Difference between revisions of "Silicate" - New World Encyclopedia
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[[Image:Kyanite crystals.jpg|thumb|300px|Crystals of kyanite, a blue silicate mineral.]] | [[Image:Kyanite crystals.jpg|thumb|300px|Crystals of kyanite, a blue silicate mineral.]] | ||
| + | [[Image:Lunar Ferroan Anorthosite 60025.jpg|thumb|250px|right|This sample of plagioclase [[feldspar]], a silicate mineral, was collected by [[Apollo 16]] from the [[Geology_of_the_Moon#Highlands_and_craters|Lunar Highlands]] near [[Descartes (crater)|Descartes Crater]].]] | ||
In [[chemistry]], a '''silicate''' is a compound containing an [[anion]] in which one or more central [[silicon]] atoms are surrounded by electronegative [[ligand]]s. This definition is broad enough to include species such as hexafluorosilicate ("fluorosilicate"), [SiF<sub>6</sub>]<sup>2−</sup>, but the silicate species that are encountered most often consist of [[silicon]] with [[oxygen]] as the ligand. Silicate anions, with a negative net electrical charge, must have that charge balanced by other cations to make an electrically neutral compound. In [[mineralogy]], '''silicate minerals''' constitute the largest and most important class of rock-forming [[mineral]]s. | In [[chemistry]], a '''silicate''' is a compound containing an [[anion]] in which one or more central [[silicon]] atoms are surrounded by electronegative [[ligand]]s. This definition is broad enough to include species such as hexafluorosilicate ("fluorosilicate"), [SiF<sub>6</sub>]<sup>2−</sup>, but the silicate species that are encountered most often consist of [[silicon]] with [[oxygen]] as the ligand. Silicate anions, with a negative net electrical charge, must have that charge balanced by other cations to make an electrically neutral compound. In [[mineralogy]], '''silicate minerals''' constitute the largest and most important class of rock-forming [[mineral]]s. | ||
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== Silicate minerals == | == Silicate minerals == | ||
| + | [[Image:KaolinUSGOV.jpg|thumb|A sample of kaolin, a phyllosilicate.]] | ||
In [[mineralogy]], '''silicate minerals''' constitute the largest and most important class of rock-forming [[mineral]]s. They are classified according to the structures of their silicate [[anion]]s, as given below. | In [[mineralogy]], '''silicate minerals''' constitute the largest and most important class of rock-forming [[mineral]]s. They are classified according to the structures of their silicate [[anion]]s, as given below. | ||
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'''Sorosilicates''': They are characterized by anions with the formula (Si<sub>2</sub>O<sub>7</sub>)<sup>6−</sup>, in which the ratio of silicon atoms to oxygen atoms is 2:7. Each anion is a double tetrahedron. Examples are members of the [[epidote]] group of minerals. | '''Sorosilicates''': They are characterized by anions with the formula (Si<sub>2</sub>O<sub>7</sub>)<sup>6−</sup>, in which the ratio of silicon atoms to oxygen atoms is 2:7. Each anion is a double tetrahedron. Examples are members of the [[epidote]] group of minerals. | ||
| − | '''Cyclosilicates''' (or '''ring silicates'''): These silicates are characterized by ring structures of different sizes, but the ratio of silicon atoms to oxygen atoms is generally 1:3. They have linked tetrahedra, and their general chemical formula is (Si<sub> | + | '''Cyclosilicates''' (or '''ring silicates'''): These silicates are characterized by ring structures of different sizes, but the ratio of silicon atoms to oxygen atoms is generally 1:3. They have linked tetrahedra, and their general chemical formula is (Si<sub>x</sub>O<sub>3x</sub>)<sup>2n-</sup>. The anions can be 3-member, 4-member, or 6-member rings. |
| + | * 3-member rings, with the formula (Si<sub>3</sub>O<sub>9</sub>)<sup>6-</sup>, are found in [[benitoite]]. | ||
| + | * 4-member rings, with the formula (Si<sub>4</sub>O<sub>12</sub>)<sup>8-</sup>, are found in [[axinite]]. | ||
| + | * 6-member rings, with the formula (Si<sub>6</sub>O<sub>18</sub>)<sup>12-</sup>, are present in [[beryl]] and [[tourmaline]]. | ||
'''Inosilicates''' (or '''chain silicates'''): They contain interlocking chains of silicate tetrahedra. They may consist of: | '''Inosilicates''' (or '''chain silicates'''): They contain interlocking chains of silicate tetrahedra. They may consist of: | ||
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'''Phyllosilicates''' (or '''sheet silicates'''): These minerals form parallel sheets of silicate tetrahedra. The general chemical formula of the anion is (Si<sub>2n</sub>O<sub>5n</sub>)<sup>2n−</sup> (2:5 ratio of Si to O atoms). Examples are the [[mica]] and [[clay]] groups of minerals. | '''Phyllosilicates''' (or '''sheet silicates'''): These minerals form parallel sheets of silicate tetrahedra. The general chemical formula of the anion is (Si<sub>2n</sub>O<sub>5n</sub>)<sup>2n−</sup> (2:5 ratio of Si to O atoms). Examples are the [[mica]] and [[clay]] groups of minerals. | ||
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'''Tectosilicates''' (or '''framework silicates'''): This is the largest group of silicates, constituting nearly 75 percent of the [[Earth]]'s [[crust (geology)|crust]]. They are characterized by a three-dimensional framework of silicate tetrahedra. Examples are members of the [[quartz]], [[feldspar]], and [[zeolite]] groups of minerals. Except for the quartz group, they are [[aluminosilicate]]s, with the general chemical formula (Al<sub>x</sub>Si<sub>y</sub>O<sub>2(x+y)</sub>)<sup>x−</sup> (1:2 ratio of Si to O atoms). | '''Tectosilicates''' (or '''framework silicates'''): This is the largest group of silicates, constituting nearly 75 percent of the [[Earth]]'s [[crust (geology)|crust]]. They are characterized by a three-dimensional framework of silicate tetrahedra. Examples are members of the [[quartz]], [[feldspar]], and [[zeolite]] groups of minerals. Except for the quartz group, they are [[aluminosilicate]]s, with the general chemical formula (Al<sub>x</sub>Si<sub>y</sub>O<sub>2(x+y)</sub>)<sup>x−</sup> (1:2 ratio of Si to O atoms). | ||
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The tectosilicates can have additional cations only if some of the silicon is replaced by a lower-charge cation such as [[aluminum]], to give a negative charge overall. This substitution can also take place in other types of silicates. | The tectosilicates can have additional cations only if some of the silicon is replaced by a lower-charge cation such as [[aluminum]], to give a negative charge overall. This substitution can also take place in other types of silicates. | ||
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== References == | == References == | ||
| − | + | ||
| + | * Cotton, F. Albert, Geoffrey Wilkinson, Carlos A. Murillo, and Manfred Bochmann. 1999. ''Advanced Inorganic Chemistry''. 6th ed. New York: John Wiley. ISBN 0471199575. | ||
| + | |||
| + | * Deer, W.A., R.A. Howie, and J. Zussman. 1996. ''An Introduction to the Rock-Forming Minerals''. 2nd ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0582300940. | ||
| + | |||
| + | * Farndon, John. 2006. ''The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks''. London: Lorenz Books. ISBN 0754815412. | ||
| + | |||
| + | * Klein, Cornelis, and Barbara Dutrow. 2007. ''Manual of Mineral Science''. 23rd ed. New York: John Wiley. ISBN 978-0471721574. | ||
| + | |||
| + | * Pellant, Chris. 2002. ''Rocks and Minerals''. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060. | ||
| + | |||
| + | * Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. ''Rocks, Gems and Minerals''. Rev. ed. New York: St. Martin's Press. ISBN 1582381321. | ||
== External links == | == External links == | ||
| + | |||
| + | * [http://www.galleries.com/minerals/silicate/class.htm The Silicate Class.] ''Amethyst Galleries''. Retrieved May 15, 2007. | ||
| + | * [http://mineral.galleries.com/minerals/by_name.htm Minerals by Name.] ''Amethyst Galleries''. Retrieved May 15, 2007. | ||
| + | *[http://www.minerals.net/index.htm The Mineral and Gemstone Kingdom.] ''Minerals.net''. Retrieved May 15, 2007. | ||
| + | *[http://www.mindat.org/index.php mindat.org Mineralogy Database.] ''Mindat.org''. Retrieved May 15, 2007. | ||
| + | *[http://webmineral.com Mineralogy Database.] ''Webmineral.com''. Retrieved May 15, 2007. | ||
[[Category:Physical sciences]] | [[Category:Physical sciences]] | ||
Revision as of 20:52, 15 May 2007
In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate ("fluorosilicate"), [SiF6]2−, but the silicate species that are encountered most often consist of silicon with oxygen as the ligand. Silicate anions, with a negative net electrical charge, must have that charge balanced by other cations to make an electrically neutral compound. In mineralogy, silicate minerals constitute the largest and most important class of rock-forming minerals.
Silica, or silicon dioxide, SiO2, is sometimes considered a silicate, although it is the special case with no negative charge and no need for counter-ions. Silica is found in nature as the mineral quartz and its polymorphs.
Notable characteristics
In the vast majority of silicates, including silicate minerals, the Si atom shows tetrahedral coordination by 4 oxygens. In different minerals the tetrahedra show different degrees of polymerization: they occur singly, joined together in pairs, in larger finite clusters including rings, in chains, double chains, sheets, and three-dimensional frameworks. The minerals are classified into groups based on these anion structures; a list is given below.
Silicon may adopt octahedral coordination by 6 oxygens at very high pressure, as in the dense stishovite polymorph of silica that is found in the lower mantle of the Earth, and which is also formed by shock during meteorite impacts. Lack of space around the oxygen atoms makes this coordination for Si very rare at normal pressure, but it is known in the hexahydroxysilicate anion, [Si(OH)6]2−, as found in the mineral thaumasite.
Silicate rock
In geology and astronomy, the term silicate is used to denote types of rock that consist predominantly of silicate minerals. Such rocks include a wide range of igneous, metamorphic and sedimentary types. Most of the Earth's mantle and crust are made up of silicate rocks. The same is true of the Moon and the other rocky planets.
On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that form and re-work the crust. These processes include partial melting, crystallization, fractionation, metamorphism, weathering and diagenesis. Living things also contribute to the silicate cycle near the Earth's surface. A type of plankton known as diatoms construct their exoskeletons, known as tests, from silica. The tests of dead diatoms are a major constituent of deep ocean sediment
Silicates have been observed in space, around evolved stars and Planetary Nebulae such as NGC 6302. They are found in both amorphous form and crystalline form, though the range of types that have been found is far smaller than those found on Earth.
Silicate minerals
In mineralogy, silicate minerals constitute the largest and most important class of rock-forming minerals. They are classified according to the structures of their silicate anions, as given below.
Nesosilicates (also known as isosilicates or orthosilicates): They contain the anion [SiO4]4− that has a tetrahedral shape. The tetrahedra are isolated but held together by interstitial cations. Examples are members of the olivine and garnet mineral groups.
Sorosilicates: They are characterized by anions with the formula (Si2O7)6−, in which the ratio of silicon atoms to oxygen atoms is 2:7. Each anion is a double tetrahedron. Examples are members of the epidote group of minerals.
Cyclosilicates (or ring silicates): These silicates are characterized by ring structures of different sizes, but the ratio of silicon atoms to oxygen atoms is generally 1:3. They have linked tetrahedra, and their general chemical formula is (SixO3x)2n-. The anions can be 3-member, 4-member, or 6-member rings.
- 3-member rings, with the formula (Si3O9)6-, are found in benitoite.
- 4-member rings, with the formula (Si4O12)8-, are found in axinite.
- 6-member rings, with the formula (Si6O18)12-, are present in beryl and tourmaline.
Inosilicates (or chain silicates): They contain interlocking chains of silicate tetrahedra. They may consist of:
- single chains, with the chemical formula (SinO3n)2n− (1:3 ratio of Si to O atoms); or
- double chains, with the formula (Si4nO11n)6n− (4:11 ratio of Si to O atoms).
The pyroxene group of minerals are examples of single-chain inosilicates; the amphibole group are examples of double-chain inosilicates.
Phyllosilicates (or sheet silicates): These minerals form parallel sheets of silicate tetrahedra. The general chemical formula of the anion is (Si2nO5n)2n− (2:5 ratio of Si to O atoms). Examples are the mica and clay groups of minerals.
Tectosilicates (or framework silicates): This is the largest group of silicates, constituting nearly 75 percent of the Earth's crust. They are characterized by a three-dimensional framework of silicate tetrahedra. Examples are members of the quartz, feldspar, and zeolite groups of minerals. Except for the quartz group, they are aluminosilicates, with the general chemical formula (AlxSiyO2(x+y))x− (1:2 ratio of Si to O atoms).
The tectosilicates can have additional cations only if some of the silicon is replaced by a lower-charge cation such as aluminum, to give a negative charge overall. This substitution can also take place in other types of silicates.
Some rare minerals have more than one type of anion coexisting in their crystal structures, or they may contain complex anions that are intermediate between the types noted above.
See also
ReferencesISBN links support NWE through referral fees
- Cotton, F. Albert, Geoffrey Wilkinson, Carlos A. Murillo, and Manfred Bochmann. 1999. Advanced Inorganic Chemistry. 6th ed. New York: John Wiley. ISBN 0471199575.
- Deer, W.A., R.A. Howie, and J. Zussman. 1996. An Introduction to the Rock-Forming Minerals. 2nd ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0582300940.
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
External links
- The Silicate Class. Amethyst Galleries. Retrieved May 15, 2007.
- Minerals by Name. Amethyst Galleries. Retrieved May 15, 2007.
- The Mineral and Gemstone Kingdom. Minerals.net. Retrieved May 15, 2007.
- mindat.org Mineralogy Database. Mindat.org. Retrieved May 15, 2007.
- Mineralogy Database. Webmineral.com. Retrieved May 15, 2007.
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