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In
chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a with other . ...

chemistry
, a silicate is any member of a family of
anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
s consisting of
silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...

silicon
and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
, usually with the general formula , where . The family includes
orthosilicate In chemistry, orthosilicate is the anion An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and ...
(),
metasilicate Metasilicates are silicates containing ions of empirical formula . Common Stoichiometry, stoichiometries include MSiO3 and MIISiO3. Common compounds containing metasilicate anion are: * Metasilicic acid, Metasilicic acid (hydrogen metasilica ...
(), and pyrosilicate (, ). The name is also used for any
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mineral is known as rock salt or halite. Sa ...
of such anions, such as
sodium metasilicate Sodium metasilicate is the chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be ...
; or any
ester An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a and an . s are s of ; they are important in biology, being one of the main classe ...

ester
containing the corresponding
chemical group In organic chemistry, a functional group is a substituent A substituent is one or a group of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and ...
, such as
tetramethyl orthosilicate Tetramethyl orthosilicate is the chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space ...

tetramethyl orthosilicate
. Silicate anions are often large
polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, repeating subunits. Due to thei ...

polymer
ic molecules with an extense variety of structures, including chains and rings (as in polymeric
metasilicate Metasilicates are silicates containing ions of empirical formula . Common Stoichiometry, stoichiometries include MSiO3 and MIISiO3. Common compounds containing metasilicate anion are: * Metasilicic acid, Metasilicic acid (hydrogen metasilica ...
), double chains (as in , and sheets (as in . In
geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, and the processes by which they change over time. Geology can ...

geology
and
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
, the term silicate is used to mean
silicate minerals Silicate minerals are rock-forming s made up of groups. They are the largest and most important class of minerals and make up approximately 90 percent of . In , (silicon dioxide) is usually considered a silicate mineral. Silica is found in ...
,
ionic solids Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in ancient Greek and Latin poetry * Ionian mode, a musical mode or a diatonic scale Places and peoples * Ionian, of or from Ionia, an ancient region in western An ...
with silicate anions; as well as
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
types that consist predominantly of such minerals. In that context, the term also includes the non-
ionic Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in ancient Greek and Latin poetry * Ionian mode, a musical mode or a diatonic scale Places and peoples * Ionian, of or from Ionia, an ancient region in western An ...
compound
silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to oxygen. An oxide () is a chemical compound that con ...
(
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

silica
,
quartz Quartz is a hard, crystalline A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together an ...

quartz
), which would correspond to in the general formula. The term also includes minerals where
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
or other
tetravalent In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they und ...
atoms replace some of the silicon atoms, as in the
aluminosilicate Aluminosilicate mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is ...
s. Such silicates comprise most of
Earth's crust 350px, Plates in the crust of Earth Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the lithosphere, a division of Earth's layers that includes the Crust (geology), ...
and mantle, as well as the other
terrestrial planets A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Su ...
, rocky
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...

moons
, and
asteroids An asteroid is a minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a ...

asteroids
. For diverse manufacturing, technological, and artistic needs, silicates are versatile materials, both natural (such as
granite Granite () is a coarse-grained (phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Lat ...

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 classifie ...

gravel
, and
garnet Garnets () are a group of s that have been used since the as s and s. All species of garnets possess similar physical properties and crystal forms, but differ in . The different species are , , , (varieties of which are or cinnamon-stone a ...

garnet
) and artificial (such as
Portland cement Portland cement is the most common type of in general use around the world as a basic ingredient of , , , and non-specialty . It was developed from other types of in England in the early 19th century by , and is usually made from . It is a f ...

Portland cement
,
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide of rutile. Ti(IV) centers are grey; oxygen ce ...

ceramic
s,
glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by ...

glass
, and waterglass). 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 .


Structural principles


Modeling as corner-sharing tetrahedra

In most commonly encountered silicates, including almost all
silicate minerals Silicate minerals are rock-forming s made up of groups. They are the largest and most important class of minerals and make up approximately 90 percent of . In , (silicon dioxide) is usually considered a silicate mineral. Silica is found in ...
, each silicon atom occupies the center of an idealized
tetrahedron In , a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular , is a composed of four , six straight , and four . The tetrahedron is the simplest of all the ordinary and the only one that has fewer than 5 faces. The t ...

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 electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
s according to the
octet rule The octet rule is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultim ...
. This structure-bonding scenario does not describe the nature of silicate minerals under high pressures, which is the situation for most terrestrial silicate minerals. These tetrahedra may occur as isolated
orthosilicate In chemistry, orthosilicate is the anion An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is equal and ...
anions , but two or more silicon atoms can be joined with oxygen atoms in various ways, to form more complex anions, such as pyrosilicate or the
metasilicate Metasilicates are silicates containing ions of empirical formula . Common Stoichiometry, stoichiometries include MSiO3 and MIISiO3. Common compounds containing metasilicate anion are: * Metasilicic acid, Metasilicic acid (hydrogen metasilica ...
ring
hexamer In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
.
Polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

Polymer
ic silicate anions of arbitrarily large sizes can have chain, double chain, sheet, or three-dimensional structures. Typically, each oxygen atom that does not contribute a negative charge to the anion is a bridge between two silicon atoms. The structure of such anions is commonly described and depicted as consisting of silicon-centered tetrahedra connected by their vertices, in such a way that each vertex is shared by at most two tetrahedra.


Silicates with non-tetrahedral silicon

Although the tetrahedron is the common coordination geometry for silicon 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 Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard conditions for temperature and pressure, standard conditions as a highly toxic, pale yellow Diatomic molecule ...

fluorine
atoms in an
octahedral In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space ...

octahedral
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 hydrateCalcium 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. Preparation Calcium silicate hydrate (also shown as C-S-H) is a result of the reacti ...
s artificially formed in
cement A cement is a binder (material), binder, a substance used for construction that solidification, 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 (constru ...
and
concrete Concrete is a composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material Material is a substance Substance may refer to: * Substance (Jainism), a ter ...

concrete
submitted to a severe sulfate attack. At very high pressure, even SiO2 adopts the six-coordinated octahedral geometry in the mineral
stishovite Stishovite is an extremely hard, dense tetragonal form ( polymorph) of silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titani ...

stishovite
, a dense polymorph of
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

silica
found in the lower mantle of the Earth and also formed by shock during
meteorite A meteorite is a solid piece of debris from an object, such as a comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varie ...
impacts.


Chemical properties

Solid silicates are generally stable and well characterized. Silicates with
alkali In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...

alkali
cations and small or chain-like anions, such as and
metasilicate Metasilicates are silicates containing ions of empirical formula . Common Stoichiometry, stoichiometries include MSiO3 and MIISiO3. Common compounds containing metasilicate anion are: * Metasilicic acid, Metasilicic acid (hydrogen metasilica ...
, are fairly soluble in water. They form several solid
hydrate In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
s when crystallized from solution. Soluble
sodium silicate Sodium silicate is a generic name for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element ...

sodium silicate
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

Silicate anions are formally the conjugate bases of
silicic acid Silicon dioxide, also known as silica, is an oxide of rutile Rutile is a mineral composed primarily of titanium dioxide (TiO2), and is the most common natural form of TiO2. Other rarer polymorphs of TiO2 are known, including anatase, akaogii ...

silicic acid
s, i.e. compounds with the Si-O-H functional group. For example, orthosilicate can be viewed as the fourfold orthosilicic acid . Silicic acids are generally weak acids. They can be isolated. They exist in aqueous solution as mixtures of condensed and partially protonated anions, in a dynamic equilibrium. The general processes in this equilibrium are
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
/
condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...
: ≡Si–O–Si≡ + ≡Si–OH + HO–Si≡ and
protonation In chemistry, protonation (or hydronation) is the addition of a proton (or hydron, or hydrogen cation), (H+) to an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance ...
/deprotonation : ≡Si–OH ≡Si– + . The equilibrium can shifted towards larger anions by increasing the silicate concentration and/or the
acidity An acid is a molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In phys ...
of the medium. The orthosilicate anion, for example, is assumed to be the predominant form of
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

silica
naturally dissolved in
seawater Seawater, or salt water, is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for al ...

seawater
, whose concentration is below 100 parts per million; and also when silica is dissolved in an excess of
sodium oxide Sodium oxide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having vol ...
at pH 12 or more.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. In higher concentrations or low , polymeric anions predominate. In the limit, the silicate anions convert to silicic acids, which condense into a tridimensional network consisting of SiO4 tetrahedra interconnected by Si-O-Si linkages. A related condensation process is seen in sol-gel processing of tetraethylsilicate.


Detection

Silicate anions in solution react with
molybdate 150px, 3D model of the molybdate ion In chemistry a molybdate is a compound containing an oxoanionAn oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are ...

molybdate
anions yielding yellow silicomolybdate complexes. In a typical preparation,
monomer In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

monomer
ic orthosilicate was found to react completely in 75 seconds; dimeric pyrosilicate in 10 minutes; and higher
oligomer In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
s in considerably longer time. In particular, the reaction is not observed with suspensions of
colloidal silica Colloidal silicas are suspensions of fine amorphous In condensed matter physics and materials science The interdisciplinary field of materials science, also commonly termed materials science and engineering, covers the design and discovery ...
.


Zeolite formation

The nature of soluble silicates is relevant to understanding
biomineralization Biomineralization, or 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; all six taxono ...
and the synthesis of aluminosilicates, such as the industrially important
catalyst Catalysis () is the process of increasing the rate of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In cla ...

catalyst
s called
zeolite Zeolites are microporous, aluminosilicate Aluminosilicate mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Ear ...

zeolite
s.


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 Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...

Carbon cycle
* Carbonate-silicate cycle *
Ocean acidification Ocean acidification is the ongoing decrease in the pH value of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of cont ...


References

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