Silicon dioxide

Silicon dioxide
Sample of silicon dioxide.jpg
IUPAC name
Silicon dioxide
Other names

Silicic oxide
Silicon(IV) oxide
Crystalline silica
Pure Silica

ECHA InfoCard100.028.678
EC Number231-545-4
E numberE551 (acidity regulators, ...)
RTECS numberVV7565000
Molar mass60.08 g/mol
AppearanceTransparent solid (Amorphous) White/Whitish Yellow (Powder/Sand)
Density2.648 (α-quartz), 2.196 (amorphous) g·cm−3[1]
Melting point 1,713 °C (3,115 °F; 1,986 K) (amorphous)[1](p4.88) to
Boiling point 2,950 °C (5,340 °F; 3,220 K)[1]
−29.6·10−6 cm3/mol
Thermal conductivity12 (|| c-axis), 6.8 (⊥ c-axis), 1.4 (am.) W/(m⋅K)[1](p12.213)
1.544 (o), 1.553 (e)[1](p4.143)
NFPA 704
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 20 mppcf (80 mg/m3/%SiO2) (amorphous)[2]
REL (Recommended)
TWA 6 mg/m3 (amorphous)[2]
Ca TWA 0.05 mg/m3[3]
IDLH (Immediate danger)
3000 mg/m3 (amorphous)[2]
Ca [25 mg/m3 (cristobalite, tridymite); 50 mg/m3 (quartz)][3]
Related compounds
Related diones
Carbon dioxide

Germanium dioxide
Tin dioxide
Lead dioxide

Related compounds
Silicon monoxide

Silicon sulfide

42 J·mol−1·K−1[4]
−911 kJ·mol−1[4]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Silicon dioxide, also known as silica, silicic acid or silicic acid anydride is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms.[5][6] In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.

Inhaling finely divided crystalline silica is toxic and can lead to severe inflammation of the lung tissue, silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis.

Uptake of amorphous silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal.[7]


Structural motif found in α-quartz, but also found in almost all forms of silicon dioxide
Relationship between refractive index and density for some SiO2 forms[8]

In the majority of silicates, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz polymorphs.

For example, in the unit cell of α-quartz, the central tetrahedron shares all four of its corner O atoms, the two face-centered tetrahedra share two of their corner O atoms, and the four edge-centered tetrahedra share just one of their O atoms with other SiO4 tetrahedra. This leaves a net average of 12 out of 24 total vertices for that portion of the seven SiO4 tetrahedra that are considered to be a part of the unit cell for silica (see 3-D Unit Cell).

SiO2 has a number of distinct crystalline forms (polymorphs) in addition to amorphous forms. With the exception of stishovite and fibrous silica, all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Silicon–oxygen bond lengths vary between the different crystal forms; for example in α-quartz the bond length is 161 pm, whereas in α-tridymite it is in the range 154–171 pm. The Si-O-Si angle also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz, the Si-O-Si angle is 144°.[9]

Fibrous silica has a structure similar to that of SiS2 with chains of edge-sharing SiO4 tetrahedra. Stishovite, the higher-pressure form, in contrast, has a rutile-like structure where silicon is 6-coordinate. The density of stishovite is 4.287 g/cm3, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648 g/cm3.[10] The difference in density can be ascribed to the increase in coordination as the six shortest Si-O bond lengths in stishovite (four Si-O bond lengths of 176 pm and two others of 181 pm) are greater than the Si-O bond length (161 pm) in α-quartz.[11] The change in the coordination increases the ionicity of the Si-O bond.[12] More importantly, any deviations from these standard parameters constitute microstructural differences or variations, which represent an approach to an amorphous, vitreous, or glassy solid.

The only stable form under normal conditions is alpha quartz, in which crystalline silicon dioxide is usually encountered. In nature, impurities in crystalline α-quartz can give rise to colors (see list). The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. Since the composition is identical, the reason for the discrepancies must be in the increased spacing in the high-temperature minerals. As is common with many substances, the higher the temperature, the farther apart the atoms are, due to the increased vibration energy.[citation needed]

The transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. 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. This is probably due to the intense compression of the atoms occurring during their formation, resulting in more condensed structure.[14]

Faujasite silica is another form of crystalline silica. It is obtained by 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 surface area (over 800 m2/g). Faujasite-silica has very high thermal and acid stability. For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid.[15]

Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water: negative temperature expansion, density maximum at temperatures ~5000 °C, and a heat capacity minimum.[16] Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C.[17]

Molecular SiO2 with a linear structure is produced when molecular silicon monoxide, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge. Dimeric silicon dioxide, (SiO2)2 has been prepared by reacting O2 with matrix isolated dimeric silicon monoxide, (Si2O2). In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si-O-Si angle of 94° and bond length of 164.6 pm and the terminal Si-O bond length is 150.2 pm. The Si-O bond length is 148.3 pm, which compares with the length of 161 pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol.[18]

Other Languages
aragonés: Silica
azərbaycanca: Lüssatit
Bân-lâm-gú: Jī-sng-hoà ke-sò͘
bosanski: Silicij-dioksid
Gaeilge: Silice
한국어: 이산화 규소
हिन्दी: सिलिका
Ido: Silico
Bahasa Indonesia: Silikon dioksida
italiano: Silice
македонски: Силициум диоксид
Bahasa Melayu: Silikon dioksida
မြန်မာဘာသာ: ကျောက်သလင်း
Nederlands: Siliciumdioxide
norsk nynorsk: Silika
Runa Simi: Ullaya
Simple English: Silicon dioxide
slovenčina: Oxid kremičitý
slovenščina: Silicijev dioksid
српски / srpski: Силицијум диоксид
srpskohrvatski / српскохрватски: Silicijum dioksid
svenska: Kiseldioxid
தமிழ்: சிலிக்கா
українська: Діоксид кремнію
Tiếng Việt: Silic điôxít
吴语: 二氧化硅
中文: 二氧化硅