Silicates tetrahedral structure

As noted earlier, cations other than silicon may occupy the tetrahedral centers. A major factor in predicting which cations will be found to substitute for silicon is ionic size. In general, cations whose size is about 0.03-0.1 nm are the best candidates. Si " has an ionic radius of about 0.041 nm. Cations such as Fe (ionic radius = 0.07 nm), Al+ (0.05 nm), Ca+ (0.1 nm), and Mg (0.065 nm) are often found in silicate-like structures and meet this requirement. 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Layer Silicates. Although the common primary minerals include island, chain, sheet, and framework silicates, the most stable and persistent silicates, which occur as weathering products (secondary minerals) in the clay fraction of soils, are sheet silicates. Figure 2.9a depicts the structure of the tetrahedral sheet in these minerals, which is comparable to the tetrahedral structure of mica. For the layer silicate clays, however, numerous structural combinations of the tetrahedral sheet with octahe-drally coordinated metal cations are possible. 

Figure 2.16. Common groups of layer silicate clay structures found in soils, pictured terms of their tetrahedral (iHk) and octahedral ( ) sheets. The usual locations of - /uctural charge and exchange cations are indicated by — and + signs.

By analogy with phyllosilicates, a group of titanium silicates whose structures are based on TOT-like layers have been called heterophyllosilicates (Ferraris et al. 1997). In these structures, rows of Ti(Nb)-octahedra (hereafter, Ti-octahedra) are introduced in a T sheet along the direction which is parallel to a pyroxene tetrahedral chain (Fig. 17). HOH layers are thus obtained where H stands for hetero to indicate the presence of the Ti-octahedra in a sheet corresponding to the T sheet of the layer silicates. Because the edges of the Ti-octahedra and Si-tetrahedra have close lengths dimensions, the insertion of the octahedra in a T sheet does not produce strain. As summarized by Ferraris (1997), three types of HOH layers (Fig. 18) are known so far. 

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

Tetrahedral networks containing equal amounts of M" and P are silicate analogs. They are most common for M = Al, but tetrahedral structures with M = B, Ga, and Fe are also known, for example, the borophosphate CSHB2P2O9. ... 

Common framework silicates have only tetrahedral structural sites with O/Tet 2. In all nonframework silicates O/Si > 2 and octahedral sites are always... 

In these crystals, the B groups (sulphate, silicate, tungstate, molybdate, etc.) have a tetrahedral structure and the central atom is sp -hybridized. Each peripheral oxygen atom carries three orbitals available for forming one or two molecular orbitals with the metal atom A, which necessarily has one or two nonbonding orbitals. 

Layered Silicates The structure of Layered silicates (LS)/clays consists of a 2-D layer of two fused silicate tetrahedral sheet with an edge-shared octahedral sheet of metal atoms, such as A1 or Mg. This model was proposed by Hofifinann et al. [19]. 

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