Amphibole, double chain silicate

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. 

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,... 

Two molecular types of silicates are referred to as asbestos. Chrysotile is a magnesium silicate built upon a layered structure of silicate rings and Mg(OH)2. The layered structure causes the sheets to roll into cylinders approximately 200A in diameter. Amphibole asbestos may contain a variety of cations but is built upon a double chain silicate structure. The chrysotile asbestos is always found as an asbestiform crystal while the amphiboles may be either acicular or asbestiform. 

Fig. 3. Silicate backbones of chrysotile and amphiboles (a) the sheet silicate structure of chrysotile, analogous to that of micas (b) the double-chain silicate...

Subclass Inosilicate - Double Chain In the double chain inosilicates, two parallel chains are linked together by their alternate tetrahedra. The basic structural group is Si40if . The amphibole group of minerals are double chain silicates, for example the tremolite-ferroactinolite series -Ca2(Mg,Fe)5Sig022(OH)3. 

Double-chain silicates, shown in Figure I5.13f, find some SiO units sharing two oxygens while others share three. The repeating unit is Si40if. Five double-chain silicates [tremolite, anthophyllite, actinolite, amosite ( brown asbestos ), crocidolite ( blue asbestos ), all members of the amphibole family of minerals] and the serpentine mineral chrysotile ( white asbestos ) are classified under the generic name asbestos. The molecular-level chain structure of these materials is reflected in... 

So far as is known such a result has not yet been seen in a silicate. However sub-unit cell structure has been resolved in amphiboles (Hutchison, Irusteta and Whittaker, 1975) which can be associated with the form of the amphibole double chain. This has permitted the direct observation of an intercalated layer of triple chains in some thin crystals. From a compositional point of view such an intercalation corresponds to a small amount of solid solution of talc in amphibole, so that we have in this case a direct... 

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. 

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).

The microscopic and macroscopic properties of asbestos fibers stem from their intrinsic, and sometimes unique, crystalline features. As with all silicate minerals, the basic building blocks of asbestos fibers are the silicate tetrahedra which may occur as double chains (SiO)6l 14, as in the amphiboles, or in sheets (SiO)i-104. as in chrysotile. 

Deer, Howie and Zussman (19J give physical and optical constants for amphibole minerals in Vol. two, Rock Forming Minerals, (chain silicates), and for Serpentine minerals (chrysotile) in volume three (sheet silicates). X-ray diffraction data is obtainable from the Powder Diffraction file, Inorganic (20). This data can be used for a double check of asbestos standards. 

S5 Infinite double chains of tetrahedra (duochain silicates) amphibole family (Fig. 3E) (Si4On) 2 and 3 SiOiVl Tremolite (OH)2Ca2Mg5 Si8022... 

Figure 9 Idealized representations of silicate chains formed by corner sharing of Si04 tetrahedra. (a) Single chains as found in the pyroxenes, (b) double chains as found in the amphibole s, and (c) infinite sheets as found in the micas...

Silicates M Si2 0s prefer the hexagonal layer to the double chain ion (see next section). Sillimanite is strictly an orthosilicate, for it contains discrete Si04 groups. However, since one-half of the A1 atoms are 4-coordinated it may alternatively be regarded as an aluminosilicate in which the Si atoms and these A1 atoms form double chains of type (d). The octahedrally coordinated A1 atoms lie between these chains, and the compound may be formulated Al(AlSi05). In the mineral world the most important compounds containing double chains are the amphiboles, of which... 

Micas. If the double-chain amphibole structure diagrammed in Figure 2.8a is extended in two dimensions by the bonding of all three basal 0 atoms of each tetrahedron with Si atoms of other tetrahedra, a sheet silicate (phyllosilicate) is formed with the structure shown in Figure 2.9a. This polymer, extended infinitely in two dimensions, has the formula (Si40to) and is the basis of the mica structure (as well as the layer silicate clays, as will be discussed later in this chapter). 

The pyroxene group and the amphibole group, respectively, are representatives of silicate minerals having single-chain and double-chain tetrahedral networks. Pyroxenes are believed to be significant components of Earth s mantle, whereas amphiboles are dark-colored minerals commonly found in continental rocks. 

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