Asilicate is any member of a family ofpolyatomic anions consisting ofsilicon andoxygen, usually with the general formula[SiO(4−2x)−
4−x]
n, where0 ≤x < 2. The family includesorthosilicateSiO4−4 (x = 0),metasilicateSiO2−3 (x = 1), andpyrosilicateSi2O6−7 (x = 0.5,n = 2). The name is also used for anysalt of such anions, such assodium metasilicate; or anyester containing the correspondingchemical group, such astetramethyl orthosilicate.[1] 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 ashexafluorosilicate[SiF6]2−. Most commonly, silicates are encountered assilicate minerals.
For diverse manufacturing, technological, and artistic needs, silicates are versatile materials, both natural (such asgranite,gravel, andgarnet) and artificial (such asPortland cement,ceramics,glass, andwaterglass).
In most silicates, a silicon atom occupies the center of an idealizedtetrahedron whose corners are four oxygen atoms, connected to it by singlecovalent bonds according to theoctet rule.[1] The oxygen atoms, which bear 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.[2]: 435 The silicates can be classified according to the length and crosslinking of the silicate anions.
Isolatedorthosilicate anions have the formulaSiO4−
4. A common mineral in this group isolivine ((Mg,Fe)2SiO4).
Two or more silicon atoms can share oxygen atoms in various ways, to form more complex anions, such aspyrosilicateSi
2O6−
7.
With two shared oxides bound to each silicon, cyclic or polymeric structures can result. The cyclicmetasilicate ringSi
6O12−
18 is ahexamer of SiO32-.Polymeric silicate anions of can exist also as long chains.
In single-chain silicates, which are a type ofinosilicate, tetrahedra link to form a chain by sharing two oxygen atoms each. A common mineral in this group ispyroxene.
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 areamphiboles.
In this group, known asphyllosilicates, 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 fall into this group. Bothmuscovite andbiotite have very weak layers that can be peeled off in sheets.
In a framework silicate, known as atectosilicate, each tetrahedron shares all 4 oxygen atoms with its neighbours, forming a 3D structure.Quartz andfeldspars are in this group.
Although the tetrahedron is a common coordination geometry for silicon(IV) compounds, silicon may also occur with higher coordination numbers. For example, in the anionhexafluorosilicateSiF2−
6, the silicon atom is surrounded by sixfluorine atoms in anoctahedral arrangement. This structure is also seen in the hexahydroxysilicate anionSi(OH)2−
6 that occurs inthaumasite, a mineral found rarely in nature but sometimes observed among othercalcium silicate hydrates artificially formed incement andconcrete structures submitted to a severesulfate attack inargillaceous grounds containingoxidizedpyrite.[3][4][5][6][7]
At very high pressure, such as exists in the majority of the Earth's rock, even SiO2 adopts the six-coordinated octahedral geometry in the mineralstishovite, a dense polymorph ofsilica found in thelower mantle of the Earth and also formed by shock duringmeteorite impacts.[8]
Silicates withalkali cations and small or chain-like anions, such assodium ortho- andmetasilicate, are fairly soluble in water. They form several solidhydrates when crystallized from solution. Solublesodium silicates and mixtures thereof, known aswaterglass are 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.
| Part of a series related to |
| Biomineralization |
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Teeth, scales, tusks etc |
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 formPortland 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.[9][10] Some plants excrete ligands that dissolve silicates, a step inbiomineralization.
Catechols can depolymerize SiO2—a component of silicates with ionic structures like orthosilicate (SiO44-), metasilicate (SiO23-), and pyrosilicate (Si2O67-)—by forming bis- and tris(catecholate)silicate dianions through coordination.[11] This complexes can be further coated on various substrates for applications such as drug delivery systems, antibacterial and antifouling applications.
Silicate anions in solution react withmolybdate anions yielding yellowsilicomolybdate complexes. In a typical preparation,monomeric orthosilicate was found to react completely in 75 seconds;dimeric pyrosilicate in 10 minutes; and higheroligomers in considerably longer time. In particular, the reaction is not observed with suspensions ofcolloidal silica.[10]
The nature of soluble silicates is relevant to understandingbiomineralization and the synthesis ofaluminosilicates, such as the industrially importantcatalysts calledzeolites.[9] Along withaluminateanions, soluble silicate anions also play a major role in the polymerization mechanism ofgeopolymers. Geopolymers areamorphous aluminosilicates whose production requires less energy than that of ordinaryPortland cement. So,geopolymer cements could contribute to limiting theCO2 emissions in the Earthatmosphere and theglobal warming caused by thisgreenhouse gas.
