SiO tetrahedra

Fig. 1. Schematic representation of basic silicate stmctures (a) modes of linkage of SiO tetrahedra (b) the corresponding bonding patterns and (c) stmctural formulas (3). The Si atoms that appear to be joined to only three O atoms are joined to a fourth also, which is above the plane of the diagram.

The stmctures in which SiO tetrahedra share all four oxygen atoms lead to the principal forms of siUca. Replacement of some siUcon atoms by aluminum gives a negatively charged framework of composition, A1 S 02. in which positive ions are accommodated in holes in the stmcture. 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

The stmcture of vitreous siUca is a continuous random network of SiO tetrahedra, linked through the sharing of corners. It differs from crystalline sihca ia having a broader distribution of Si—O—Si bond angles and a more random distribution of one tetrahedron with respect to another (44). The density is 2.2 g/cm. ... 

SiO tetrahedra link together to form a sheet-like structure... 

SiO tetrahedra per silicate ion without referring to the ionicity of chemical bonds ... 

If all the four corners of the SiO tetrahedra are shared, three-dimensional networks result. The different forms of silica (quartz, tridymite and cristobalite) discussed earlier in this section are typical examples. Feldspars are generated when part of the Si is... 

The basic structural units in layer silicates are silica sheets and brucite or gibbsite sheets. The former consist of SiO tetrahedra connected at three corners in the same plane forming a hexagonal network. The tips of the tetrahedra all point in the same direction. This unit is called the tetrahedral sheet. The bijucite or gibbsite sheet consists of two planes of hydroxyl ions between which lies a plane of magnesium or aluminum ions which is octahedrally coordinated by the hydroxyls. This unit is known as the octahedral sheet. These sheets are combined so that the oxygens at the tips of the tetrahedra project into a plane of hydroxyls in the octahedral sheet and replace two-thirds of the hydroxyls. This combination of sheets forms a layer. 

Continuous 3-D network of SiO tetrahedra with small degree of thermal bond breaking... 

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