Amphibole Structures

Figure 4.14 Portion of the amphibole structure projected down the a axis showing (a) bands of edge-shared octahedra extending along the c axis, and (b) metal-metal distances and site occupancies in glaucophane. Cell parameters and interatomic distances from Papike and Clark (1968).

Cation distributions in amphibole structures, the site occupancy data for which were obtained mainly from X-ray crystal structure refinements, Mossbauer spectroscopy and hydroxyl stretching frequency measurements in the infrared region, have been critically reviewed (Strens, 1974 Hawthorne, 198 la,b 1983). 

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 fibrous texture of asbestos results from the silicate double chains of the amphibole structure. 

Mechanical (101) [101] twins have been identified in experimentally deformed hornblende single crystals, as well as dislocations on the (100)[001] slip system [333,334]. In hornblendes from naturally deformed rocks dislocations on (hkO) planes were documented, mainly [001] screws [335-338]. A systematic investigation of dynamically recrystallized hornblende from a high-temperature shear zone discovered microstructures typical of dislocation creep, with subgrain boundaries and free dislocations [313]. The primary slip system is (100)[001] consistent with experimental results. Secondary, slip systems are (010)[100] and 110)5<110>. There is evidence for cross-slip of [0 01] screws producing heUcal microstructures [Fig. 13(b)]. Amphibole structures are intermediate between pyroxenes and sheet silicates and indeed chain multiplicity faults have been described [339] and transitional structures may be facilitated by movement of partial dislocations [340]. 

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. 

Amperometric cells, sensors using, 22 271 Amperometric measurements, 14 612 Amphetamine, 3 89-90 Amphibole asbestos, 1 803 3 288 crystal structure, 3 297-298 exposure limits, 3 316 fiber morphology, 3 294-295 silicate backbone, 3 296 Amphibole potassium fluorrichterite, glass- ceramics based on, 12 637 Amphiphile-oil-water-electrolyte phase diagram, 16 427-428 Amphiphile-oil-water phase diagrams,... 

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).

The chemical classification of amphiboles groups the various terms according to their chemical compositions independent of the represented structural classes. 

The structure of amphibole admits a large number of isomorphous substitutions, and the group shows a marked heterogeneity in natural specimens. Table... 

Evaluation of the effects of chemistry on volume properties of amphiboles is comphcated by the high number of sites in which isomorphous substitutions take place. Limiting ourselves to the main structural classes C2lm, Pnma, and P2, m, we present the following evaluations. 

Table 5.46 Structural data on amphibole end-members (from Smyth and Bish, 1988). Molar volume in cm /mole molar weight in g/mole cell edges in A cell volume in A. ...

Figure 5,38 Structural parameters of amphiboles belonging to various chemical groups. From Hawthorn (1981a). Reprinted with permission of The Mineralogical Society of America.

Hawthorne F. C. (1976). The crystal chemistry of the amphiboles, V The structure and chemistry of arfvedsonite. Canadian Mineral, 346-356. 

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. 

The crystal structures of all amphibole minerals, including the asbestiform varieties, are most easily understood as variations on a basic structural unit called an I-beam. The term I-beam alludes to the cross-sectional shape of the three-part structure consisting of corner-linked (Si04) and/or (A104) tetrahedra (T) linked together into a double-tetrahedral chain that sandwiches a layer of edge-shared and R Og octahedra (O). R elements, which... 

Further explication of the complexities of the compositions and the crystal structures of the amphiboles can be found in works by Papike and Ross... 

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