Silicon dioxide

The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula . Siliceous is an adjective meaning "referring to silica"

In nature

Silica is found in nature in several forms, including quartz and opal. In fact, 35 crystalline forms have been identified. ^[1]

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 which are extremely rare and may be found almost exclusively in the base and ejecta of large meteor impact craters such as Meteor Crater.

Silica structures can be synthesized for biological and nanotech applications in structures such as ormosil. Additionally, many forms of life also 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 also used in plaster, and 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.

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.
A raw material for many whiteware ceramics such as earthenware, stoneware and porcelain.
A raw material for the production 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)
A food additive, primarily as a flow agent in powdered foods, or to absorb water (see the ingredients list for. In addition, silica is also used as a cosmetic additive in loose powders such as Bare Essentuals.
The natural ("native") oxide coating that grows on silicon is hugely beneficial in microelectronics. It is a superior electric insulator, possessing high chemical stability. 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 (thermal oxidation).
Raw material for aerogel in the Stardust spacecraft
Used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
Added to medicinal anti-foaming agent, like Simethicone, with a small portion to enhance defoaming activity.

Health effects

Inhaling finely divided crystalline silica dust in significant quantities can lead to silicosis or (much more rarely) cancer, as the dust becomes lodged in the lungs and continuously irritates them (silica does not dissolve over time). This effect can be an occupational hazard for people working with sandblasting equipment, products that contain powdered silica, and so on. In all other respects, silicon dioxide is inert and harmless. When silica is ingested orally, it passes unchanged through the gastrointestinal tract, exiting in the feces, leaving no trace behind. Small pieces of silicon dioxide are equally harmless, as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining. Silicon dioxide produces no fumes and is insoluble in vivo. It is indigestible, with zero nutritional value and zero toxicity.

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 has covalent bonding and forms a network structure (also known as lattice or continuous).

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

External links

(Tridymite)
(Quartz)
(Cristobalite)
NIOSH Pocket Guide to Chemical Hazards (amorphous)
NIOSH Pocket Guide to Chemical Hazards (crystalline, as respirable dust)
Quartz SiO2 piezolelctric properties

Citations

[1] http://www.minsocam.org/MSA/collectors_corner/arc/silicanom.htm Nomenclature of Silica

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