Silicates, dense

Figure 2.12 Plot of the area per T-atom vertex SI) versus the average ring size, n, for a variety of zeolites, silica clathrasils and dense silicates. (All zeolites have a silicon aluminium ratio exceeding three, so that the approximate stoichiometry of all these frameworks is Si02). Zeolite and clathrasil frameworks are labelled by the code adopted by the International Zeolite Association [18]). The shaded domain indicates the window of geometrically accessible values of as a function of the ring size. Despite the allowed geometric variability, the value of D is close to 12.2A2 for all these "silicates", regardless of the ring size and consequent intrinsic curvature.

The present Collection serves as a source of reference patterns for pure zeolite phases. The data will be helpful in establishing the structural purity of experimental phases and in indexing their diffraction patterns. The data will also aid in the determination of changes in the lattice parameters with changing composition, assessing preferred orientation effects, background evaluation, and line broadening. We have also included diffraction patterns of several common dense silicate phases to facilitate their detection in mixed phase syntheses. 

The increasingly indispensable role of atomistic and quantum mechanical simulations in inorganic crystallography is perhaps no more strikingly illustrated than in the field of silicates and zeolitic materials. The two classes of material with which we shall be concerned in this chapter, namely microporous zeolites (including both aluminosilicate-based materials and their sister compounds, the aluminophosphates (ALPOs)), and the dense silicate materials and related oxides which constitute the bulk of the Earth s mantle, have in common the fact that their structural properties may be difficult to determine by conventional experimental means. Yet highly detailed and accurate structural information is critical in understanding the properties of both these important types of material. 

The function of these metal and organic cations has been the subject of prolonged debate. Their primary role is to stabilise open silicate frameworks with respect to dense silicates. This has been demonstrated by calorimetric data, for example by Petrovic et al. who showed that all zeolitic silicas are only 7-14 kJ(molSi02) less stable than quartz, the stability decreasing with decreasing density (Figure 4.3). They concluded that this relatively small difference would be made up by the non-bonding interactions... 

The main difference between the two types are that the reaction products of the silico fluoride types are less soluble in water and are also harder, which may give better in-service performance but at a slightly higher material cost. However, with recent developments in floor-laying techniques, the concrete substrates for industrial floors are laid with much more dense low-porosity surfaces, so that neither silicate nor silico fluoride treatments are as effective as they used to be, when the concrete used had a slightly more open finish and hence was more receptive to these treatments. With modern concrete floors, it is imperative to wash any material not absorbed into the surface within a short period. Otherwise, unpleasant white alkaline deposits, which are difficult to remove, may occur. 

Silicates produce hard, dense, gray to grayish brown scales of variable composition. Silicates are complex materials and usually are associated with several cations, including sodium, magnesium, iron, and calcium. 

At blast furnace temperatures, calcium silicate is a liquid, called slag. Being less dense than iron, slag pools on the surface of the molten metal. Both products are drained periodically through openings in the bottom of the furnace. 

Consists of clay, mud, and silt, mainly aluminum silicates Dense fine-grained rock containing mainly clay Contains silicates secondary product... 

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

Salt glands of plants from Atriplex genus contain inclusions in the form of crystals of siliceous or sulphate salts of calcium and magnesium (Fahn, 1979). Usually the crystal particles also include phenols (see Chapter 7). The crystals are seen as dark dense spots within the structures on OCM images of the optical slices from the gland (Fig. 4). Profiles of signal intensity along... 

Multimedia filters, which consist of a top layer of coarse and low density anthracite, layers of silica, and then dense finest medium vitreous silicate, remove about 98% of particulates >20 tm. These filters are regularly back-washed to avoid buildup of particulates. Finer filters (S-lO tm) are used to remove suspended matter and colloidal materials. To prevent scaling due to water hardness, sodium ions generated from brine are exchanged with calcium and magnesium ions in the water. Activated carbon or metabisulfite is used to remove chlorine. 

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 lb of lime must be used to manufacture a ton of steel. 

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