Mineral silicate structures

The ion radius of oxygen (O2-) is much larger than that of most cations found in silicates. The oxygen ion constitutes 50-70% of the mass, and over 90% of the volume, of most common silicate minerals. Silicate structures are largely determined by the manner in which the oxygen ions pack together. 

Companson with the mineral silicates is instructive since there is a l l correspondence between the two sets of compounds, the methyl groups in the silicones being replaced by the formally iso-electronic 0 in the silicates (see p. 366). This reminds us of the essentially covalent nature of the Si 0-Si linkage, but the analogy should not be taken to imply identity of structures in detail, particularly for the more highly condensed polymers. Some aspects of the technology of silicones arc summarized in the concluding Panel. 

Faujasite is a naturally occurring mineral, having a specific crystalline, alumina-silicate structure, used in the manufacturing of the FCC catalyst. Zeolite faujasite is a synthetic form of the mineral. 

Layer-silicate structure, as in other silicate minerals, is dominated by the strong Si-O bond, which accounts for the relative insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH. The spatial arrangement of Si and these metals with O and OH results in the formation of tetrahedral and octahedral sheets (see Fig. 8-2). The combination of the tetrahedral and octahedral sheets in different groupings, and in conjunction with different metal oxide sheets, generates a number of different layer silicate clays (see Table 8-1). 

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

Clay minerals are structure of aluminum- and silicate-oxides. Such structures have free bond and vacancies on selective site of the mineral structure allowing them to participate in both physical adsorption and chemical reactions modifying the available concentration of the chemicals in the environment. 

To the extent that the surfaces of the kaolinite layers resemble the surfaces of other silicate minerals, the structure of the adsorbed hole and associated water can serve as a useful model. To determine the applicability of our model to a specific mineral, it will be necessary to know in some detail the structure of the external surfaces of that mineral. 

Biogeochemists use the terms dissolved silica (DSi) or dissolved silicate to collectively refer to all of the dissolved silicon. Silicic acid exhibits tetrahedral geometry with the silicon atom at the center and a hydroxyl group occupying each of the four corners. This structure is similar that of the mineral silicate tetrahedra (Figure 14.3c). Chemical weathering of the silicate minerals is the major source of DSi to the ocean, giving rise to the term dissolved silicate, which is usually abbreviated to just silicate. ... 

Veblen, D. R., P. R. Buseck, and C. W. Burnham (1977). Asbestiform chain silicates, new minerals and structural groups. Science 198 359-365. 

The block iron silicate Si2 xFex Fe3ijFe 05(0H)4, cronstedtite, is representative of several mixed-valent minerals. Its structure consists of alternate layers of comer-shared tetrahedra and edge-shared octahedra. The tetrahedra Si " and Fe ions are coordinated by four oxide ions, the octahedral Fe " and Fe " ions share three common oxide ions with tetrahedral-site layers on one side and on the other are coordinated by three hydroxyl ions that hydrogen bond to the next tetrahedral-site layer. Mossbauer data at 35 K are able to resolve distinguishable Fe " and Fe " ions on the octahedral sites but in the range 300 < T < 500 K a rapid (t), < 10 s) charge transfer was found ... 

It should be born in mind that this is a overall clay composition. When we compare the representations of a clay composition in both tables, we notice that chemists apply the existing nomenclature rules (chapter 3, Chemistry). Furthermore the above represention proves that clay contains a number of silicate minerals. The structure of these silicate minerals and of quartz was already discussed in the chapter on Geology and Minerology. In the next paragraph we will concentrate on a certain group of silicates, the so-called clay minerals. Without these minerals clay would not possess its specific clay properties. 

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