silicon dioxide

silicon dioxide, commonly known as silica, is a chemical compound consisting of silicon and oxygen atoms. It is one of the most abundant minerals in the Earth's crust, found in numerous crystalline and amorphous forms. Its most prevalent natural form is quartz, a major component of sand and many types of rock. The material is fundamental to many industrial processes and modern technologies.

Properties

The physical properties vary significantly between its different structural forms. Crystalline polymorphs include quartz, tridymite, and cristobalite, each stable at different temperatures and pressures. The crystal structure of α-quartz, for instance, is trigonal and is piezoelectric, a property utilized in devices like the Quartz clock. Amorphous forms, such as fused quartz and silica gel, lack long-range order and exhibit different characteristics, including high transparency to ultraviolet light. It is generally hard, chemically inert, and has a high melting point, though these traits can be modified by impurities or structural defects. Its electrical properties range from being an excellent insulator to, in the form of very pure fused silica, having extremely low attenuation for optical signals.

Occurrence and production

Naturally, it is a primary constituent of many igneous, sedimentary, and metamorphic rocks, including granite and sandstone. Major commercial deposits are mined as industrial minerals, often from sources like the Badlands or the Sahara. Quartz sand is a key raw material, extracted through operations like those of Sibelco. Synthetic production is vital for high-purity applications; methods include vaporizing silicon tetrachloride in an oxyhydrogen flame or hydrolyzing tetraethyl orthosilicate. The Sol–gel process, pioneered by researchers like Samuel Kistler, is another important route to creating amorphous gels and glasses. The global market is influenced by companies such as Evonik Industries and Cabot Corporation.

Applications

Its uses are extraordinarily diverse, spanning ancient and modern industries. Historically, it was essential for making pottery and glass, with early examples found in Ancient Egypt. Today, high-purity forms are critical in the semiconductor industry, where they serve as an insulating layer on wafers in facilities operated by TSMC and Intel. Fused quartz is used for optics in telescopes like the Hubble Space Telescope and for crucibles in crystal growth. As a food additive (E551), it acts as an anti-caking agent. It is a reinforcing filler in products from Michelin tires to silicone rubber, and is the precursor for producing elemental silicon and silicon carbide.

Chemical reactions

While notably resistant to most acids, it is attacked by hydrofluoric acid, a reaction utilized in etching processes and glass frosting. At high temperatures, it reacts with basic metal oxides and carbonates to form silicates, fundamental reactions in glassmaking and metallurgy, such as in blast furnaces. It can be reduced to elemental silicon using strong reducing agents like carbon in an electric arc furnace, a process commercialized by companies like Wacker Chemie. In aqueous chemistry, it slowly hydrates to form silicic acid, and its surface chemistry is crucial for the function of chromatographic media like those from Waters Corporation.

Health and safety

Crystalline forms, particularly cristobalite, are classified as hazardous by agencies like the Occupational Safety and Health Administration due to their ability to cause silicosis, a serious lung disease historically associated with miners in Potosi. Prolonged inhalation of fine dust, a risk in industries like sandblasting or mining, is the primary exposure route. In contrast, amorphous forms are generally considered less pathogenic. Regulations, such as those enforced by the Health and Safety Executive in the United Kingdom, mandate exposure limits and protective measures like respirators. The International Agency for Research on Cancer lists crystalline silica as a Group 1 carcinogen.