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Silicate mineral

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File:Spodumene-sd305c.jpg
Lithium aluminium silicate mineral spodumene

Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=deer>Template:Cite book</ref><ref name=hurlbut>Template:Cite book</ref>

In mineralogy, the crystalline forms of silica (silicon dioxide, Template:Chem2) are usually considered to be tectosilicates, and they are classified as such in the Dana system (75.1). However, the Nickel-Strunz system classifies them as oxide minerals (4.DA). Silica is found in nature as the mineral quartz, and its polymorphs.

On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that have been forming and re-working the crust for billions of years. These processes include partial melting, crystallization, fractionation, metamorphism, weathering, and diagenesis.

File:Diatomaceous Earth BrightField.jpg
Diatomaceous earth, a biogenic form of silica as viewed under a microscope. The imaged region measures approximately 1.13 by 0.69 mm.

Living organisms also contribute to this geologic cycle. For example, a type of plankton known as diatoms construct their exoskeletons ("frustules") from silica extracted from seawater. The frustules of dead diatoms are a major constituent of deep ocean sediment, and of diatomaceous earth.Template:Citation needed

General structureEdit

A silicate mineral is generally an inorganic compound consisting of subunits with the formula [SiO2+n]2n−. Although depicted as such, the description of silicates as anions is a simplification. Balancing the charges of the silicate anions are metal cations, Mx+. Typical cations are Mg2+, Fe2+, and Na+. The Si-O-M linkage between the silicates and the metals are strong, polar-covalent bonds. Silicate anions ([SiO2+n]2n−) are invariably colorless, or when crushed to a fine powder, white. The colors of silicate minerals arise from the metal component, commonly iron.

In most silicate minerals, silicon is tetrahedral, being surrounded by four oxides. The coordination number of the oxides is variable except when it bridges two silicon centers, in which case the oxide has a coordination number of two.

Some silicon centers may be replaced by atoms of other elements, still bound to the four corner oxygen corners. If the substituted atom is not normally tetravalent, it usually contributes extra charge to the anion, which then requires extra cations. For example, in the mineral orthoclase Template:Chem, the anion is a tridimensional network of tetrahedra in which all oxygen corners are shared. If all tetrahedra had silicon centers, the anion would be just neutral silica Template:Chem. Replacement of one in every four silicon atoms by an aluminum atom results in the anion Template:Chem, whose charge is neutralized by the potassium cations Template:Chem.

Main groupsEdit

In mineralogy, silicate minerals are classified into seven major groups according to the structure of their silicate anion:<ref>Deer, W.A.; Howie, R.A., & Zussman, J. (1992). An introduction to the rock forming minerals (2nd edition ed.). London: Longman Template:ISBN</ref><ref>Hurlbut, Cornelius S.; Klein, Cornelis ||1985). Manual of Mineralogy, Wiley, (20th edition ed.). Template:ISBN</ref>

Major group Structure Chemical formula Example
Nesosilicates isolated silicon tetrahedra [SiO4]4− olivine, garnet, zircon...
Sorosilicates double tetrahedra [Si2O7]6− epidote, melilite group
Cyclosilicates rings [SinO3n]2n− beryl group, tourmaline group
Inosilicates single chain [SinO3n]2n− pyroxene group
Inosilicates double chain [Si4nO11n]6n− amphibole group
Phyllosilicates sheets [Si2nO5n]2n− micas and clays
Tectosilicates 3D framework [AlxSiyO(2x+2y)]x− quartz, feldspars, zeolites

Tectosilicates can only have additional cations if some of the silicon is replaced by an atom of lower valence such as aluminum. Al for Si substitution is common.

Nesosilicates or orthosilicatesEdit

File:Silicate-tetrahedron-3D-balls.png
Orthosilicate anion Template:Chem. The grey ball represents the silicon atom, and the red balls are the oxygen atoms.
File:Nesosilicates exhibit, Museum of Geology, South Dakota.jpg
Nesosilicate specimens at the Museum of Geology in South Dakota

Template:Main category Nesosilicates (from Greek Template:Wikt-lang Template:Transliteration 'island'), or orthosilicates, have the orthosilicate ion, present as isolated (insular) Template:Chem2 tetrahedra connected only by interstitial cations. The Nickel–Strunz classification is 09.A –examples include:

File:Kyanite crystals.jpg
Kyanite crystals (unknown scale)

SorosilicatesEdit

File:Silicate-double-tetrahedra-3D-balls.png
Pyrosilicate anion Template:Chem
File:Sorosilicates exhibit, Museum of Geology, South Dakota.jpg
Sorosilicate exhibit at Museum of Geology in South Dakota

Template:Main category Sorosilicates (from Greek Template:Wikt-lang Template:Transliteration 'heap, mound') have isolated pyrosilicate anions Template:Chem, consisting of double tetrahedra with a shared oxygen vertex—a silicon:oxygen ratio of 2:7. The Nickel–Strunz classification is 09.B. Examples include:

CyclosilicatesEdit

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File:Cyclosilicate exhibit, Museum of Geology, South Dakota.jpg
Cyclosilicate specimens at the Museum of Geology, South Dakota
File:Pezzottaite-256889.jpg
Pezzottaite
File:Bazzite - Fibbia Ticino Switzerland.jpg
Bazzite

Cyclosilicates (from Greek Template:Wikt-lang Template:Transliteration 'circle'), or ring silicates, have three or more tetrahedra linked in a ring. The general formula is (SixO3x)2x−, where one or more silicon atoms can be replaced by other 4-coordinated atom(s). The silicon:oxygen ratio is 1:3. Double rings have the formula (Si2xO5x)2x− or a 2:5 ratio. The Nickel–Strunz classification is 09.C. Possible ring sizes include:

Some example minerals are:

The ring in axinite contains two B and four Si tetrahedra and is highly distorted compared to the other 6-member ring cyclosilicates.

InosilicatesEdit

Template:Main category Inosilicates (from Greek Template:Wikt-lang Template:Transliteration [genitive: Template:Wikt-lang Template:Transliteration] 'fibre'), or chain silicates, have interlocking chains of silicate tetrahedra with either Template:Chem2, 1:3 ratio, for single chains or Template:Chem2, 4:11 ratio, for double chains. The Nickel–Strunz classification is 09.D – examples include:

Single chain inosilicatesEdit

Double chain inosilicatesEdit

  • Pyroxen-chain.png

    Inosilicate, pyroxene family, with 2-periodic single chain Template:Chem2, diopside

  • Tremolite-chain.png

    Inosilicate, clinoamphibole, with 2-periodic double chains Template:Chem2, tremolite

  • Wollastonite-chain.png

    Inosilicate, unbranched 3-periodic single chain of wollastonite

  • Rhodonite-chain.png

    Inosilicate with 5-periodic single chain, rhodonite

  • Pellyite-chain.png

    Inosilicate with cyclic branched 8-periodic chain, pellyite

PhyllosilicatesEdit

Template:Main category Phyllosilicates (from Greek Template:Wikt-lang Template:Transliteration 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedra with Template:Chem2 or a 2:5 ratio. The Nickel–Strunz classification is 09.E. All phyllosilicate minerals are hydrated, with either water or hydroxyl groups attached.

File:KaolinUSGOV.jpg
Kaolinite

Examples include:

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  • Muskovite.sheet.png

    Phyllosilicate, mica group, muscovite (red: Si, blue: O)

  • Apophyllite.sheet.png

    Phyllosilicate, single net of tetrahedra with 4-membered rings, apophyllite-(KF)-apophyllite-(KOH) series

  • Pyrosmalite.sheet.png

    Phyllosilicate, single tetrahedral nets of 6-membered rings, pyrosmalite-(Fe)-pyrosmalite-(Mn) series

  • Zeophyllite.sheet.png

    Phyllosilicate, single tetrahedral nets of 6-membered rings, zeophyllite

  • Carletonite.sheet.png

    Phyllosilicate, double nets with 4- and 6-membered rings, carletonite

TectosilicatesEdit

Template:Main category

File:Beta-quartz-CM-2D-balls.png
Silica family Template:Chem2 3D network), β-quartz
File:Zeolite-ZSM-5-vdW.png
Aluminosilicate family, the 3D model of synthetic zeolite ZSM-5
File:Quartz oisan.jpg
Quartz
File:Lunar Ferroan Anorthosite (60025).jpg
Lunar ferroan anorthosite (plagioclase feldspar) collected by Apollo 16 astronauts from the Lunar Highlands near Descartes Crater

Tectosilicates, or "framework silicates," have a three-dimensional framework of silicate tetrahedra with Template:Chem2 in a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth.<ref name=deer4B>Template:Cite book</ref> Tectosilicates, with the exception of the quartz group, are aluminosilicates. The Nickel–Strunz classifications are 9.F (tectosilicates without zeolitic Template:Chem2), 9.G (tectosilicates with zeolitic Template:Chem2), and 4.DA (quartz/silica group). Below is a list of tectosilicate minerals and their chemical formulas, organized by groups and series:

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  • Scapolite group<ref>{{#invoke:citation/CS1|citation

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See alsoEdit

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ReferencesEdit

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External linksEdit

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