Silicates infinite chain 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. 

More than 90% of the rocks and minerals found in the earth s crust are silicates, which are essentially ionic Typically the anion has a network covalent structure in which Si044-tetrahedra are bonded to one another in one, two, or three dimensions. The structure shown at the left of Figure 9.15 (p. 243), where the anion is a one-dimensional infinite chain, is typical of fibrous minerals such as diopside, CaSi03 - MgSi03. Asbestos has a related structure in which two chains are linked together to form a double strand. 

There are also chloro complexes M Be h, M = K, Rb, NFL, that have p.-Cl atoms in a chain structure quite unlike the MIBe2F5 analogs that have infinite sheet anions in which there are hexagonal rings of BeF4 tetrahedra sharing comers (cf. silicate anions Si202-, Section 8-7). 

Fig. 16.1 Various silicate structure (a) Si04 tetrahedron. When carrying a —4 charge, this s the orthoslknte ion. (b) The dbalicalc anion- (c) Portion of an infinite single chain. (SiO ) "-- (d) Portion of an infinite double clcun or band. Si40,itff. (e) Portion of a sheet or layer structure. (Si2Oj 2, —.

The structural roles of phosphate, arsenate and vanadate ions are similar to each other in the crystal structures of rare earth minerals. Like the carbonate ion, these anions almost always exist as isolated anions in the crystal structures, and do not form infinite anionic groups such as chains. A few silicates structurally related to the phosphates are also included in this section. 

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