Tetrahedra, isolated

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

Not all silicate minerals that form fibers have structures based on rings, sheets, or chains of linked tetrahedra. Several species whose structures are based on isolated tetrahedra commonly occur in acicular or fibrous form. 

Figure 4.18 The spinel crystal structure adopted by magnetite, Fe2+Fe3+204. Note the three-dimensional infinite chains of edge-shared [FeOe] octahedra extending along [110] directions, which accommode Fe2+ and Fe3+ ions separated by 297 pm. Fe3+ ions also occur in isolated tetrahedra linking the octahedral chains.

Ba4SiAs4 Ba4GeAs4 SiAs4 " isolated tetrahedra Ba4SiAs4... 

Si-0 bond overlap populations for isolated tetrahedra. Amer. Mineral. 57, 1614-42. 

In the case of 2-D polymerization, stmctures comprised of 2-dimensional arrangements of uranium polyhedra will be considered. The vanadium polyhedra (isolated tetrahedra or dimers of edge-sharing tetragonal bipyramids) populating holes within the uranium network do not directly participate in the stmctural organization. 

It was early realised that the acidity of zeolites is affected by their composition. Zeolites with a low aluminium content, hence a low density of lattice anions, present isolated acidic sites, which strength is not diminished by mutual interaction [30]. In this way, catalytic applications usually demand zeolites with a lower aluminium content than applications in water softening or in the separation of air gases, for which a high density of cations is demanded. The aluminium content of zeolites can be controlled by post-synthesis treatment, like in the dealumination treatments which lead to the ultra-stable Y (USY) used in the TCC catalysts [31]. The aluminium content can also be controlled by modifying the conditions of synthesis, albeit each zeolite structure presents a preferential field of composition. It was early shown that the Si/AI ratio is affected by the pi I of the synthesis system, the lowest Si/Al ratio being obtained in the most alkaline systems, in which silica is largely depolymerised and is incorporated as isolated tetrahedra [32],... 

The relationship between the O/Si ratio, which can only vary between 2 and 4, and the structure of a silicate is illustrated in Table 3.4. Depending on the shape of the repeat units, these structures have been classified as three-dimensional networks, infinite sheets, chains, and isolated tetrahedra. Each of these structures is discussed in some detail below. 

Table 5. Phosphate minerals based on isolated tetrahedra and octahedra and finite clusters of tetrahedra and octahedra.

Tetrahedral chambers in capped deltahedra require localized bonding like isolated tetrahedra. Tetrahedral chambers formed by capping deltahedra having all vertices of degree > 3 may be regarded as islands of localization in an otherwise delocalized system. 

Of these silicates, the most structurally simple ones involve isolated tetrahedra (Figure 12.12a). For example, forsterite (Mg2Si04) has the equivalent of two Mg ions associated with each tetrahedron in such a way that every ion has six oxygen nearest neighbors. 

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