Ribbon silicates

Fig. 3.2-1 Schematic representation of the arrangement of the [Si04] tetrahedra in planar silicate crystal structures 1, nesosilicate 2, sorosilicate 3, cyclosilicate 4 and 5, inosilicates 6 and 7, ribbon silicates 8, layered silicate or phyllosilicate...

A layer of tetrahedra can be considered as being built up by linking parallel chains. That this is not a mere formalism is shown by the existence of intermediate stages. Two linked silicate chains result in a ribbon of the composition [Si4Of ] it has two kinds of tetrahedra, one kind being joined via three and the other kind via two vertices, [SiO Oj SiOj/jOj ]3 Silicates of this type are termed amphiboles. They are fibrous and also used to be used as asbestos. 

Both sepiolite and palygorskite contain tetrahedral silicate sheets (with a variety of substitutions for the Si " ), but the apicies of the tetrahedra are thought to point up or down with the transition areas containing Ca and Mg ions, and bound or associated H2O. The arrangement produces a continuous basal oxygen plane that is compartmentalized. Laths or ribbons three chains wide in sepiolite and two chains wide in palygorskite are separated by discontinuous octahedral areas (Fig. 2.16A and B). 

Substitution and variations in the tetrahedral sites change the manner of side linkages for the ribbons, effecting the octahedral cation and water associations. In addition, different ribbon widths can lead to different numbers of octahedral cations. Variation in the width of chains and substitution of cations and water are easily accomplished, which means that accurate and consistent chemical and crystal structural data on these minerals are difficult or, at best, approximate. However, the minerals do form fibers with a consistent fiber axis repeat of about 0.512 nm (Preisinger, 1959 Rautureau et al., 1972). Sepiolite and palygorskite represent the widest possible structural and chemical diversity among fibrous silicate minerals. 

Chain metasilicates or ino -silicates Chains or ribbons of [Si04] tetrahedra sharing two oxygen atoms Jadeite, NaAl(Si2Ofi) Wollastonite, Ca3(Si309) Tremolite, Ca2Mg5(Si40,)2(OH)2... 

The nodules are found in gray-black commercial slate having fine grain size, uniform color and texture, and well developed slaty cleavage. Such slate occurs in thick strata marked only by thin black bands ( ribbons ) of somewhat coarser texture, and by rare, disseminated knots, siliceous nodules of foreign material. The slate is a mixture of quartz, illite, chlorite, caldte, and muscovite, with minor amounts of pyrite, carbonaceous matter, and heavy mineral grains. The dark color is attributed to finely disseminated carbon and pyrite. An analysis of the slate is given in Table I. 

Fig. 3.2. Graph showing the dependence of magnitude Ah on the kind of cations binding silica chains and ribbons in minerals of aliphatic silicate subgroup. (After Povarennykh, 1963)...

Amphiboles. The crystalline structure common to amphibole minerals consists of two ribbons of silicate tetrahedra placed back to back. 

We referred earlier to the silylation of silicates to give trimethylsilyl derivatives corresponding to the Si—0—Si systems in the original minerals. A development of this idea is to treat chrysotile with a mixture of HCl and ClSi(CH3)3 when the Mg and OH ions are stripped off the outside of the layer and replaced by -OSi(CH3)3, giving a gel which on treatment with water forms a fibrous mass of ribbons. When dry these curl up to form fibres similar in shape to those of the original chrysotile. ... 

A number of silicate and germanate fluoroamphiboles of the richterite series, Na2Mg2[(Si ,Ge8 B)022]F2 (n = 8, 7.7, 4, or 0), in which the anions form a ribbon structure, have been synthesized and their properties studied as a function of Si—Ge content 312 it is concluded from the data that a broad chemical crystallographic analogy exists between the silicates and ger-manates. 

Such clusters of FeOe octahedra also exist in other inorganic compounds. For instance they exist in ilvaite (CaFe TFe 2Si20700H) where silicate layers replace phosphate anions and FeOs octahedra form ribbons. It is interesting to note that such a material is active and selective for the reaction although to a lesser extent than the previous hydroxyphosphates [24]. The lower catalytic behaviour may be due to the presence of Ca " cations in the structure and/or to the silicate counter anion whose basicity in the sense of Pearson is different from that of the phosphate anion and/or the infinite size of Fe06 octahedra clusters. 

Amphiboles. The crystalline structure common to amphibole minerals consists of two ribbons of silicate tetrahedra placed back to back as shown in Figure 5. The plane of anionic valency sites created by this double ribbon arrangement is neutralized by the metal cations. The unit cell has seven cationic sites of three different types these sites can host a large variety of metal cations without substantial disruption of the lattice. 

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