Silicon dioxide amorphous structure

FIGURE 12.3 Segment of amorphous silicon dioxide (Si02) structure. 

Not all amorphous solids are glasses. For example, liquid silicon dioxide is structurally identical to window glass the bonding mechanisms, short-range order, electrical properties, density, and so forth, are the same. Pure elemental silicon, on the other hand, has a liquid state that is very different from the amorphous solid state. The liquid is a metal, and correspondingly dense, whereas the amorphous solid has a much less dense, tetrahedrally bound structure akin to that of diamond and is not metallic. If silicon could be made to form a glass, it would be a dense metal, like the liquid. 

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... 

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. 

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. 

A source of error in chemical analyses of montmorillonites (and in other clays) that is not commonly checked is the presence of amorphous material, particularly Si and Al. Table XXXII lists structural formulas given by Osthaus (1955) for montmorillonites which were purified by size fraction and by extraction with 0.5 N NaOH to remove amorphous Si and Al. In six analyses dissolved silica ranged from 3.6 to 8.4% and alumina from 0.6 to 2.25%. Amorphous silicon dioxide should be expected in most montmorillonites derived from volcanic material. The source glass has more Si than is required for the 2 1 layer and the excess must be leached from the glass. Much of the Si is deposited in the sediments underlying the bentonite bed in the form of chert but it is to be expected that the extraction would not be complete and a portion of the colloidal Si would remain in the bentonite bed. 

An amorphous silicon monoxide could be formed as a structure between metallic silicon (oxidation state zero) and silicon dioxide (oxidation state four). Structures like those shown above seem to be possible. If we summarize all available facts about SiO in the solid state, the following picture is obtained ... 

Silica. Amorphous silicon dioxide (glass). It is a structural component in many organisms, such as diatoms and horsetails. 

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. 

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.

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