Silicas

Silicas, or silica materials, are compounds composed mainly of silicon and oxygen with the formula SiO2. They occur in natural minerals such as quartz, cristobalite, and tridymite, and in amorphous forms including glassy silica, opal, and silica gel. Silica is the most abundant mineral in Earth's crust.

Structure and forms: In crystalline silicas, silicon-oxygen tetrahedra are arranged in repeating three-dimensional lattices (quartz, cristobalite, tridymite). Amorphous silicas lack long-range order and include fused silica, silica gel, and opal. Natural and synthetic silicas differ in purity, porosity, and water content.

Occurrence and production: Silica occurs widely in sand and rocks. Commercial silica is obtained from quartz-rich ores, processed to produce fused silica, precipitated silica, fumed silica, and hydrated forms. Natural silica deposits include quartz veins and sandstone.

Uses: Silicas are core materials in glassmaking and ceramics; in cement and concrete as fillers and performance enhancers; as desiccants (silica gel); as reinforcing fillers in rubber and plastics; in catalysts supports; and in optical fibers and specialty coatings.

Properties and safety: Silica has high hardness (Mohs 7) and a high melting point; it is chemically inert and insoluble in water, but dissolves in hydrofluoric acid. Inhalation of crystalline silica dust is a health risk (silicosis and related diseases); appropriate controls are required in industry.