Silicate infinite chain

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.)...

More than 90% of the rocks and minerals found in the earth s crust are silicates, which are essentially ionic Typically the anion has a network covalent structure in which Si044-tetrahedra are bonded to one another in one, two, or three dimensions. The structure shown at the left of Figure 9.15 (p. 243), where the anion is a one-dimensional infinite chain, is typical of fibrous minerals such as diopside, CaSi03 - MgSi03. Asbestos has a related structure in which two chains are linked together to form a double strand. 

Silicate lattices. The red circles represent oxygen atoms. The black dot in die center of die red circle represents the Si atom, which is at the center of a tetrahedron. (Left) Diopside has a one-dimensional infinite chain. (Right) A portion of the talc structure, which is composed of infinite sheets. 

Si04 units share two corners to form infinite chains (Figure 1.52(c)). The repeat unit is SiOs . Minerals with this structure are called pyroxenes (e.g., diopside (CaMg(S 103)2) and enstatite (MgSiOs)). The silicate chains lie parallel to one another and are linked together by the cations that lie between them. 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

The two main types of infinite chain anions are the pyroxenes, which contain singlestrand chains of composition (SiO ) (Fig. 8-4) and the amphiboles, which contain double-strand, cross-linked chains or bands of composition (SLOif),. 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 called metasilicates, especially in older literature. There is actually neither metasilidc add nor any discrete metasilicate anion. With the exception of the few metasilicates with cyclic anions, such compounds contain infinite chain anions. 

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]. 

The cyclie groups occur at metaborates, as the rings of (B O ), for example, K B O, or as groups in infinite chains of B-0 bonding, with the infinite repetition of the (B02) formula, for example, CaB20. Note that the stratified structures are less spread to borates than to silicates. 

Chemically, the uranyl silicates form three groups depending on the uranium/silicon ratio. The most populated group, the 1 1 group, is one of the best studied. Stohl and Smith and Sidorenko and co-workers reviewed the crystal chemistry of these minerals. They showed that all 1 1 minerals have essentially the same basic structural unit [(U02)Si04]J" , an infinite chain of edge-shared uranyl pentagonal dipyramids and silicate... 

Wollastonite is calcium silicate with triclinic crystal system (P21). It has infinite-chain structure, with three tetrahedra per unit sell arranged parallel to y, this repeat unit consisting of a pair of tetrahedra joined apex to apex as in the [Si07] group, alternating with a single tetrahedron with one edge parallel to the chain direction. Steady-state luminescence of wollastonite has been previously studied and luminescence of Mn ", Fe " and supposedly Cr " has been proposed (Min ko et al. 1978). 

---