It is found in nature in several forms, including quartz and opal. In fact, silica has 17 crystalline forms (see Nomenclature of Silica). Also, many forms of life include silica structures, including microorganisms such as diatoms, plants such as horsetail, and animals such as hexactinellid sponges.

It is manufactured in several forms including glass (in colorless high purity form called fused silica), synthetic amorphous silica and silica gel (used e.g. as desiccants in brand new clothes and leather goods). Silica, with alumina, is a crucial ingredient in clay and allows for the development of an interlocking crystal matrix after firing in earthenware, stoneware and porcelain ceramic processes. Silica is a major ingredient of Portland cement. The ceramic re-entry heat protection tiles mounted on the bottom side of the Space Shuttles are made mostly of silica, as are the firebricks used in steel processing.

The most common constituent of sand in inland continental settings and non-tropical coastal settings is silica, usually in the form of quartz because the considerable hardness of this mineral resists erosion. However, the composition of sand varies according to local rock sources and conditions.

Inhaling crystalline silica dust can lead to silicosis.

Variants found in high-pressure impacts are coesite and stishovite.

Silica is also used as a food additive, primarily as a flow agent in powdered foods, or to absorb water. See the ingredients list for Burger King.

The chemical stability of silicon dioxide and its electrical insulation properties are a major reason why silicon is the dominant material for semiconductor devices. It is used to separate the active regions of devices and to form insulating surfaces.

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Chemistry

Silicon dioxide can be formed when silicon is exposed to oxygen (or air) at extremely high temperatures. This can occasionally happen naturally in fires, or when lightning strikes onto sand.

Silicon dioxide is attacked by hydrofluoric acid (HF). HF is used to remove or pattern silicon dioxide in the semiconductor industry.

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