Silicates, SiO

The affinity between the tetrahedrally arranged orthophosphate oxyanion, P04, and hexava-lently coordinated metal cations lends itself to a classification of phosphate minerals in a scheme similar to silicates (SiO -) framework, insular, chain, and layer phosphates. Examples of this scheme, advanced by Povarennykh (1972) and further elaborated by Lindsay Vlek (1977), include berlinite (AIPO4 framework) hydroxyapatite (insular) monetite (CaHP04, chain) and vivianite (Fe4(P04)2-2H2O, layer). 

The silicate (SiOs-Si), phosphate (PO4-P), and inorganic nitrogen (DIN) are the nutrients of planktons. Their concentrations vary with the areas and seasons. Silicate and DIN concentrations decrease from north to south. While the highest phosphate concentration is found in the south and the lowest value in the north. In spring, SiOs-Si, PO4-P, and DIN concentrations in surface waters are 0.59 13.6, 0.1 0.28, and 1.91 11.6 pmol/L, with averages of 3.43, 0.16, and 3.73 pmol/L, respectively. In summer, SiOs-Si, PO4-P, and DIN con-... 

The most important ion in rocks is the silicate SiO " (in analogy to sulfate S04 ) from the weak orthosilicic acid H4Si04 = Si (OH)4. It is in equilibrium with silicon dioxide (Si02) by condensation (and liberation of H2O) via metasilicic acid (H2Si03) ... 

In the three dimensional neutral silicate structure, the silicon ions Si may be isomorphically replaced by Al ions. In other words, a silicate (SiO ) is replaced by an aluminate (AlO ) ion. In such a case, in order to maintain neutrality, a monovalent cation need to be introduced. Such minerals are called aluminosilicates. 

Simple Silicates. The most important simple siUcate glass-ceramics are based on lithium metasiUcate [10102-24-6] Li SiO lithium disiUcate... 

Fig. 1. Schematic representation of basic silicate stmctures (a) modes of linkage of SiO tetrahedra (b) the corresponding bonding patterns and (c) stmctural formulas (3). The Si atoms that appear to be joined to only three O atoms are joined to a fourth also, which is above the plane of the diagram.

Silicate Glasses. Synthetic siHcates and siHca are composed of oligomers of the orthosiHcate ion, SiO. OrthosiHcate monomers have a... 

The physical and chemical properties of silicate glasses depend on the composition of the material, ion size, and cation coordination number (9). A melt or glass having a Si02/Na20 ratio of 1, ie, sodium metasiUcate [1344-09-8] is expected to possess a high proportion of (SiO ) chains. At a ratio of 2, sheets might predominate. However, litde direct evidence has been shown for a clear predominance of any of these stmctures. The potential stmctures of sihcate melts of different ratios are discussed in detail elsewhere (10—12). 

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

Dicalcium silicate (2CaO SiO ) is very important in the final strength of the cement. This compound hydrates very slowly. The average dicalcium silicate content is 25% to 35%. 

NaOH sodium hydroxide Mg(OH)t magnesium hydroxide Al(OH)3 aluminum hydroxide Si(OH) silicic acid (usually written H[Pg.375]

Elemental phosphorus is prepared by the reduction of calcium phosphate, Ca3(P04)o, with coke in the presence of sand, SiO>. The products are phosphorus, calcium silicate, CaSiOa, and carbon monoxide. 

A. Determination of silica in a soluble silicate Discussion. Most of the silicates which come within the classification of soluble silicates are the orthosilicates formed from SiO units in combination with just one or two cations. More highly condensed silicate structures give rise to the insoluble silicates. 

Most common silicate deposits, such as analcite (NajOAljO SiO HjO), do not dissolve when tested with concentrated HC1. 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

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