Silicates chain

For an 0/Si ratio of 3, infinite chains or ring structures result. An example for a silicate material having this structure is asbestos. Chains are held together by weak electrostatic forces. 

Structure of mica showing the position of alkali ions. 

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Fig. 16.4. Silicate structures, (a) The Si04 monomer, (b) The Si207 dimer with a bridging oxygen. ( ) A chain silicate. (d) A sheet silicate. Each triangle is the projection of on Si04 monomer.

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

Chain silicates, consisting of connected metasilicate units, (SiOg) " (Q ), and of an open structure. Wollastonite Ca(SiOg) reacted completely with poly(acrylic add), but the cement was much affected by water. 

In studies of amphiboles (44), isolated strips of triplechain silicates were discovered embedded in the double-chain parent structure. It was later realized that new types of silicate structures, composed of recurrent triple chains, existed in nature. The part that HREM played in the identification of this new family of triple-chain silicates, which constitute a further step in the progression pyroxene, amphibole,. .. mica, was crucial. 

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.)...

In addition, many mineral species, such as the chain silicates, contain identical basic chemical units, and hence exhibit very similar crystal structures but have different chemical compositions. These minerals may be members of a mineral series if the chemistry varies predictably. The variations, known as atomic substitution, are based on Pauling s rules. 

The pyroxenes are chemically complex but common rock-forming minerals. They resemble the amphiboles in many ways, but are actually single-chain silicates. The tetrahedral basic unit of the pyroxenes, [(Al,Si)20g] , was schematically depicted in Fig. 2.1C. The general formula for the group is Ai (B, C)i+ TjOfi, where A = Ca" Fe Li+, Mg "", Na-" B = Mg ", Fe Mn" Sc+ C = Fe A Cr" Ti " and T = Si Al+l Within the group are several mineral series and several species that often occur in acicular or fibrous forms. One species that occurs in fibrous form is jadeite, [Na(Al,Fe )Si206], a relatively familiar name because of the popularity of this material with Oriental sculptors. 

Mallinson, L. G., J. L. Hutchinson, D. A. Jefferson, and J. M. Thomas (1980). Internal structure of nephrite experimental and computational evidence for the coexistence of multiple chain silicates within an amphibole host. Phil. Trans. Roy. Soc. London 295 537-552. 

Veblen, D. R., P. R. Buseck, and C. W. Burnham (1977). Asbestiform chain silicates, new minerals and structural groups. Science 198 359-365. 

The objective of the synthesis would be to maximise the first reaction (leading to ferrlslllcates) and suppress the formation of hydroxides of iron by the latter reaction by operating at low pH, using aluminium free source of Si and adjusting the reaction conditions to maximise the concentration of monomeric/short chain silicate species. Usually the... 

Chain silicate minerals with MgSiOg (En), FeSiOg (Fs), and CaSiOg (Wo) as the major components, where En is enstatite, Fs is ferrosilite, and Wo is wollastonite. 

Double-chain silicates. Double-chain silicates, (Si40n)n6n, are known as amphiboles, such as tremolite, Ca2Mg5(0H)2[(Si40n)2]. These include the true asbestoses, such as crocidolite or blue asbestos,... 

Pyroxenes e.g., Mg2[Si2Ofi] = 2MgSi03, enstatite) and amphiboles (double chain silicates containing OH groups) form chains of Si04 tetrahedra. By interlinking the chains we arrive at layer silicates. 

In solids of known composition containing more than one type of Q unit, separate lines were observed in the requisite intensity ratio. For instance, the spectrum of xonotlite, Ca6(OH)2(Si6Ol7), a double-chain silicate branched at every third silicon,... 

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