Silicon dioxide
Encyclopedia
The chemical compound
silicon dioxide, also known as silica (from the Latin
silex
), is an oxide
of silicon
with the chemical formula
. It has been known for its hardness since antiquity. Silica is most commonly found in nature as sand
or quartz
, as well as in the cell walls of diatoms.
Silica is manufactured in several forms including fused quartz
, crystal
, fumed silica
(or pyrogenic silica, trademarked Aerosil or Cab-O-Sil), colloidal silica
, silica gel
, and aerogel
.
Silica is used primarily in the production of glass
for windows, drinking glasses, beverage bottles, and many other uses. The majority of optical fibers for telecommunications are also made from silica. It is a primary raw material for many whiteware ceramics such as earthenware
, stoneware
, porcelain
, as well as industrial Portland cement
.
Silica is a common additive in the production of foods, where it is used primarily as a flow agent in powdered foods, or to absorb water in hygroscopic applications. It is the primary component of diatomaceous earth
which has many uses ranging from filtration to insect control. It is also the primary component of rice husk ash which is used, for example, in filtration and cement manufacturing.
Thin films of silica grown on silicon wafers via thermal oxidation
methods can be quite beneficial in microelectronics
, where they act as electric insulators with high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.
A silica-based aerogel was used in the Stardust spacecraft
to collect extraterrestrial particles. 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
. As hydrophobic silica
it is used as a defoamer component. In hydrated form
, it is used in toothpaste
as a hard abrasive to remove tooth plaque.
In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric. In cosmetics, it is useful for its light-diffusing properties and natural absorbency. Colloid
al silica is used as a wine and juice fining agent. In pharmaceutical products, silica aids powder flow when tablets are formed. Finally, it is used as a thermal enhancement compound in ground source heat pump industry.
In the vast majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz crystalline form of silica SiO2. In each of the most thermodynamically stable crystalline forms of silica, on average, all 4 of the vertices (or oxygen atoms) of the SiO4 tetrahedra are shared with others, yielding the net chemical formula: SiO2.
For example, in the unit cell of α-quartz, the central tetrahedron shares all 4 of its corner O atoms, the 2 face-centered tetrahedra share 2 of their corner O atoms, and the 4 edge-centered terahedra 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 7 SiO4 tetrahedra which 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 (polymorph
s) 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°.
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. 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.
The change in the coordination increases the ionicity of the Si-O bond. But more important is the observation that any deviations from these standard parameters constitute microstructural differences or variations which represent an approach to an amorphous
, vitreous or glassy solid.
Note that the only stable form under normal conditions is α-quartz and this is the form in which crystalline silicon dioxide is usually encountered. In nature impurities in crystalline α-quartz can give rise to colors (see list).
Note also that both high temperature minerals, cristobalite and tridymite, have both a lower density and index 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 due to the increased vibration energy.
The high pressure minerals, seifertite
, stishovite
, and coesite
, on the other hand, have a higher density and index of refraction when compared to quartz. This is probably due to the intense compression of the atoms that must occur during their formation, resulting in a more condensed structure.
Faujasite silica is another form of crystalline silica. It is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with a combined acid and thermal treatment. The resulting product contains over 99% silica, has high crystallinity and high 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.
Molten silica exhibits several peculiar physical characteristics that are similar to the ones observed in liquid water
: negative temperature expansion, density maximum, and a heat capacity minimum. When molecular silicon monoxide
, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge, molecular SiO2 is produced which has a linear structure. 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.
temperature of pure SiO2 is about 1600 K (1330 °C or 2420 °F). Like most of the crystalline polymorph
s the local atomic structure in pure silica glass is regular tetrahedra of oxygen atoms around silicon atoms. The difference between the glass and the crystals arises in the connectivity of these tetrahedral units. SiO2 glass consists of a non-repeating network of tetrahedra, where all the oxygen corners connect two neighbouring tetrahedra. Although there is no long range periodicity in the glassy network there remains significant ordering at length scales well beyond the SiO bond length. One example of this ordering is found in the preference of the network to form rings of 6-tetrahedra.
Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very shallow layer (approximately 1 nm
or 10 Å
) of so-called native oxide is formed on the surface when silicon is exposed to air under ambient conditions. Higher temperatures and alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry or wet oxidation with O2
or H2O, respectively. The depth of the layer of silicon replaced by the dioxide is 44% of the depth of the silicon dioxide layer produced.
Alternative methods used to deposit a layer of SiO2 include
Pyrogenic silica (sometimes called fumed silica or silica fume), which is a very fine particulate form of silicon dioxide, is prepared by burning SiCl4
in an oxygen rich hydrocarbon flame to produce a "smoke" of SiO2:
Amorphous silica, silica gel, is produced by the acidification of solutions of sodium silicate
to produce a gelatinous precipitate that is then washed and then dehydrated to produce colorless microporous silica.
Quartz exhibits a maximum solubility in water at temperatures about 340 °C. This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel which is cooler at the top. Crystals of 0.5–1 kg can be grown over a period of 1–2 months. These crystals are a source of very pure quartz for use in electronic applications.
Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) react much less readily.
Silicon dioxide is attacked by hydrofluoric acid
(HF) to produce hexafluorosilicic acid:
HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide:
Silicon dioxide reacts with basic metal oxides (e.g. sodium oxide
, potassium oxide
, lead(II) oxide
, zinc oxide
, or mixtures of oxides forming silicate
s and glasses as the Si-O-Si bonds in silica are broken successively). As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:
Na2O + SiO2 → glass.
Examples of such glasses have commercial significance e.g. soda lime glass, borosilicate glass
, lead glass
. In these glasses, silica is termed the network former or lattice former.
With silicon at high temperatures gaseous SiO is produced:
Prominent examples include:
Crystalline minerals formed in the physiological environment often show exceptional physical properties (e.g. strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order over a range of scales. The minerals are crystallized from an environment that is undersaturated with respect to silicon, and under conditions of neutral pH and low temperature (0–40 °C).
Formation of the mineral may occur either within the cell wall of an organism (such as with phytoliths), or outside the cell wall, as typically happens with tests. Specific biochemical reactions exist for mineral deposition. Such reactions include those that involve lipids, proteins, and carbohydrates.
It is unclear in what ways silica is important in the nutrition of metazoa. This is a challenging field of research, because silica is ubiquitous in the environment and in most circumstances dissolves in trace quantities only. All the same it certainly does occur in the living body, leaving us with the problem that it is hard to create proper silica-free controls for purposes of research. This makes it difficult to be sure when the silica present has had operative beneficial effects, and when its presence is coincidental, or even harmful. The current consensus is that it certainly seems important in the growth, strength, and management of many connective tissues. This is true not only for hard connective tissues such as bone and tooth.
Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1 mg/m3) over time can lead to silicosis
, bronchitis
, or cancer
, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities. (In the body crystalline silica particles do not dissolve over clinically relevant periods of time.) This effect can create an occupational hazard for people working with sandblasting equipment, products that contain powdered crystalline silica and so on. Children, asthmatics of any age, allergy sufferers, and the elderly (all of whom have reduced lung capacity) can be affected in much less time. Amorphous silica, such as fumed silica is not associated with development of silicosis, but may cause irreversible lung damage in some cases. Laws restricting silica exposure with respect to the silicosis hazard specify that they are concerned only with silica that is both crystalline and dust-forming.
Note however, that plant materials with high silica phytolith content, appear to be of importance to grazing animals, from chewing insects to ungulates. It is known that it accelerates tooth wear at least, and has been doing so for hundreds of millions of years.
A study which followed subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia
. The study found that with an increase of 10 milligram-per-day of the intake of silica in drinking water, the risk of dementia
dropped by 11%.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
silicon dioxide, also known as silica (from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
silex
Silex
Silex is any of various forms of ground stone. In modern contexts the word refers to a finely ground, nearly pure form of silica or silicate....
), is an oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
. It has been known for its hardness since antiquity. Silica is most commonly found in nature as sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
or quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, as well as in the cell walls of diatoms.
Silica is manufactured in several forms including fused quartz
Fused quartz
Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...
, crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
, fumed silica
Fumed silica
Fumed silica, also known as pyrogenic silica because it is produced in a flame, consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. The resulting powder has an extremely low bulk...
(or pyrogenic silica, trademarked Aerosil or Cab-O-Sil), colloidal silica
Colloidal silica
Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase.-Properties:Usually they are suspended in an aqueous phase that is stabilized electrostatically...
, silica gel
Silica gel
Silica gel is a granular, vitreous, porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid. It is a naturally occurring mineral that is purified and processed into either granular or beaded form...
, and aerogel
Aerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...
.
Silica is used primarily in the production of glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
for windows, drinking glasses, beverage bottles, and many other uses. The majority of optical fibers for telecommunications are also made from silica. It is a primary raw material for many whiteware ceramics such as earthenware
Earthenware
Earthenware is a common ceramic material, which is used extensively for pottery tableware and decorative objects.-Types of earthenware:Although body formulations vary between countries and even between individual makers, a generic composition is 25% ball clay, 28% kaolin, 32% quartz, and 15%...
, stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...
, porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
, as well as industrial Portland cement
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
.
Silica is a common additive in the production of foods, where it is used primarily as a flow agent in powdered foods, or to absorb water in hygroscopic applications. It is the primary component of diatomaceous earth
Diatomaceous earth
Diatomaceous earth also known as diatomite or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to...
which has many uses ranging from filtration to insect control. It is also the primary component of rice husk ash which is used, for example, in filtration and cement manufacturing.
Thin films of silica grown on silicon wafers via thermal oxidation
Thermal oxidation
In microfabrication, thermal oxidation is a way to produce a thin layer of oxide on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rate of oxide growth is often predicted by the Deal-Grove model...
methods can be quite beneficial in microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
, where they act as electric insulators with high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.
A silica-based aerogel was used in the Stardust spacecraft
Stardust (spacecraft)
Stardust is a 300-kilogram robotic space probe launched by NASA on February 7, 1999 to study the asteroid 5535 Annefrank and collect samples from the coma of comet Wild 2. The primary mission was completed January 15, 2006, when the sample return capsule returned to Earth...
to collect extraterrestrial particles. Silica is also used in the extraction of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
due to its ability to bind to the nucleic acids under the presence of chaotropes
Chaotropic agent
A denaturating agent is a substance which disrupts the three dimensional structure in macromolecules such as proteins, DNA, or RNA and denatures them. A denaturating agent is a chaotropic agent, but chaotropic agents aren't necessarily denaturating agents...
. As hydrophobic silica
Hydrophobic silica
Hydrophobic silica is a silica that has hydrophobic groups chemically bonded to the surface. Hydrophobic silica can be made both from fumed and precipitated silica. The hydrophobic groups are normally alkyl or polydimethylsiloxane chains.-Manufacturing:...
it is used as a defoamer component. In hydrated form
Hydrated silica
Hydrated silica is a form of silicon dioxide, which has a variable amount of water in the formula. It is also known as silicic acid, a term usually used for its form dissolved in water. It is found in nature, as opal, which has been mined as a gemstone for centuries and in the cell walls of...
, it is used in toothpaste
Toothpaste
Toothpaste is a paste or gel dentifrice used with a toothbrush as an accessory to clean and maintain the aesthetics and health of teeth. Toothpaste is used to promote oral hygiene: it serves as an abrasive that aids in removing the dental plaque and food from the teeth, assists in suppressing...
as a hard abrasive to remove tooth plaque.
In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric. In cosmetics, it is useful for its light-diffusing properties and natural absorbency. Colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
al silica is used as a wine and juice fining agent. In pharmaceutical products, silica aids powder flow when tablets are formed. Finally, it is used as a thermal enhancement compound in ground source heat pump industry.
Crystal structure
SiO2 has a number of distinct crystalline forms (polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
s) in addition to amorphous forms. With the exception of stishovite
Stishovite
Stishovite is an extremely hard, dense tetragonal form of silicon dioxide. It was long considered the hardest known oxide; however, boron suboxide has recently been discovered to be much harder...
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°.
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
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
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. 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.
The change in the coordination increases the ionicity of the Si-O bond. But more important is the observation that any deviations from these standard parameters constitute microstructural differences or variations which represent an approach to an amorphous
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....
, vitreous or glassy solid.
Note that the only stable form under normal conditions is α-quartz and this is the form in which crystalline silicon dioxide is usually encountered. In nature impurities in crystalline α-quartz can give rise to colors (see list).
Note also that both high temperature minerals, cristobalite and tridymite, have both a lower density and index 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 due to the increased vibration energy.
The high pressure minerals, seifertite
Seifertite
Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of quartz. It has only been found in Martian and lunar meteorites where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the...
, stishovite
Stishovite
Stishovite is an extremely hard, dense tetragonal form of silicon dioxide. It was long considered the hardest known oxide; however, boron suboxide has recently been discovered to be much harder...
, and coesite
Coesite
Coesite[p] is a form of silicon dioxide SiO2 that is formed when very high pressure , and moderately high temperature , are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953. In 1960, coesite was found by Edward C. T...
, on the other hand, have a higher density and index of refraction when compared to quartz. This is probably due to the intense compression of the atoms that must occur during their formation, resulting in a more condensed structure.
Faujasite silica is another form of crystalline silica. It is obtained by dealumination of a low-sodium, ultra-stable Y zeolite with a combined acid and thermal treatment. The resulting product contains over 99% silica, has high crystallinity and high 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.
Molten silica exhibits several peculiar physical characteristics that are similar to the ones observed in liquid water
Water (properties)
Water is the most abundant compound on Earth's surface, covering about 70%. In nature, it exists in liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at standard temperature and pressure. At room temperature, it is a tasteless and odorless liquid,...
: negative temperature expansion, density maximum, and a heat capacity minimum. When molecular silicon monoxide
Silicon monoxide
Silicon monoxide is the chemical compound with the formula SiO. In the vapour phase it is a diatomic molecule. It has been detected in stellar objects and it has been described as the most common oxide of silicon in the universe....
, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge, molecular SiO2 is produced which has a linear structure. 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.
| Form | Crystal symmetry Pearson symbol Pearson symbol The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8... , group No. |
Notes | Structure |
|---|---|---|---|
| α-quartz | rhombohedral (trigonal) hP9, P3121 No.152 |
Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K | |
| β-quartz | hexagonal Hexagonal crystal system In crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families... hP18, P6222, No.180 |
closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active; β-quartz converts to β-tridymite at 1140 K | |
| α-tridymite Tridymite Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo,... |
orthorhombic oS24, C2221, No.20 |
metastable form under normal pressure | |
| β-tridymite | hexagonal hP12, P63/mmc, No. 194 |
closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K | |
| α-cristobalite Cristobalite The mineral cristobalite is a high-temperature polymorph of silica, meaning that it has the same chemical formula, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group which also include coesite, tridymite and stishovite... |
tetragonal tP12, P41212, No. 92 |
metastable form under normal pressure | |
| β-cristobalite | [Cubic crystal system|cubic]] cF104, Fd3m, No.227 |
closely related to α-cristobalite; melts at 1978 K | |
| faujasite Faujasite Faujasite is a mineral group in the zeolite family of silicate minerals. The group consists of faujasite-Na, faujasite-Mg and faujasite-Ca. They all share the same basic formula: 3.5[Al7Si17O48]·32 by varying the amounts of sodium, magnesium and calcium... |
ubic cF576, Fd3m, No.227 |
sodalite Sodalite Sodalite is a rich royal blue mineral widely enjoyed as an ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent... cages connected by hexagonal prisms; 12-membered ring pore opening; faujasite structure. |
|
| melanophlogite Melanophlogite Melanophlogite is a rare silicate mineral and a polymorph of silica . It has a zeolite-like porous structure which results in relatively low and not well-defined values of its density and refractive index. Melanophlogite often overgrows crystals of sulfur or calcite and typically contains a few... |
cubic (cP*, P4232, No.208) or tetragonal (P42/nbc) | Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces-a clathrasil | |
| keatite | tetragonal tP36, P41212, No. 92 |
Si5O10, Si4O14, Si8O16 rings; synthesised from glassy silica and alkali at 600–900K and 40–400 MPa | |
| moganite Moganite Moganite is a silicate mineral with the chemical formula SiO2 that was discovered in 1984. It crystallises in the monoclinic crystal system... |
monoclinic mS46, C2/c, No.15 |
Si4O8 and Si6O12 rings | |
| coesite Coesite Coesite[p] is a form of silicon dioxide SiO2 that is formed when very high pressure , and moderately high temperature , are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953. In 1960, coesite was found by Edward C. T... |
monoclinic mS48, C2/c, No.15 |
Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa | |
| stishovite Stishovite Stishovite is an extremely hard, dense tetragonal form of silicon dioxide. It was long considered the hardest known oxide; however, boron suboxide has recently been discovered to be much harder... |
Tetragonal tP6, P42/mnm, No.136 |
One of the densest (together with seifertite) polymorphs of silica; rutile Rutile Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:... -like with 6-fold coordinated Si; 7.5–8.5 GPa |
|
| fibrous | orthorhombic oI12, Ibam, No.72 |
like SiS2 consisting of edge sharing chains | |
| seifertite Seifertite Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of quartz. It has only been found in Martian and lunar meteorites where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the... |
orthorhombic oP, Pbcn |
One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa. |
Quartz glass
When silicon dioxide SiO2 is cooled rapidly enough, it does not crystallize but solidifies as a glass. The glass transitionGlass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...
temperature of pure SiO2 is about 1600 K (1330 °C or 2420 °F). Like most of the crystalline polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
s the local atomic structure in pure silica glass is regular tetrahedra of oxygen atoms around silicon atoms. The difference between the glass and the crystals arises in the connectivity of these tetrahedral units. SiO2 glass consists of a non-repeating network of tetrahedra, where all the oxygen corners connect two neighbouring tetrahedra. Although there is no long range periodicity in the glassy network there remains significant ordering at length scales well beyond the SiO bond length. One example of this ordering is found in the preference of the network to form rings of 6-tetrahedra.
Chemistry
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
or 10 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
) of so-called native oxide is formed on the surface when silicon is exposed to air under ambient conditions. Higher temperatures and alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry or wet oxidation with O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
or H2O, respectively. The depth of the layer of silicon replaced by the dioxide is 44% of the depth of the silicon dioxide layer produced.
Alternative methods used to deposit a layer of SiO2 include
- Low temperature oxidation (400–450 °C) of silaneSilaneSilane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
-
- SiH4 + 2 O2 → SiO2 + 2 H2O.
- Decomposition of tetraethyl orthosilicateTetraethyl orthosilicateTetraethyl orthosilicate is the chemical compound with the formula Si4. Often abbreviated TEOS, this molecule consists of four ethyl groups attached to SiO44- ion, which is called orthosilicate. As an ion in solution, orthosilicate does not exist. Alternatively TEOS can be considered to be the...
(TEOS) at 680–730 °C
-
- Si(OC2H5)4 → SiO2 + 2 H2O + 4 C2H4.
- Plasma enhanced chemical vapor deposition using TEOS at about 400 °C
-
- Si(OC2H5)4 + 12 O2 → SiO2 + 10 H2O + 8 .
- Polymerization of tetraethyl orthosilicate (TEOS) at below 100 °C using amino acid as catalyst.
Pyrogenic silica (sometimes called fumed silica or silica fume), which is a very fine particulate form of silicon dioxide, is prepared by burning SiCl4
Silicon tetrachloride
Silicon tetrachloride is the inorganic compound with the formula SiCl4. It is a colourless volatile liquid that fumes in air. It is used to produce high purity silicon and silica for commercial applications.-Preparation:...
in an oxygen rich hydrocarbon flame to produce a "smoke" of SiO2:
- SiCl4 + 2 H2 + O2 → SiO2 + 4 HCl.
Amorphous silica, silica gel, is produced by the acidification of solutions of sodium silicate
Sodium silicate
Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...
to produce a gelatinous precipitate that is then washed and then dehydrated to produce colorless microporous silica.
Quartz exhibits a maximum solubility in water at temperatures about 340 °C. This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel which is cooler at the top. Crystals of 0.5–1 kg can be grown over a period of 1–2 months. These crystals are a source of very pure quartz for use in electronic applications.
Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) react much less readily.
Silicon dioxide is attacked by hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
(HF) to produce hexafluorosilicic acid:
- SiO2 + 6 HF → H2SiF6 + 2 H2O.
HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide:
- SiO2 + 2 NaOH → Na2SiO3 + H2O.
Silicon dioxide reacts with basic metal oxides (e.g. sodium oxide
Sodium oxide
Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
, potassium oxide
Potassium oxide
Potassium oxide is an ionic compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive compound...
, lead(II) oxide
Lead(II) oxide
Lead oxide is the inorganic compound with the formula PbO. Lead oxide occurs in two polymorphs, red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure...
, zinc oxide
Zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
, or mixtures of oxides forming silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s and glasses as the Si-O-Si bonds in silica are broken successively). As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:
- 2 Na2O + SiO2 → Na4SiO4;
- Na2O + SiO2 → Na2SiO3;
Na2O + SiO2 → glass.
Examples of such glasses have commercial significance e.g. soda lime glass, borosilicate glass
Borosilicate glass
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
, lead glass
Lead glass
Lead glass is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40 weight% lead oxide , while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO...
. In these glasses, silica is termed the network former or lattice former.
- SiO2 + Si → 2 SiO (gas).
Biomaterials
Silicification in and by cells has been common in the biological world for well over a billion years. In the modern world it occurs in bacteria, single-celled organisms, plants, and animals (invertebrates and vertebrates).Prominent examples include:
- Tests or frustules of Diatoms and Radiolaria.
- Silica Phytoliths in the cells of many plants, including EquisetaceaeEquisetaceaeEquisetaceae, sometimes called the horsetail family, is the only extant family of the class Equisetales, with one surviving genus, Equisetum, which comprises about twenty species.- Evolution and systematics :...
, practically all grasses, and a wide range of dicotyledons. - The spicules which form the skeleton of many sponges.
Crystalline minerals formed in the physiological environment often show exceptional physical properties (e.g. strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order over a range of scales. The minerals are crystallized from an environment that is undersaturated with respect to silicon, and under conditions of neutral pH and low temperature (0–40 °C).
Formation of the mineral may occur either within the cell wall of an organism (such as with phytoliths), or outside the cell wall, as typically happens with tests. Specific biochemical reactions exist for mineral deposition. Such reactions include those that involve lipids, proteins, and carbohydrates.
It is unclear in what ways silica is important in the nutrition of metazoa. This is a challenging field of research, because silica is ubiquitous in the environment and in most circumstances dissolves in trace quantities only. All the same it certainly does occur in the living body, leaving us with the problem that it is hard to create proper silica-free controls for purposes of research. This makes it difficult to be sure when the silica present has had operative beneficial effects, and when its presence is coincidental, or even harmful. The current consensus is that it certainly seems important in the growth, strength, and management of many connective tissues. This is true not only for hard connective tissues such as bone and tooth.
Health effects
Silicosis
Silicosis, also known as Potter's rot, is a form of occupational lung disease caused by inhalation of crystalline silica dust, and is marked by inflammation and scarring in forms of nodular lesions in the upper lobes of the lungs...
, bronchitis
Bronchitis
Acute bronchitis is an inflammation of the large bronchi in the lungs that is usually caused by viruses or bacteria and may last several days or weeks. Characteristic symptoms include cough, sputum production, and shortness of breath and wheezing related to the obstruction of the inflamed airways...
, or cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities. (In the body crystalline silica particles do not dissolve over clinically relevant periods of time.) This effect can create an occupational hazard for people working with sandblasting equipment, products that contain powdered crystalline silica and so on. Children, asthmatics of any age, allergy sufferers, and the elderly (all of whom have reduced lung capacity) can be affected in much less time. Amorphous silica, such as fumed silica is not associated with development of silicosis, but may cause irreversible lung damage in some cases. Laws restricting silica exposure with respect to the silicosis hazard specify that they are concerned only with silica that is both crystalline and dust-forming.
Note however, that plant materials with high silica phytolith content, appear to be of importance to grazing animals, from chewing insects to ungulates. It is known that it accelerates tooth wear at least, and has been doing so for hundreds of millions of years.
A study which followed subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia
Dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...
. The study found that with an increase of 10 milligram-per-day of the intake of silica in drinking water, the risk of dementia
Dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...
dropped by 11%.
See also
- Fused silica
- Mesoporous silicaMesoporous silicaMesoporous silica is a form of silica and a recent development in nanotechnology. The most common types of mesoporous nanoparticles are MCM-41 and SBA-15. Research continues on the particles, which have applications in catalysis, drug delivery and imaging....
- Thermal oxidationThermal oxidationIn microfabrication, thermal oxidation is a way to produce a thin layer of oxide on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rate of oxide growth is often predicted by the Deal-Grove model...
- Silicon carbideSilicon carbideSilicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
External links
- Tridymite,
- Quartz,
- Cristobalite,
- amorphous, NIOSH Pocket Guide to Chemical Hazards
- crystalline, as respirable dust, NIOSH Pocket Guide to Chemical Hazards
- Formation of silicon oxide layers in the semiconductor industry. LPCVD and PECVD method in comparison. Stress prevention.
- Quartz SiO2 piezoelectric properties
- Silica (SiO2) and Water