SiO2

Silicon dioxide, SiO2, commonly called silica, is a widely occurring compound in the Earth's crust. It exists as crystalline minerals such as quartz, as well as in amorphous forms including fused silica and silica glass. Hydrated forms such as opal and silica gels are also encountered.

In its crystalline form, SiO2 comprises a three-dimensional network of SiO4 tetrahedra in which each silicon atom is covalently bonded to four oxygen atoms and each oxygen bridges between two silicon centers. This network confers high hardness, high melting temperature, and chemical inertness. Quartz has a hexagonal crystal structure; cristobalite and tridymite are high-temperature polymorphs. Amorphous silica lacks long-range order and exhibits a glassy structure.

Occurrence and natural form: Silica is the second most abundant oxide in the Earth's crust and is ubiquitous in sand and rocks. Quartz is the most common crystalline form; chert, flint, and opal represent other natural varieties.

Industrial production and processing: Natural silica is mined as quartz or sand. It is refined to high-purity fused silica by melting and processing into glassy materials. Silicon metal is produced by reducing silica with carbon at high temperature, a step used to obtain elemental silicon for semiconductors and other applications.

Applications: Silica is used in glass production (soda-lime and borosilicate glasses), glass fibers and optics, cement and refractories, ceramics, and as a desiccant (silica gel). It also serves as a filler and catalyst support in chemistry.

Safety and environment: Respirable crystalline silica dust poses serious health risks, including silicosis and other lung diseases. Exposure is regulated in many countries; controls include water suppression, ventilation, and personal protective equipment. Responsible mining and proper disposal are emphasized to mitigate environmental impact.