Silica crystalline forms

The inorganic materials involve the silica, montmorillonite, allapulgite, FCjO, ZnCl, CuCl, etc. The as-prepared composites can absorb versatile organic and inorganic pollutants from aqueous solutions. Different silica crystalline forms were utilized to fabricate various PANI/silica composites for water purification. PANI were coated on silica gel to modify its surface morphology and to improve its adsorption capacity for phenol [35] and acid green 25 [36] adsorption from aqueous solutions. 

The irradiation of tetra-/-butylcyclopentadienone with 254 nm light at 77 K produced a tricyclopentanone which, upon extended irradiation, lost carbon monoxide. Tetra-f-butyltetrahedrane was formed. This derivative of the second fundamental hydrocarbon of molecular formula (CH), namely tetrahedrane, is stable at room temperature and could be isolated after chromatography on silica gel in crystalline form (G. Maier, 1978). 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

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 crystalline form of free silica, extremely hard and inert chemically very resistant to heat. Quartz in refractory bricks and amorphous silica in diatomaceous 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. 

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. 

At pH levels above 9.0, there is an increased tendency for the soluble silica to form silicate anions and react with magnesium (and to a lesser extent with calcium) to form insoluble forms of magnesium silicate that precipitate to form crystalline scales and sludge. 

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. 

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... 

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... 

Since it is the major constituent of the Earth s crust, silica is very abundant in nature in amorphous or crystalline forms. Silica can be manufactured, for instance, by high-temperature processing (glasses) or by the sol-gel process. This latter occurs through a two-step reaction hydrolysis followed by condensation. Silica gels are good candidates for use in enzyme immobilization. They display good optical and... 

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... 

Silica Silicon oxide (Si02(s)), which can occur in crystalline form, e.g., quartz, or noncrystaUine form, e.g., opal. 

A variety of mineral-type materials are inorganic polymers [Ray, 1978]. Silica [(SiCLL] is found in nature in various crystalline forms, including sand, quartz, and agate. The various crystalline forms of silica consist of three-dimensional, highly crosslinked polymer chains composed of Si04 tetrahedra where each oxygen atom is bonded to two silicon atoms and each silicon atoms is bonded to four oxygen atoms. Silicates, found in most clay, rocks, and... 

Silica Refractories. This type consists mainly of silica in three crystalline forms cristobalite [1446446-1]> tridymite [1546-32-3]> and quartz [14808-60-7]. Quartzite sands and silica gravels are the main raw materials, although lime and iron oxides are added to increase the mineralization of the tridymite and cristobalite. Uses include roof linings, refractories for coke ovens, coreless induction foundry furnaces, and fused-silica technical ceramic products. Consumption of silica refractories has declined dramatically since the 1960s as a result of the changes in the steel industry. 

The principal health hazard that may be associated with silicon and silicon alloys is caused by the crystalline form of the oxide, ie, quartz, used as a raw material. Silica in its crystalline form is the chief cause of disabling pulmonary fibrosis, such as silicosis. Over a period of years, the breathing of air containing excessive amounts of crystalline silica can cause shortness of breath (30). 

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