Silica is one of the most complex and abundant families of materials, existing as a compound of several minerals and as a synthetic product.

The starkly different structures of the dioxides of carbon and silicon are a manifestation of the double bond rule.

[10] Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous).

The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz.

Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit.

[13] The high-pressure minerals, seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz.

[17] Faujasite silica, another polymorph, is obtained by the dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment.

The resulting product contains over 99% silica, and has high crystallinity and specific surface area (over 800 m2/g).

For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid.

Plant materials with high silica phytolith content appear to be of importance to grazing animals, from chewing insects to ungulates.

[24][25] Silica is also the primary component of rice husk ash, which is used, for example, in filtration and as supplementary cementitious material (SCM) in cement and concrete manufacturing.

[26] Silicification in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates).

Crystalline silica is used in hydraulic fracturing of formations which contain tight oil and shale gas.

Both processes result in microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles, a white powder with extremely low bulk density (0.03-0.15 g/cm3) and thus high surface area.

Silica fume is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production.

[41] Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production.

Colloidal silica is used as a fining agent for wine, beer, and juice, with the E number reference E551.

It consists of the silica shells of microscopic diatoms; in a less processed form it was sold as tooth powder.

It is commonly used to manufacture metal–oxide–semiconductor field-effect transistors (MOSFETs) and silicon integrated circuit chips (with the planar process).

[52] Silica is used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.

[15] The idealized equation involving a trisilicate and sulfuric acid is: Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.

[58][59] The native oxide layer is beneficial in microelectronics, where it acts as electric insulator with high chemical stability.

The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:[63] Being highly stable, silicon dioxide arises from many methods.

This reaction is analogous to the combustion of methane: However the chemical vapor deposition of silicon dioxide onto crystal surface from silane had been used using nitrogen as a carrier gas at 200–500 °C.

While silicon dioxide is only poorly soluble in water at low or neutral pH (typically, 2 × 10−4 M for quartz up to 10−3 M for cryptocrystalline chalcedony), strong bases react with glass and easily dissolve it.

Silicon dioxide reacts in heated reflux under dinitrogen with ethylene glycol and an alkali metal base to produce highly reactive, pentacoordinate silicates which provide access to a wide variety of new silicon compounds.

[22] A 2008 study following subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia.

[71] When fine silica particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk of systemic autoimmune diseases such as lupus[72] and rheumatoid arthritis compared to expected rates in the general population.

Children, asthmatics of any age, those with allergies, and the elderly (all of whom have reduced lung capacity) can be affected in less time.

Inhalation of amorphous silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal.