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In chemistry, a silicate is any member of a family of
polyatomic anion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zero. The term molecule may or may not ...
s consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used for any salt of such anions, such as
sodium metasilicate Sodium metasilicate is the chemical substance with formula , which is the main component of commercial sodium silicate solutions. It is an ionic compound consisting of sodium cations and the polymeric metasilicate anions ��–sub>''n''. It is ...
; or any ester containing the corresponding
chemical group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...
, such as tetramethyl orthosilicate. The name "silicate" is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as hexafluorosilicate .Most commonly, silicates are encountered as silicate minerals. For diverse manufacturing, technological, and artistic needs, silicates are versatile materials, both natural (such as granite,
gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
, and garnet) and artificial (such as Portland cement, ceramics, glass, and waterglass).


Structural principles

In all silicates, silicon atom occupies the center of an idealized tetrahedron whose corners are four oxygen atoms, connected to it by single
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
s according to the octet rule. The oxygen atoms, which bears some negative charge, link to other cations (Mn+). This Si-O-M-O-Si linkage is strong and rigid, which properties are manifested in the rock-like silicates. The silicates can be classified according to the length and crosslinking of the silicate anions.


Isolated silicates

Isolated orthosilicate anions have the formula . A common mineral in this group is olivine (). Ttwo or more silicon atoms can share oxygen atoms in various ways, to form more complex anions, such as pyrosilicate .


Chains

With two shared oxides bound to each silicon, cyclic or polymeric structures can result. The cyclic metasilicate ring is a hexamer of SiO32-. Polymeric silicate anions of can exist also as long chains. In single-chain silicates, which are a type of inosilicate, tetrahedra link to form a chain by sharing two oxygen atoms each. A common mineral in this group is pyroxene. Double-chain silicates, the other category of inosilicates, occur when tetrahedra form a double chain (not always but mostly) by sharing two or three oxygen atoms each. Common minerals for this group are
amphiboles Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...
.


Sheets

In this group, known as phyllosilicates, tetrahedra all share three oxygen atoms each and in turn link to form two-dimensional sheets. This structure does lead to minerals in this group having one strong cleavage plane.
Micas Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
fall into this group. Both
muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage ...
and
biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...
have very weak layers that can be peeled off in sheets.


Framework

In a framework silicate, known as a tectosilicate, each tetrahedron shares all 4 oxygen atoms with its neighbours, forming a 3D structure. Quartz and feldspars are in this group.


Silicates with non-tetrahedral silicon

Although the tetrahedron is a common coordination geometry for silicon(IV) compounds, silicon may also occur with higher coordination numbers. For example, in the anion hexafluorosilicate , the silicon atom is surrounded by six
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
atoms in an
octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...
arrangement. This structure is also seen in the hexahydroxysilicate anion that occurs in thaumasite, a mineral found rarely in nature but sometimes observed among other
calcium silicate hydrate Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials (e.g. concrete). Preparation When water is added to cement, each of the compounds ...
s artificially formed in
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
and concrete submitted to a severe
sulfate attack Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C3S) (alite), and dicalcium silicate (C2S) (belite). Upon hydration, the main reaction products are calcium silicate hydrates (C-S-H) and calcium ...
. At very high pressure, such as exists in the majority of the earth's crust, even SiO2 adopts the six-coordinated octahedral geometry in the mineral stishovite, a dense polymorph of silica found in the
lower mantle The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2900 km below Earth's surface; between the transition zone and the outer core. The preliminary ...
of the Earth and also formed by shock during meteorite impacts.


Chemical properties

Silicates with
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of ...
cations and small or chain-like anions, such as sodium ortho- and metasilicate, are fairly soluble in water. They form several solid hydrates when crystallized from solution. Soluble
sodium silicate Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...
s and mixtures thereof, known as waterglass are in fact important industrial and household chemicals. Silicates of non-alkali cations, or with sheet and tridimensional polymeric anions, generally have negligible solubility in water at normal conditions.


Reactions

Silicates are generally inert chemically. Hence they are common minerals. Their resiliency also recommends their use as building materials. When treated with calcium oxides and water, silicate minerals form Portland cement. Equilibria involving hydrolysis of silicate minerals are difficult to study. The chief challenge is the very low solubility of SiO44- and its various protonated forms. Such equilibria are relevant to the processes occurring on geological time scales.G. B. Alexander (1953): "The Reaction of Low Molecular Weight Silicic Acids with Molybdic Acid". ''Journal of the American Chemical Society, volume 75, issue 22, pages 5655–5657. Some plants excrete ligands that dissolve silicates, a step in
biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; ...
.


Detection

Silicate anions in solution react with
molybdate In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, altho ...
anions yielding yellow silicomolybdate complexes. In a typical preparation, monomeric orthosilicate was found to react completely in 75 seconds;
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
ic pyrosilicate in 10 minutes; and higher oligomers in considerably longer time. In particular, the reaction is not observed with suspensions of
colloidal silica {{Unreferenced, date=November 2021Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase. It may be produced by Stöber process from Tetraethyl orthosilicate (TEOS). Properties Us ...
.


Zeolite formation

The nature of soluble silicates is relevant to understanding
biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. It is an extremely widespread phenomenon; ...
and the synthesis of aluminosilicates, such as the industrially important
catalyst Catalysis () is the process of increasing the reaction rate, rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the ...
s called zeolites.


See also

* * Alkali-silica reaction *
Carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major componen ...
* Carbonate-silicate cycle *
Ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxide ...


References

{{DEFAULTSORT:Silicon-oxygen tetrahedron Silicates