The structure of silicate minerals

Unlike Si(IV), Cd(II) often takes an octahedral configuration as well as a tetrahedral one it is rather difficult for Si(IV) to take an octahedral configuration under ambient conditions. The isopolycyanocadmate(II) systems built of the CN-linkages among tetrahedral and octahedral Cd atoms can mimic the structures of silicate minerals composed of the tetrahedral Si and other octahedral cations. 

Silicate minerals are oxides of silicon and a small number of elements from the first three columns of the periodic table and the transition elements. As such they closely mirror the abimdance of the elements in the crust of the Earth (Table 3.1). Since the number of different elements which play a major role in the structure of silicate minerals is small, it is not surprising that the fundamental building blocks of these minerals, and many other non-silicate minerals, are few. 

Since silicates and aluminosilicates are by far the predominant rock-forming minerals, the crystal structures of most species have been determined. Liebau (1980) presents an overview of the structures of silicate and aluminosilicate minerals, and one can consult Berry, Mason, and Dietrich (1983, especially pp. 382-389) or other mineralogy texts for an introduction to the subject. The multivolume work of Eitel (1965) provides a general treatment of the crystal chemistry of all types of silicate materials. 

Ionic radius. The wide variation of metal-oxygen distances within individual coordination sites and between different sites in crystal structures of silicate minerals warns against too literal use of the radius of a cation, derived from interatomic distances in simple structures. Relationships between cation radius and phenocryst/glass distribution coefficients for trace elements are often anomalous for transition metal ions (Cr3+, V3+, Ni2+), which may be attributed to the influence of crystal field stabilization energies. 

The electronic structures of silicate minerals of polymerization intermediate between nesosilicates and tektosilicates have been studied to a lesser extent than have SiOj or the olivines. The complexity of their crystal structures makes calculation difficult, and their diversity in terms of local chemical environment makes phenomenological assignment of their spectra difficult. Nonetheless, some recent comparative studies have given valuable electronic structure information on such materials. 

M. Taylor, and G.E. Brown, Structure of silicate mineral glasses. I. The feldspar glasses, NaTUSisOs, KTUSisOg, Ca7U2Si20g. Geochim. Cosmochim. Acta 43, pp. 61-77(1979). 

Asbestos, which is a term for a group of hydrous magnesium silicate minerals with a fibrous texture, is another silicate of technological significance. The structure of these minerals is characterized by long silicate chains. The most important variety is chrysotile Mg3Si205(0H)4, a fibrous form of serpentine. 

Layer silicate structure, as for the rest of silicate minerals, is dominated by the strong Si—O bond which accounts for the insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH groups. The spatial arrangement of Si and the above metals with O and OH results in the formation of the tetrahedral and octahedral sheets. The combination of the tetrahedral and octahedral sheets forms a layer, or the units of layer silicates. A number of layer silicate structures can be generated with different arrangements of the tetrahedral and octahedral sheets or other hydroxide layers (Table 7-4). 

The key feature that allows the ionic model to be successful in modelling many materials is the inclusion of ion polarizability. According to how the electron density is partitioned, it is possible to view many features of semi-ionic materials equal as well as covalency effects or ion polarization. Hence, providing the necessary polarizability terms are included, it is possible to get good results with formal charges despite the fact that a solid may generally be viewed as appreciably covalent. An example of such a case is the family of silicate minerals (Sanders et al. 1984). The inclusion of polarization is also the mechanism by which low symmetry phases become stable as opposed to regular close packed structures. 

Nesosilicates is the class of silicate minerals in which the structure is built up by independent tetrahedra of SiOa. This class contains some mineral groups like the olivines, the silicate garnets, the epidote group and some separate minerals. 

Metals and metalloids on the surface of silicate minerals are also connected by oxide or hydroxide ion bridges, which undergo acid-base reactions with the adjacent aqueous solution. The results of these acid-base reactions are measurable as surface charge, which varies in concentration with solution pH. (The situation is a little more complicated than this, especially for minerals that have a structural charge due to uncompensated cation substitutions. The reader is directed to, for example, Schindler and Stumm 1988.) The enhancement of dissolution rates via adsorption of hydrogen ions is referred to as the proton-promoted pathway for dissolution (Furrer and Stumm 1986) and is analogous to the proton-promoted pathway for the dimer dissociation discussed above. 

What follows is a very brief discussion of the crystal structures of silicate minerals and a few other geologically important minerals. The aim is to present the reader with an introduction to the subject, sufficient to allow... 

Si04 tetrahedra are bonded together in three-dimensional structures. Three-dimensional arrays of tetrahedra result when each Si atom shares all four O atoms with the Si atoms in four adjoining tetrahedra (as shown in Figure 21-31a). This is the most common arrangement, occurring in silica (quartz) and in the majority of silicate minerals. 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

The structures of aluminum silicates of divalent metals which are simplest from the coordination standpoint are shown to correspond to the formulas R3 + +Al2Si30i2 and R3++Al2Si60i8, which include the most important minerals of this class, the garnets and beryl. 

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