Silicates and aluminosilicates

When three corners of each Si04 tetrahedron are shared, a two-dimensional sheet 

O oxygen O hydroxyl group O water molecule 0 oxygen + hydroxyl (in projection) 

Structures of typical groups are shown in Fig. 1.21. Further subdivision is possible on the basis of the nature of the octahedral sheets (di- or tri-octahedral) and stacking sequences. 

If all the four corners of the SiO tetrahedra are shared, three-dimensional networks result. The different forms of silica (quartz, tridymite and cristobalite) discussed earlier in this section are typical examples. Feldspars are generated when part of the Si is 

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The metals which form silicate and aluminosilicate minerals are the more electropositive metals, i.e. those in Groups 1, 2 and the 3d transition series (except Co), together with Y, La and the lanthanoids, Zr, Hf, Th, U and to a much lesser extent the post-transition elements Sn", Pb", and Bi ". 

Thus an acid-base reaction involves the transfer of an oxide ion (compared with the transfer of a proton in the Bronsted theory) and the theory is particularly applicable in considering acid-base relationships in oxide, silicate and aluminosilicate glasses. However, we shall find that it is subsumed within the Lewis definition. 

Tetrahedra linked via three vertices correspond to a composition MX1 1X3 2 or MX2 5 = M2X5. Small units consisting of four tetrahedra are known in P4O10, but most important are the layer structures in the numerous sheet silicates and aluminosilicates with anions of the compositions and [AlSiOj-]. Because the terminal vertices of the single... 

Jambor, J.L., Dutrizac, J.E., Groat, L.A., Raudsepp, M. 2002. Static tests of neutralization potentials of silicate and aluminosilicate minerals. Environmental Geology, 43, 1-17. 

Humus/SOM enter into a wide variety of physical and chemical interactions, including sorption, ion exchange, free radical reactions, and solubilization. The water holding capacity and buffering capacity of solid surfaces and the availability of nutrients to plants are controlled to a large extent by the amount of humus in the solids. Humus also interacts with solid minerals to aid in the weathering and decomposition of silicate and aluminosilicate minerals. It is also adsorbed by some minerals. 

The concentrations of dissolved species in natnral waters depend ultimately on the dissolution of basic rocks-carbonates, silicates and aluminosilicates-induced by the action of weak acids in the water derived from dissolved gases-e.g. H2CO3 derived from CO2. Anions produced in acid-base reactions balance cations produced in dissolution reactions. The charge balance is ... 

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).

Since silicates and aluminosilicates are by far the predominant rock-forming minerals, the crystal structures of most species have been determined. Liebau (1980) presents an overview of the structures of silicate and aluminosilicate minerals, and one can consult Berry, Mason, and Dietrich (1983, especially pp. 382-389) or other mineralogy texts for an introduction to the subject. The multivolume work of Eitel (1965) provides a general treatment of the crystal chemistry of all types of silicate materials. 

SILICATE AND ALUMINOSILICATE MINERALS THAT FORM FIBERS... 

In addition to the asbestos varieties many other examples of silicate and aluminosilicate minerals occur in fibrous form. In the following section we present data for a few selected species, indicating the types of chemical... 

From every chemical group mentioned in Dana s System of Mineralogy there are minerals that form as fibers. We began with the most commonly encountered minerals, the silicate and aluminosilicate groups, and now briefly mention a few from other chemical classes. 

We have recorded 388 minerals (Appendix 1) that occur, at least occasionally, as fibers some minerals are found only in fibrous form. This number includes more than 92 silicate and aluminosilicate species, most of them common rock-forming minerals. This list, only a fraction of the 3000 minerals known, probably represents only a sampling of naturally occurring fibers. 

The following list is a presentation of all minerals, mineral series, and mineral groups described in standard mineralogy texts and references as occurring in fibrous, acicular, or needlelike forms. The presentation follows the outline given in The System of Mineralogy of J. D. Dana and E. S. Dana (7th ed. Palache, Berman, and Frondel, eds.). The silicates and aluminosilicates follow the classifications of Rock Forming Minerals (Deer, Howie, and Zussman, eds.). 

Mg NMR can be used to provide information on the thermal decomposition of silicates and aluminosilicates such as for the asbestos mineral chrysotile [Mg3Si205(0H)4]. Combining Mg and Si NMR showed the... 

Iron oxides present in coal are generally stable for the relatively short period of time that they are exposed to combustion temperatures. Therefore, siderophile elements (e.g., Ni, Co, Mo, Pt, Pd, Au) that are incorporated within iron oxides are also expected to remain stable, and escape any significant thermal transformation reactions (Bums 2003). Similarly, lithophile elements (e.g., Ba, B, Cr, Mn, Sr, V) that are initially found in association with silicates and aluminosilicates in coal are expected to be incorporated within the glassy fraction of coal ash upon thermal transformation of their parent minerals (Bums 2003). 

Pederson, L. R., McGrail, B. P., McVay, G. L., Petersen-Villalobos, D. A. Settles, N. S. 1993. Kinetics of alkali silicate and aluminosilicate glass reactions in alkali chloride solutions Influence of surface charge. Physics and Chemistry of Glasses, 34, 140-148. 

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