Silicon dioxide crystalline structure

Flint is a hard and easily split variety of the mineral quartz (composed of silicon dioxide), which occurs not only as flint but also in a wide range of other varieties. Some of these exhibit different colors and colored patterns and have characteristic crystalline structures, while others are amorphous (see Textbox 21). In all its varieties, nevertheless, the hardness of quartz is very high, being graded as 7 on the Mohs scale (see Textbox 23). Almost all varieties are either transparent or translucent and display a distinctive luster. These properties made quartz an attractive material for making ornamental... 

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... 

FIGURE 12.4 Structure of crystalline silicon dioxide (Si02) tetrahedra found in quartz (a) structural formula and (b) ball and stick model of one unit cell. 

Amorphous silica, ie, silicon dioxide [7631-86-9]9 Si02, does not have a crystalline structure as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quartz at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Fig. I. Silicon dioxide (Si02) iai crystalline, and (b) glassy state, iCourse structure is shown. Some authorities have recently suggested that, when studied at a much liner structure (such as by neutron scattering techniques), glass shows a much more orderly structure)...

Transmission electron microscopy (tern) is used to analyze the structure of crystals, such as distinguishing between amorphous silicon dioxide and crystalline quartz. The technique is based on the phenomenon that crystalline materials are ordered arrays that scatter waves coherently. A crystalline material diffracts a beam in such a way that discrete spots can be detected on a photographic plate, whereas an amorphous substrate produces diffuse rings. Tern is also used in an imaging mode to produce images of substrate grain structures. Tern requires samples that are very thin (10—50 nm) sections, and is a destructive as well as time-consuming method of analysis. 

In this chapter we have considered three-dimensional ceramic constructions and nature s three-dimensional crystalline structures and glasses. There are many similarities between the construction of a ceramic work of art and the construction of a crystalline solid or glass. In both cases, units are built into a structure. The units are bonded together in a variety of ways. Both can result in pleasing works for the viewer to enjoy. A quartz crystal, made of silicon dioxide (Si02), can be just as beautiful as a carefully crafted ceramic piece. Particular elements of design make these objects beautiful. When considering ceramic pieces and natural crystals, texture is an important element. 

In crystalline solids, the particles are in a highly ordered, repeating pattern. Substances that solidify slowly tend to form crystalline solids. The most important substance in glass is silicon dioxide. When solid glass melts and then cools, the molecular structure of the original solid is not reestablished. This can occur because liquid glass cools rapidly. 

The refractive index of a pigment at each wavelength is determined by its crystal structure. Titanium dioxide pigments (rutile and anatase) differ from each other and from other white pigments or crystalline substances, like silicon dioxide, in the proportion of radiant energy that is transmitted, absorbed, or re-... 

Dupree, R. (2000) in Structure and Impeifections in Amorphous and Crystalline Silicon Dioxide, eds Devine, R.A.B., Duraud, J.-P. Dooryhee, E., John Wiley Sons, Chichester, p. 107. 

Cross-sectional view of a three-dimensional map of dopant atoms (light blue spheres) implanted into a typical silicon transistor structure. Red dots represent the silicon atoms (only 2% are shown for clarity) and the gray spheres represent a native silicon dioxide layer located at the interface between the crystalline silicon substrate and layer of deposited polycrystalline silicon. 

Silica or silicon dioxide SiC>2, is a much more complex material than imagined from its simple stoichiometry. In nature, silica can be found in a wide range of both crystalline and amorphous structures depending on... 

Figure 12.33 Crystalline and amorphous silicon dioxide. A, The atomic arrangement of cristo-balite, one of the many crystalline forms of silica (SI02), shows the regularity of cubic closest packing. B, The atomic arrangement of a quartz glass is amorphous with a generally disordered structure.

Mercury(II) halides can be inserted into porosils (crystalline microporous silicon dioxide modifications) via the vapor phase [60-63]. The guest molecules HgX2 (X Cl, Br, I) are disordered within the voids of the host and the positions of their atoms cannot be determined by diffraction methods. EXAFS as a local spectroscopic tool is well suited to gather information about the local structure of the guest species. 

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