[edit] In nature

Silica is found in nature in several forms, including quartz and opal. In fact, it has 17 crystalline forms (see Nomenclature of Silica).

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.

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

Many forms of life contain silica structures (biogenic silica), including microorganisms such as diatoms, plants such as horsetail, and animals such as hexactinellid sponges. It is present in the cell walls of various plants (including edible ones) to strengthen their structural integrity. Silica is used in plaster, this can have health implications for plasterers' children when work clothes and children's clothes are washed together. Microscopic particles of glass are held in the clothing and rubbed against the skin causing abrasion scars that go undiagnosed at the doctors.

[edit] Applications

Silica is manufactured in several forms including:

glass (a colorless, high-purity form is called fused silica)
synthetic amorphous silica
silica gel (used e.g. as desiccants in new clothes and leather goods)

It is used in the production of various products.

Inexpensive soda-lime glass is the most common and typically found in drinking glasses, bottles, and windows.
Silica, along with alumina (silica-alumina), forms a major part of the crystal lattice of clay minerals. These decompose on firing and form part of the microstructure of clay based ceramics such as earthenware, stoneware and porcelain.
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 (see HRSI), as are the firebricks used in steel processing.
It is the substance upon which silica aerogels are based.
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 natural skin, or oxide coating, that grows on silicon is hugely beneficial in microelectronics. It is a superior insulator, possessing high chemical stability, and in electrical applications it can protect the silicon, store charge, block current, and even act as a controlled pathway to allow small currents to flow through a device. At room temperature, however, it grows extremely slowly, and so to manufacture such oxide layers on silicon, the traditional method has been the deliberate heating of silicon in high temperature furnaces within an oxygen ambient.
Silica is the central component in most glass optical fibers.
Silica in the form of Silicon Dioxide Ph. Eur. 6x. is also used as a homeopathic remedy to treat impure blood, brittle nails and lack lustre hair.
Silica is also used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
Silica is added to medicinal anti-foaming agent, like Simethicone, with a small portion to enhance defoaming activity.

[edit] Health effects

Inhaling crystalline silica dust can lead to silicosis or cancer.

[edit] Homeopathic use

Silicon dioxide (silica) is widely used as a homeopathic remedy to treat impure blood, boils, brittle nails and lack-lustre hair. Dr Samuel Hahnemann first introduced the homeopathic usage of silicon dioxide into medicine in the 19th century. Dr. Wilhelm Heinrich Schüßler (1821-1898) further developed its use through his biochemic method of healing.[1]

[edit] Chemistry

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 nanometer or 10 angstroms) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions. Higher temperatures and alternate environments are used to grow well-controlled layers of silicon dioxide on silicon.

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