicates. The micas—one has the chemical formula... 

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