silica

Silica, or silicon dioxide, is a chemical compound with the formula SiO2. It occurs widely in the Earth's crust as a constituent of many minerals, most notably quartz, cristobalite, and tridymite, and in amorphous forms such as fused silica and silica gel. It is a major component of sand, sandstone, and many rocks, and can form in hydrated forms such as opal.

Chemically, silica is relatively inert. In its crystalline forms it is hard and chemically resistant; in amorphous form it is less crystalline but still chemically inert. Quartz, the most common form, has a Mohs hardness of 7 and a melting point of about 1,713°C. Silica appears in nature as crystals and as granular, glassy material. It has a density around 2.65 g/cm3 and a visible-light refractive index near 1.46.

Industrial uses are diverse. The largest application is in glass production, but silica is also used in cement and ceramics, as a filler or reinforcing agent in plastics and rubber, and as a raw material for producing silicon metal. It is processed into silica gel for desiccants, fumed silica as a thickening agent, and diatomaceous earth for filtration. Silica glass and fused silica are valued in optics and high-temperature applications, and silica is a key component in fiber optics.

Health and safety considerations are significant. Inhalation of crystalline silica dust can cause silicosis and other lung diseases; regulatory exposure limits exist in many regions. Proper controls, such as ventilation and protective equipment, are important in workplaces. Silica is abundant in the environment, but mining and processing can impact landscapes and water quality.