Pyroxene, single chain silicate

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.)...

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. 

Single-chain silicates. Single-chain silicates, (Si03)n , are called pyroxenes. (Note that n is not intended to mean the chain length, which is infinite. ) Examples are enstatite (Mg[Si03[) and diopside, MgCa[(Si03)2]. 

Figure 4.5 Structure of pyroxene minerals (a) demonstration of the end view of the single silicate chain (b) end view of the stacking arrangement of single chains, showing the position of the metal cations. There are two different cationic environments, Ml and M2. (After Putnis, 1992 Figure 6.11, by permission of Cambridge University Press.)...

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).

Although there are many chain silicates, we will consider only the pyroxene and amphibole families, and the sheet silicates related to talc. The pyroxenes contain single chains of Si04 tetrahedra linked by octahedra to give a formula /tSiOj, or (/l,.6)Si20e, where A and B are usually small octahedrally co-ordinated cations. A simplified diagram of the structure is shown in Figure 9 (a). Typical examples of... 

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...

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. 

The ionsilicates (chain silicates) are comprised of either single or double chains of linked silica tetrahedrons. The single chains (pyroxene group) have the formula and consist of a chain of tetrahedrons that are linked together by two of their corner oxygens. The double chains (amphibole group) have the formula... 

Chain silicates Enstatite Pyroxenes are single-chain AI2O3 3.96... 

Match the Si 0 ratio to the type of silicate in Table 22.2. A 1 3 ratio is a single chain, a pyroxene (or inosihcate). A 2 7 ratio is a double tetrahedron, a pyrosilicate (or sorosilicate). 

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]. 

Infinite Chain Anions. These are of two main types, the pyroxenes, which contain single-strand chains of composition (SiO ) (Fig. 11-3) and the amphiboles which contain double-strand, cross-linked chains or bands of composition (Si Of ),. Note that the general formula of the anion in a pyroxene is the same as in a silicate with a cyclic anion. Silicates with this general stoichiometry are often, especially in older literature, called metasilicates. There is actually neither metasilicic acid nor any discrete meta-silicate anion. With the exception of the - few— -metasilicates with-cychc -anions, such compounds contain infinite chain anions. 

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