Silicates with chain structures

Silicates with chain structures—pyroxenes and amphiboles,... 

Pyroxene-type. The yttrium compound MgYSi20sN, which can be regarded as being derived from diopside [CaMg(Si03)2]n with replacement of Ca by Y and of one O by N is the only known example so far of nitrided silicate with chain structure of pyroxene-type (Patel and Thompson 1988). 

Amphiboles refer to the class of silicates with chain-ribbon structure. They crystallize in monoclinic and rhombic crystal systems. The long-prismatic crystals of these compounds possess an absolute cleavage. 

Figure 4.4 Infinite chain silicates (single, double, and sheet) (a) infinite single chain silicate with two corners shared per tetrahedron (pyroxene structure) (b) infinite double chain, with alternate two and three corners shared (am-phibole structure) (c) infinite sheet structure, with each tetrahedron sharing three corners (sheet silicates). (From Putnis, 1992 Figure 6.3, by permission of Cambridge University Press.)...

Si04 units share two corners to form infinite chains (Figure 1.52(c)). The repeat unit is SiOs . Minerals with this structure are called pyroxenes (e.g., diopside (CaMg(S 103)2) and enstatite (MgSiOs)). The silicate chains lie parallel to one another and are linked together by the cations that lie between them. 

The phenomenon of increased hardness occurs principally in minerals of sheet and chain structures, which link together through the cations (silicates and aluminosilicates, as well as hydrated sheet minerals, such as glauconite, melilite and gypsum—M ranging from 0 to about 1.25), and also in minerals of skeletal structures (borates, phosphates, sulphates, nitrates, carbonates, such as calcite, dolomite and others—Ah from 0 to about 1.15). For this reason, the hardness analysis of minerals with weak bonds demands consideration of the fact that just as the basic crystallo-chemical factors, so is hardness influenced by the form of domains (component parts of structures) in all anisodesmic minerals of chain, sheet or skeletal structure. Depending on the form of domain (and also according... 

Although these anions are simpler than the structures deduced on the basis of transport measurements, they agree with the structure (see Section 5.Q. Chains are present, as are O and SiOj. However, up to 1997, no peaks have yet been registered that are characteristic of the ring structures known to exist in the corresponding soUds, the suggested presence of which in the liquid silicates fits stoichiometry, bond angles, and the behavior of the heat of activation for viscous flow as a function of composition (Fig. 5.71). 

Ring and chain structures of Alj X owere deduced by Blander and Saboungi in 1992 and are given in diagrams in the text. Compare the structure shown there (Al the coordinating ion) with those deduced by MacKenzie and Lowe for the liquid silicates in 1955 (also given in the text). Differences Similarities Why... 

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