Amorphous or noncrystalline silica is silicon dioxide (
SiO
2
) that does not have a crystalline structure. Amorphous silica can be naturally occurring or synthetic, and has a wide range of applications.
The chemical structure and chemical, physical, and particulate characteristics of amorphous silica are given. Polymerization processes and further modifications and important applications, including silica sols and colloidal silica, silica gel, precipitated silica, pyrogenic silica (fumed silica), and fused silica and fused quartz, are discussed. Formation, properties, processing, modification approaches, and applications of naturally occurring amorphous silica are presented as well. Finally, health effects and safety issues of amorphous silica materials are introduced, including exposure and uptake, biological effects, and classification and labeling according to international regulations.