The crystalline forms of silica

At atmospheric pressure silica exists in three crystalline forms, which are stable in the temperature ranges indicated below  

Plan of the structure of/ quartz. Small black circles represent Si atoms. The oxygen atoms lie at different heights above the plane of the paper, those nearest the reader being drawn with heaviest lines. Each atom is repeated at a certain distance above (and below) the plane of the paper along the normal to that plane so that the SiaOa rings in the plan represent helical chains. 

In Fig. 23.8 the structures of 3-tridymite and 3-cristobalite are shown with the 0 atoms midway between, and on the straight lines joining, the pairs of Si atoms. In fact, the 0 atoms lie off the lines Joining pairs of nearest Si atoms, giving 

The idealized structures of (a) p-tridymite, and (b) p-cristobalite (see text). Small 

Tridymite presents a more complex picture. The less dense structures of tridymite and cristobalite are presumably maintained at high temperatures by the thermal movements of the atoms, and it is possible that they are stabilized at lower temperatures by the inclusion of foreign atoms. 

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Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

According to Wyckoff [7], the crystalline forms of silica are the largest group of tetrahedral structures. Each of the three main polymorphs of silica formed at atmospheric pressure in nature (quartz, tridymite, and cristobalite) has a low and high temperature modification. The unit cell of low (a) quartz has three molecules and similar dimensions as the high ((3) quartz structure. The difference between low and high forms of quartz arises from small shifts of atom positions. Table 1 lists structural data for both quartz structures. 

Soluble silica does not cause silicosis. The crystalline form of silica, stishovite, dissolves in water to give a concentration of soluble silica much more concentrated than from quartz (309). If soluble silica is the active agent, then stishovite should be even more toxic than quartz. Instead, it is harmless. ... 

No better success has attended the efforts of Urry (17), or of Rayleigh (15) to measure the helium permeability of quartz, the densest of the crystalline forms of silica, and of beryl. The... 

Asbestos and silica are naturally occurring minerals that are mined in various forms for numerous commercial purposes (1,2). Asbestos ore breaks down into fiber forms with a crystalline backbone, while silica can be in crystalline or amorphous (noncrystal) particle form. Asbestos is divided mineralogically into the serpentine form known as chrysotile and the amphibole forms that include crocidolite and amosite. Chrysotile constitutes about 95% of the world s use, and the amphibole fibers, approximately the other 5% (1). All the asbestos varieties have been established as causative agents of all the asbestos-induced diseases (i.e., scarring, lung cancer, and mesothelioma), while it is only the crystalline form of silica that causes disease (1,2). 

The substitution of aluminum for silicon in a silica covalent network leads to a charge unbalance, which must be compensated by extra-framework cations, mostly alkaline. This occurs in the cases of the so-called stuffed silicas these materials have structures strictly related to the crystalline forms of silica, but with cations in the interstices to counterbalance the presence of A1 ions substituting for Si. This is the case, for example, of Eucriptite (LaAlSi04, a stuffed /3-quartz) or nepheline (NaAlSiOa, a stuffed tridymite). A similar mechanism also occurs in the amorphous networks of glasses [175]. 

Silica occurs in different crystal forms and impurity levels. Its low cost and widespread availability are offset by its density (2.65 g/cm ), hardness (7 on mho scale), and concern about toxicity of some but not all grades. Its wide geographic distribution makes it available in all parts of the world. So it is widely used. The crystalline form of silica causes silicosis or white lung when inhaled in excess as occurs in some... 

Schematic representation of cristobalite, one of the crystalline forms of silica, which is based on tetrahedral Si04 groups joined at the apices. Quartz, the more common form of Si02, has a similar, but more condensed, structure.

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. 

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. 

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... 

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

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 has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

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... 

Results obtained at high temperatures indicate that the solubilities of the crystalline modifications of silica are in the order tridymite > cristobalite > quartz, an order that parallels to some extent the chemical reactivity of these forms. Lower values for solubility of crystalline as compared to amorphous silica are consistent with the free-energy differences between them. 

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). 

Owing to this activation threshold, the first precipitation product from aqueous solutions of silicic acids will be an amorphous silica of some degree of hydration, while at room temperature the growth of vitreous and crystalline forms of silica from the precipitate (and thus the approach toward the absolute equilibrium) will proceed extremely slowly. With this understanding the data in Figure 1 are said to represent, an equilibrium—i.e., the reversible equilibrium between silicic acids in aqueous solution and metastable hydrated silica or polymeric silicic acid as precipitate. 

Quartz, a crystalline form of silica, is much less soluble than amorphous silica, as shown by the K value of its dissolution reaction ... 

Except for water, silica is the most extensively studied MX2 compound. One of the challenges in studying silica is its complex set of structures. Silica has several common polymorphs under different conditions of temperature [1] and pressure [4], as seen in Figs. 2 and 3. For instance, cristobalite is the crystalline silica polymorph at atmospheric pressure above 1,470°C. It is built on an fee lattice with 24 ions per unit cell. This structure is, in fact, the simplest form of silica. In addition to five polymorphs (quartz, coesite, stishovite, cristobalite, tridymite) that have thermodynamic stability fields, a large and increasing number of metastable polymorphs have been synthesized. These include vitreous silica, clathrasils, and zeolites [2], Except for stishovite, all these structures are based on frameworks of... 

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