Amphibole, double chain

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. 

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. 

Figure 532 Structural features of amphiboles. (A) Double chain [T40n] seen along axis c (a) and in perspective (b). (B) Double chain seen from terminal part and various cationic positions (compare with figure 5.19, for analogies with the pyroxene structure).

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).

Polymerization into (Si40,i) units gives rise to minerals characterized as having double chains. The configuration depicted in Fig. 2. IF is common and typical of the amphiboles, a mineral group of special interest because five amphiboles have been mined as asbestos. 

These few examples illustrate the range of habits exhibited by minerals whose basic structural units are single chains. In addition, the pyroxenes, which have compositions quite similar to those of the amphiholes, are often found intimately intergrown with the latter double-chain minerals in what appears to be a single large crystal. An intergrowth of amphibole and pyroxene as a fiber, or in a fibrous aggregate, has not yet been described, but it is a likely occurrence. 

Both single- and double-stranded chains are found. The most important members of single chains are the pyroxenes including diopside. The most important double-chained minerals are the amphiboles. Some of these contain hydroxyl and fluoride ions, bonded directly to the metal cation and not to the silicon atom. 

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,... 

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. 

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. 

In the double chains or ribbons, there are different kinds of tetrahedra sharing two and three vertices. The most numerous amphiboles and asbestos minerals, such as tremolite, Ca2Mgs(Si40n)2(0H)2, adopt the [Si40n] 6 doublechain structure, as shown in Fig. 14.4.9(f). 

More important than the simple metasilicate chain, in the structure of crystals, is the double chain found in the amphiboles. This structure is shown in Fig. XXVI-3, looking down on the top. A unit of the chain contains four silicons, eleven oxygens, as we see by counting, and it has... 

Inosilicates contain single or double chains of tetrahedra. In a single chain structure, the Si to O ratio is 1 3, and in a double chain, it is 4 11. This structural class contains many common rock-forming minerals, including the amphibole and pyroxene groups. Both jade minerals, jadeite, and nephrite, fall within this group. 

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

Si40 ) - 5.87 Double chains [e.g., amphiboles such as tremolite, (0H)jCa2Mg5(Si40 )2]... 

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