Quartz, Si02 polymorphs

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

As previously described silica, which is an important catalyst support, is polymorphous (figure 3.36). Forms of silica other than quartz (Si02), can be stabilized chemically for use as catalyst support materials. Here we describe EM studies of the chemical stabilization of the cristobalite form of silica. It can be used as a stable catalyst support. 

When temperature is gradually increased, the reactions and transformations first take place inside the individual components. Kaolin (clay) is dehydrated producing transient products while separating Si02- Quartz undergoes polymorphic transformations. However, these processes are not vital to porcelain formation. The most significant processes will begin to take place at temperatures above 1100 °C, namely ... 

There is a third possible variation for acidic zeolite catalysts - the composition of the framework. The active Si-0(H)-A103 site in a high-sUica zeolite is formally created from a nanoporous Si02 polymorph by replacing Si by Al/H. If we consider an AIPO4 framework instead of an Si02 framework (as we do when we go from the mineral quartz to berlinite) and then replace P by Si/H (see formula above), we obtain a catalyst with the same Si-0(H)-A103 active site but a different framework composition. 

The actual values of lattice energies can also be calculated. Comparison of such values for pure silica polymorphs of known zeolite structure types or of aluminophosphates reveal clear trends when normalised to framework cation content and plotted against framework density (Figure 4.2). The framework stability of the observed structures with respect to dense phases such as quartz (Si02) or berlinite (AIPO4) decreases linearly as the framework density... 

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

Different modifications of a compound are frequently designated by lower case Greek letters a, j3,..., e.g. a-sulfur, j3-sulfur, or by roman numerals, e.g. tin-I, tin-II etc. Polymorphic forms of minerals have in many cases been given trivial names, like a-quartz, P-quartz, tridymite, cristobalite, coesite, keatite, and stishovite for Si02 forms. 

Polymorphism in Si02 leads to approximately 20 different forms of the compound. Among others, it occurs in quartz, tridymite, and cristobalite, each of which exists in a and (3 forms. The structure for C02 ... 

Fig. 26.7. Variation in silica concentration (top) and saturation indices (log Q/K) of the silica polymorphs (bottom) over the course of the reaction path shown in Figure 26.6. The dashed lines in the top diagram show Si02(aq) concentrations in equilibrium with quartz, cristobalite, and amorphous silica.

Si02 is one of the most abundant compounds in nature. It forms a number of minerals and several varieties whose names are quite familiar agate, car-nelian, sard, amethyst, chalcedony, flint, and chert. All are composed of Si02 with only small or trace amounts of other elements or compounds included during crystallization. In many cases it is the additional components that impart the peculiar color, optical, or physical properties to these minerals. The names are familiar because since ancient times these minerals were used or commonly set in jewelry. All of the names are varieties of the mineral quartz, the stable form of SiOj, formed at ordinary temperatures and pressures. Si02 also forms several polymorphs, which are mentioned below. 

As must be obvious, we have only touched on the natural occurrences of crystalline fibrous Si02- For example, Si02, usually quartz, often replaces fibrous minerals of another composition as rock masses are altered. Si02 also occurs in fibrous form within some plants (phytoliths) and in the soil. To summarize, in addition to the usual and common occurrences of the mineral quartz and its varieties, several silica polymorphs occur as fibers in a variety of biologic and geologic environments. 

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