Introduction and Historic Overview
Quartz and the silicas are composed of silicon and oxygen, the two most ubiquitous elements in the earth’s crust  (Fig. 1). The widespread presence of the various forms of SiO2 in common geological materials is a manifestation of this fact. Along this line, many common geological silicates (SiO2-based materials such as rocks, clays, and sand) hold a detailed historical record of high-pressure and elevated t emperature conditions with significant implications in materials science, engineering, geology, planetary science, and physics . As a result, a discussion of pressure-related structure and properties will be included in this chapter.
Silica (SiO2) is the most important and versatile ceramic compound of MX2 stoichiometry. As noted above, it is widely available in raw materials in the earth’s surface, and silica is a fundamental constituent of a wide range of ceramic products and glasses;
J. F. Shackelford and R. H. Doremus (eds.), Ceramic and Glass Materials: 71
Structure, Properties and Processing.
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Fig. 1 The relative abundance of elements in the earth’s crust illustrates the common availability of quartz and the silicas 
its properties permit it to be used in high-temperature and corrosive environments and as abrasives, refractory materials, fillers in paints, and optical components.
Vitreous silica (high purity SiO2) is a technologically important amorphous material used in a myriad of applications including gas transport systems, laser optics, fiber optics, waveguides, electronics, vacuum systems, and furnace windows. During service, glass may experience elevated conditions of pressures and temperatures that can alter its properties. For instance, a vitreous silica lens may undergo drastic structural changes if pressure and temperature vary greatly in laser optics components. On the other hand, vitreous silica may undergo beneficial structural modifications under controlled conditions, e. g. during waveguide fabrication when femtosecond lasers are applied to induce a desired index of refraction in this glass .