Crystallization of silicates

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... 

Silicates comprise more than 95% by weight of the earth s crust and mantle, and are widely used in glasses, ceramics, sieves, catalysts, and electronic devices. Crystals of silicates are often hard, and may show considerable extinction in their diffraction pattern, which means not only that small samples must be used, but also that ambient temperatures may be adequate for charge density studies.2... 

The results of X-ray determinations of the crystal structures of silicates with complex cations are disciissed. Vfaterso-luble single crystals of silicates with Ni(en), Cu(en)p Co(en), UCCH,). and cations have been studied. It is shown that in all structures determined the silicate anions have the foim of double trigonal and tetragonal rings... 

Figure 14.36 Reprinted from Mallamaci, M.P, Bentley, J., and Carter, C.B. (1997) In-Situ TEM crystallization of silicate-glass films on AI2O3, Acta Mater. 46, 283. Copyright 1997, with permission from Elsevier. See also Mallamaci, M.P. and Carter, C.B. (1999) Crystallization of pseudo-orthorhombic anorthite on basal sapphire, J. Am. Ceram. Soc. 82, 33.

Fig. 11. Single crystal of siliceous ferrierite. (a) As-synthesized and (b) calcinated crystal viewed along the lOMR channels ([100]-direction top) and 8MR channels ([010]-direction, bottom), respectively. Oxygen atoms of the

Crystallization of Silicic Acid. — Several authors have succeeded in obtaining crystals at higher temperatures from both the gel and the... 

Because gemstones are mostly single crystals of silicate minerals, there is generally no need for a long exposure time. This makes gemstone analysis quick and efficient. Difficulties may, however, arise under certain circumstances ... 

Figure 10.22 Crystals of silicate minerals aquamarine, heliodor, morganite and emerald.

Together with the structural principles established by the Bragg school concerning the many types of silicates, Goldschmidt s ideas were taken further by Linus Pauling in California to establish the modern science of crystal chemistry. A good early overview of the whole field can be found in a book by Evans (1939, 1964). 

NaY zeolite is produced by digesting a mixture of silica, alumina, and caustic for several hours at a prescribed temperature until crystallization occurs (Figure 3-8). Typical sources of silica and alumina are sodium silicate and sodium aluminate. Crystallization of Y-zeolite typically takes 10 hours at about 210°F (100°C). Production of a quality zeolite requires proper control of temperature, time, and pH of the crystallization solution. NaY zeolite is separated after filtering and water-washing of the crystalline solution. 

As a result of its unique chemical and physical properties, silica gel is probably the most important single substance involved in liquid chromatography today. Without silica gel, it is doubtful whether HPLC could have evolved at all. Silica gel is an amorphous, highly porous, partially hydrated form of silica which is a substance made from the two most abundant elements in the earth s crust, silicon and oxygen. Silica, from which silica gel is manufactured, occurs naturally, either in conjunction with metal oxides in the form of silicates, such as clay or shale, or as free silica in the form of quartz, cristobalite or tridymite crystals. Quartz is sometimes found clear and colorless, but more often in an opaque form, frequently colored... 

To overcome the limitations of natural zeolites a whole range of synthetic zeolites have been manufactured since the 1950s. These have tailored pore sizes and tuned acidities, as well as often incorporating other metal species. The basic synthesis involves mixing a source of silica, usually sodium silicate or colloidal Si02, with a source of alumina, often sodium aluminate, and a base such as sodium hydroxide. The mixture is heated at temperatures up to 200 °C under autogenous pressure for a period of a few days to a few weeks to allow crystallization of the zeolite. 

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