Form of silica

Noncrystalline Silicas. The noncrystalline forms of silica iaclude bulk vitreous silica and a variety of other amorphous types, which are of substantial commercial importance (see SiLiCA, AMORPHOUS SILICA SiLiCA, VITREOUS SILICA). A review discussiag the occurrence, synthesis, properties, and apphcations of the various forms of silica adsorbents is available (71). 

Pure silica contains no metal ions and every oxygen becomes a bridge between two silicon atoms giving a three-dimensional network. The high-temperature form, shown in Fig. 16.3(c), is cubic the tetrahedra are stacked in the same way as the carbon atoms in the diamond-cubic structure. At room temperature the stable crystalline form of silica is more complicated but, as before, it is a three-dimensional network in which all the oxygens bridge silicons. 

A microcrystalline form of silica. An impure form of flint used in abrasives. 

A microcrystalline impure form of silica similar to chert. Used for decorative purposes. Rare in industry. 

Amorphous silica The noncrystalline forms of silica or quartz. 

Although vitreous silica is nominally a homogeneous isotropic amorphous material, and should normally remain so during its service life, it is in fact in a metastable condition. The tendency to revert to crystalline forms with attendant deterioration in mechanical durability places severe limitations on the range of applications. Figure 18.2 illustrates the polymorphic forms of silica, and the dimensional changes accompanying each transition. 

FIGURE 5.17 (Left) Quartz is a crystalline form of silica, Si02, with the atoms in an orderly network represented here in two dimensions. (Right) When molten silica solidifies in an amorphous arrangement, it becomes glass. Now the atoms form a disorderly network. 

FIGURE 14.33 Three common forms of silica (Si02) (a) quartz (b) quartzite and (c) cristobalite. The black parts of the sample of cristobalite are obsidian, a volcanic rock that contains silica. Sand consists primarily of small pieces of impure quartz. 

FIGURE 14.36 Impure forms of silica amethyst (left), in which the color is due to I e impurities agate (center) and onyx (right). 

The dimensions of the unit in the basal plane closely approximate those for the similarly pseudohexagonal crystal hydrargillite, Al(OH)3, as well as of the hexagonal layers in two forms of silica, /3-tridymite and 0-cristo-balite. The monoclinic (pseudohexagonal) unit of structure of hydrargillite8 has a = 8.70 A, b = 5.09 A, c = 9.76 A, and 0 = 85°29, and contains 8A1(0H)3. The crystal is composed of layers of octahedra,... 

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... 

Quartz, a common form of silica, is a network of Si—O bonds. Silicon and oxygen both have tetrahedral electron group geometry. All the silicon atoms have tetrahedral shapes and all the oxygen atoms have bent shapes. 

More than 20 different forms of silica exist, because the bonds and lone pairs around the oxygen atoms can be arranged in various ways. Each arrangement creates a different stmctural form for the silica network. Quartz, the most common form of silica, is found in granite, sandstone, and beach sand. 

This is caused by the fact that the Si - is tied up in the form of silica tetrahedra where some of the oxygen atoms are shared within the structure and are not free to move. A representation of this is given as follows ... 

The application of infrared photoacoustic spectroscopy to characterize silica and alumina samples is reported. High quality infrared photoacoustic spectra illuminate structural changes between different forms of silica and alumina, as well as permit adsorbate structure to be probed. Adsorption studies on aerosil suggest adsorbed species shield the electric fields due to particle-particle interactions and induce changes in the vibrational spectra of the adsorbates as well as in the bulk phonon band. It is shown that different forms of aluminum oxides and hydroxides could be distinguished by the infrared spectra. 

The spectrum in Figure Id is for a crystalline form of silica, silicalite (Union Carbide S-115, see ref. 13). The structure is comprised of twelve silica tetrahedra linked into five pentasil groups and one hexasil group. This building block is repeated... 

The form of silica in the matrix is at present unknown. In the freshly prepared cement there are appreciable amounts of silicic acid present which decline as the cement ages (Crisp, Lewis Wilson, 1976d). In the set cement silica could be present as a polymeric silicic acid, a siliceous gel or even a hydrated silicate gel, such as the tobermorite gel present in Portland cements (Taylor, 1966). 

The most common inorganic adsorbents used in TLC are silica gel, alumina and kieselguhr silica gel being by far the most important. Kieselguhrs are made from natural diatomites and are an impure form of silica, being about 90% or so silica with the... 

Silica gel. Silica gel is a porous amorphous form of silica (Si02) and is manufactured by acid treatment of sodium silicate solution and then dried. The silica gel surface has an affinity for water and organic material. It is primarily used to dehydrate gases and liquids. 

Figure 4 The structure of the two forms of silica cristobalite (crystalline, left) and silica glass (amorphous, right). (Reprinted with permission from Ref. 4.)...

Chert Cristobalite A mir rocryslalline form of silica. An impure form of flint used in abrasives. A crystalline form of free silica, extremely hard and inert chemically very resistant to heal. Quartz in refractory bricks and amorphous silica in rlialomaceous earth are altered to cristobalite when exposed to high temperatures (calcined). Cristobalite is extensively used in precision casting by the hot wax process, dental laboratory work, and certain speciality ceramics. 

These two anciently served as a means of ignition. They still can. Although the sparking of flint and steel is strictly a mechanical phenomenon, any subsequent fires will not be. Other forms of silica still serve as igniters to this day, via the piezoelectric effect. 

A variety of minerals are prized for their exquisite beauty, rarity, and exceptional durability. These extraordinary materials are classified as gemstones. One such mineral, silica, with a chemical composition of SK>2 (silicon dioxide), exhibits several crystal structures. Several gemstones are crystalline forms of silica, including amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zircon J l... 

In addition, silicon (Si), which also can be found in coal gas, has been studied (35) as a contaminant. It is believed to accumulate on the fuel electrode in the form of silica (Si02). The deposition of the Si throughout the cell has been found to be enhanced by high (-50%)... 

Stishovite is very interesting because it has the rutile structure with octahedral coordination of silicon. In all other forms of silica, each silicon atom is surrounded tetrahedrally by four oxygen atoms. 

Fibrosis was first recognized in certain occupational settings. One of the well-known conditions of this type is silicosis, which is brought about by long-term, uncontrolled exposure to certain crystalline forms of silica (SiOi), and certain related substances called silicates. These minerals are widespread on earth, in fact most of the inorganic, non-aqueous earth consists of silica and silicates. Many of these minerals (e.g., quartz) have major industrial uses. It is important to emphasize that silica and silicates occur in both crystalline and non-crystalline... 

Biogenic silica A mineral form of silica that is amorphous in structure and deposited by marine organisms such as diatoms and radiolaria. Also called opal or opaline silica. 

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