Silicates cyclic anions

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. 

Beryl contains the cyclic anion SifiOis. Other silicate minerals have anions with very long chains or double chains. 

This, however, is an oversimplification, since these monomeric anions are in rapid equilibrium with oligomers (small polymers) such as the dimer (cf. Fig. 7.2b), cyclic trimer (7.2c), and other small silicate oligomers, 22 of which have been identified by silicon-29 magnetic resonance methods and many of which have no counterparts in solid silicate minerals9 ... 

Polymeric chain, band- and sheet silicate structures have been discussed previously (page 742), and it should not be surprising to learn that cyclic silicate anions, such as [Si3Oy]ft and [Si601B]l2 (Fig. 16.28) are known. These anions are sometimes referred to as metasilicates in line with the older system of nomenclature, which assigned vrtlto to the most fully hydrated species [as in orthosilicic acid." Si(QH)J and metu to the acid (and anion) from which one mole of water has been removed [either in fact or formally for example, "metasilicic acid," OSi(OH)2]. 

Figure 10.7. Cyclic silicate anions. The repeated SiO unit is marked by a dashed line.

Figure 8-3 Examples of cyclic silicate anions Si309 and SifiO l. ...

From the development of resonance lines in the different spectra, a qualitative reaction mechanism can be extracted for the oligomerization process. In this mechanism, formation of cyclic structures is a predominant phenomenon. In the presence of TMA cations, no linear trimeric silicate anions are found in the silicate solutions, as can be deduced from Figures Id through lh. The use of alkali metal hydroxides as bases shows, aside from the cyclic trimeric silicate anion, the linear structure the relative concentration of each depends on the alkalinity and the alkali metal used. The formation of cyclic trimeric silicate anions occurs before formation of the linear trimeric species (6). The formation of double-cyclic silicate anions can be observed in Figure le two cyclic trimeric silicate anions combine to form the prismatic hexameric silicate anion (Q36). This prismatic hexameric silicate anion forms, through addition of two monomers or one dimer, the well-known cubic octameric silicate anion. This cubic octameric silicate anion has been proposed to be predominantly... 

---