Silicates chain structures

Figure 4.5 Structure of pyroxene minerals (a) demonstration of the end view of the single silicate chain (b) end view of the stacking arrangement of single chains, showing the position of the metal cations. There are two different cationic environments, Ml and M2. (After Putnis, 1992 Figure 6.11, by permission of Cambridge University Press.)...

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

Minerals and mineral series with the same basic chemical units, such as the silicate polymerized ions, and very similar crystal structures are related and referred to collectively as mineral groups. The amphiboles are a group composed of several mineral series, two of which were cited in the preceding examples. The several series that make up the amphibole group reflect the changes in the size and location of cations associated with the polymerized silicate chains. Because several amphibole species occur in fibrous fonn, we discuss this group in much greater detail, and include an idealized crystal structure. 

Fig. 2.16 Schematic representations of the structures suggested for sepiolite and palygorskite. The ribbonlike arrangement of silicate chains alternates with hydroxyl and water areas. (A) Sepiolite, the (001) projection, showing the cross section of three 2 1 silicate chains and associated water and hydroxyl groups. (B) Palygorskite, the (100) projection, showing the cross section of two silicate chains and associated water and hydroxyl groups.

The sorption of Pb(II) to C-S-H has been investigated by Moulin (1999) and Pointeau (2000). In fact XAFS studies indicate that, as for Zn(II), Pb(II) is adsoibed to the silicate chains within the C-S-H structure (Rose et al. 2000). Figure 7 shows sorption isotherms for C-S-H of different Ca Si ratios. It appears that sorption is greater at a lower Ca Si ratio. 

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... 

It has been suggested that the —Si—0 portions of the polymers sit tightly on the structurally similar silicate chains on the surface of the glass, leaving the organic sections of the silicones extended outward to form a hydrocarbonlike barrier that water molecules cannot penetrate. 

Silicate chains are of two general types. Long chains of SiC>4 tetrahedra are present in minerals known as pyroxenes that can be considered as a repeating pattern of Si03 units giving rise to a structure such as that shown here having the formula (SiC 2-), (see also Figure 11.1(c)) ... 

Fig. 5.8 Silicate chain of the type present in 1.4-nm tobermorite and jennite (dreierkette). In the tobermorite structure, the oxygen atoms at the bottom of the figure are also part of the central CaOj layer. The tetrahedra in the lower row are described as paired and those in the upper row as bridging. A bridging tetrahedron is missing (at X). Suggested positions of hydrogen atoms and negative charges balanced by interlayer cations are included. Taylor (T24).

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). 

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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