Discrete silicate anions

Potassium silicate solutions are equally complex for example, an aqueous solution prepared from KOH and Si02 in which the ratio K Si is 1 1 contains 22 different discrete silicate anions as identified by Si COSY nmr studies. 

The frictional force, which hinders the relative motion of the neighboring liquid layers, is brought about by the momentum transport between silicate anions. This means that the structural units, the discrete silicate anions, are also the flow units. 

The two main types of infinite chain anions are the pyroxenes, which contain singlestrand chains of composition (SiO ) (Fig. 8-4) and the amphiboles, which contain double-strand, cross-linked chains or bands of composition (SLOif),. Note that the general formula of the anion in a pyroxene is the same as in a silicate with a cyclic anion. Silicates with this general stoichiometry are often called metasilicates, especially in older literature. There is actually neither metasilidc add nor any discrete metasilicate anion. With the exception of the few metasilicates with cyclic anions, such compounds contain infinite chain anions. 

Ring stability might be expected to lessen with increase in size and increasing proportion of SiOj because the silicate polyanions that correspond to compositions approaching 10 mol% of M O, would be very long. The critical 10% composition at which there is a radical change in many properties may be explained as that composition in the region of which a discrete polymerized anion type of structure becomes... 

These ideas about the discrete-polyanion model for liquid-silicate stmctures are summarized in Table 5.50. As in most models, the description is highly idealized. Thus, all the silicate anions may not have the Si-O-Si angle of the crystalline state only the mean angle may have the crystalline value. Furthermore, the discrete anion suggested for a particular composition is intended to be that which is dominant, though not exclusive. Mixtures of polymerized anions may be present at a given composition, the proportions of which vary with composition. 

Silicates and aluminosilicates are the most abundant minerals, which are main constituents of various rocks in the crust and mantle. Si" " combines with 0 to form silicate anion, SiOa" " in silicate crystal. The SiOa" has tetrahedral form (Fig. 1.11). The SiOa tetrahedron is the basic building block of silicates in which silicon is situated at the center of a tetrahedron of four oxygen atoms. SiOq" tetrahedra exist as discrete units or is joined via the O atoms, sharing their particles with other tetrahedra (Fig. 1.12). Silicate minerals mainly consisting of oxygen and... 

SiO units can also condense themselves to form discrete polysilicate anions examples are shown in Fig. 14.4. These possibilities suggest the wide variety in which silicates form minerals. 

Silicate minerals are all structurally derived from the tetrahedral bonding of silicon to oxygen. For a relatively small group of these minerals, the structure consists of discrete orthosilicate anions Si04 , but, in the vast majority, the Si04 tetrahedra... 

A few silicates are known in which there are simple, discrete orthosilicate, Si044 anions. In such components, the associated cations are coordinated by the oxygen atoms, and various structures are found depending on the coordination number of the cations. There are a number of compounds of the type M2Si04, where M2+ is Mg2+, Fe2+, Mn2+ or some other cation with a preferred coordination number of 6, in which the Si044 anions are arranged to provide an interaction with six oxygen atoms in an octahedron symmetry. 

The discrete-polyanion model is a speculative one because no direct proof of the existence of its ring-polymeric anions, for example, is available. It provides a much more consistent qualitative account of facts concerning the behavior of liquid silicates than does the network model. It predicts the observed marked changes in properties near 10% MjO (Fig. 5.72), the relatively small variation in over the concentration range of 10 to 50% (Fig. 5.71), etc. The suggestions for the stmcture of the anions... 

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