Silicate structures, building block

Range I The first range comprises distances and angles within the structural building blocks of the respective amorphous solid, for instance, the Si-O and 0-0 distances in the S04-tetrahedra in the covalent network of silicate... 

Zeolites are crystalline alumina-silicates having a regular pore structure. Their basic building blocks are silica and alumina tetrahedra. Each tetrahedron consists of silicon or aluminum atoms at the center of the tetrahedron with oxygen atoms at the comers. Because silicon and aluminum are in a +4 and +3 oxidation state, respectively, a net charge of -1 must be balanced by a cation to maintain electrical neutrality. 

The Si04 tetrahedra can be arranged into several silicate units, e.g. squares, six-or eight-membered rings, called secondary building blocks. Zeolite structures are then built up by joining a selection of building blocks into periodic structures. 

The crystal structures of all the minerals in the serpentine group contain the same basic building blocks. The basic unit is composed of a silicate sheet of composition (Si205) ", in which three of the O atoms in each tetrahedron are shared with adjacent tetrahedra (Fig. 2.2A), and a nonsilicate sheet of... 

Let us briefly examine modelling of solids, taking silicates as a case study. In order to model silicate structures, it is necessary to develop accurate potentials for the various silicate forms. Such potentials should be able to predict the known structures of zeolites and related materials accurately. Attempts have been made by a number of workers in this direction. The Si04 tetrahedron is the basic building block in silicates. While in... 

An alternative hypothesis, developed from studies of the synthesis of Linde A zeolite carried out by Kerr (5) and Ciric (6), pointed to growth occurring from solution. The gel was believed to be at least partially dissolved in solution, forming active aluminosilicate species as well as silicate and aluminate ions. These species linked to form the basic building blocks of the zeolite structure and returned to the solid phase. Aiello et al. (7) followed the synthesis from a highly alkaline clear aluminosilicate solution by electron microscopy, electron diffraction, and x-ray diffraction. These authors observed the formation of thin plates (lamellae) of amorphous aluminosilicates prior to actual crystal formation. 

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. 

Fig. 1. In phosphorus nitrides PN4 tetrahedra occur as typical building blocks, which may be linked via common vertices exhibiting structural analogies to silicates and phosphates.

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