Chemistry:Silicate mineral

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Short description: Rock-forming minerals with predominantly silicate anions
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.[1][2][3]

In mineralogy, silica (silicon dioxide, SiO
2
) is usually considered a silicate mineral rather than an oxide mineral. 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.

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.[citation needed]

General structure

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 [KAlSi3O8]n, 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 [SiO2]n. Replacement of one in every four silicon atoms by an aluminum atom results in the anion [AlSi3O8]n, whose charge is neutralized by the potassium cations K+.

Main groups

In mineralogy, silicate minerals are classified into seven major groups according to the structure of their silicate anion:[4][5]

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 orthosilicates

Orthosilicate anion SiO4−4. The grey ball represents the silicon atom, and the red balls are the oxygen atoms.
Nesosilicate specimens at the Museum of Geology in South Dakota

Nesosilicates (from Greek νῆσος nēsos 'island'), or orthosilicates, have the orthosilicate ion, present as isolated (insular) [SiO
4
]4−
tetrahedra connected only by interstitial cations. The Nickel–Strunz classification is 09.A –examples include:

Kyanite crystals (unknown scale)

Sorosilicates

Pyrosilicate anion Si2O6−7
Sorosilicate exhibit at Museum of Geology in South Dakota

Sorosilicates (from Greek σωρός sōros 'heap, mound') have isolated pyrosilicate anions Si2O6−7, 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:

Cyclosilicates

Cyclosilicate specimens at the Museum of Geology, South Dakota
Pezzottaite
Bazzite

Cyclosilicates (from Greek κύκλος kýklos '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:

  • 3-member single ring
  • 4-member single ring
  • 6-member single ring
  • 9-member single ring
    • EudialyteNa15Ca6(Fe,Mn)3Zr3SiO(O,OH,H2O)3(Si3O9)2(Si9O27)2(OH,Cl)2
  • 6-member double ring

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

Inosilicates

Inosilicates (from Greek ἴς is [genitive: ἰνός inos] 'fibre'), or chain silicates, have interlocking chains of silicate tetrahedra with either SiO
3
, 1:3 ratio, for single chains or Si
4
O
11
, 4:11 ratio, for double chains. The Nickel–Strunz classification is 09.D – examples include:

Single chain inosilicates

Double chain inosilicates

Phyllosilicates

Phyllosilicates (from Greek φύλλον phýllon 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedra with Si
2
O
5
or a 2:5 ratio. The Nickel–Strunz classification is 09.E. All phyllosilicate minerals are hydrated, with either water or hydroxyl groups attached.

Kaolinite

Examples include:

  • Serpentine subgroup
  • Clay minerals group
    • 1:1 clay minerals (TO)
    • 2:1 clay minerals (TOT)
      • PyrophylliteAl
        2
        Si
        4
        O
        10
        (OH)
        2
      • TalcMg
        3
        Si
        4
        O
        10
        (OH)
        2
      • Illite(K,H
        3
        O)(Al,Mg,Fe)
        2
        (Si,Al)
        4
        O
        10
        [(OH)
        2
        ,(H
        2
        O)]
      • Montmorillonite (smectite) – (Na,Ca)
        0.33
        (Al,Mg)
        2
        Si
        4
        O
        10
        (OH)
        2
         · nH
        2
        O
      • Chlorite(Mg,Fe)
        3
        (Si,Al)
        4
        O
        10
        (OH)
        2
         · (Mg,Fe)
        3
        (OH)
        6
      • Vermiculite(Mg,Fe,Al)
        3
        (Al,Si)
        4
        O
        10
        (OH)
        2
         · 4H2O
    • Other clay minerals
      • SepioliteLua error: Internal error: The interpreter exited with status 1.
      • Palygorskite (or attapulgite) – Lua error: Internal error: The interpreter exited with status 1.
  • Mica group
    • BiotiteLua error: Internal error: The interpreter exited with status 1.
    • FuchsiteLua error: Internal error: The interpreter exited with status 1.
    • MuscoviteLua error: Internal error: The interpreter exited with status 1.
    • PhlogopiteLua error: Internal error: The interpreter exited with status 1.
    • LepidoliteK(Li,Al)2–3(AlSi3)O10(OH)2
    • MargariteLua error: Internal error: The interpreter exited with status 1.
    • GlauconiteLua error: Internal error: The interpreter exited with status 1.

Tectosilicates

Silica family Lua error: Internal error: The interpreter exited with status 1. 3D network), β-quartz
Aluminosilicate family, the 3D model of synthetic zeolite ZSM-5
Quartz
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 Lua error: Internal error: The interpreter exited with status 1. in a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth.[6] Tectosilicates, with the exception of the quartz group, are aluminosilicates. The Nickel–Strunz classifications are 09.F and 09.G, 04.DA (Quartz/ silica family). Examples include:

  • 3D-Silicates, quartz family
    • QuartzLua error: Internal error: The interpreter exited with status 1.
    • TridymiteLua error: Internal error: The interpreter exited with status 1.
    • CristobaliteLua error: Internal error: The interpreter exited with status 1.
    • CoesiteLua error: Internal error: The interpreter exited with status 1.
    • StishoviteLua error: Internal error: The interpreter exited with status 1.
    • MoganiteLua error: Internal error: The interpreter exited with status 1.
    • ChalcedonyLua error: Internal error: The interpreter exited with status 1.
  • Tectosilicates, feldspar group
    • Alkali feldspars (potassium feldspars)
      • MicroclineLua error: Internal error: The interpreter exited with status 1.
      • OrthoclaseLua error: Internal error: The interpreter exited with status 1.
      • AnorthoclaseLua error: Internal error: The interpreter exited with status 1.
      • SanidineLua error: Internal error: The interpreter exited with status 1.
    • Plagioclase feldspars
      • AlbiteLua error: Internal error: The interpreter exited with status 1.
      • OligoclaseLua error: Internal error: The interpreter exited with status 1.     (Na:Ca 4:1)
      • AndesineLua error: Internal error: The interpreter exited with status 1.     (Na:Ca 3:2)
      • LabradoriteLua error: Internal error: The interpreter exited with status 1.     (Na:Ca 2:3)
      • BytowniteLua error: Internal error: The interpreter exited with status 1.     (Na:Ca 1:4)
      • AnorthiteLua error: Internal error: The interpreter exited with status 1.
  • Tectosilicates, feldspathoid family
    • NoseanLua error: Internal error: The interpreter exited with status 1.
    • CancriniteLua error: Internal error: The interpreter exited with status 1. • 2H2O
    • LeuciteLua error: Internal error: The interpreter exited with status 1.
    • NephelineLua error: Internal error: The interpreter exited with status 1.
    • SodaliteLua error: Internal error: The interpreter exited with status 1.
    • Hauyne(Na,Ca)4–8Al6Si6(O,S)24(SO4,Cl)1–2
      • LazuriteLua error: Internal error: The interpreter exited with status 1.
  • Tectosilicates, scapolite group
    • MarialiteLua error: Internal error: The interpreter exited with status 1.
    • MeioniteLua error: Internal error: The interpreter exited with status 1.
  • Tectosilicates, zeolite family
    • NatroliteLua error: Internal error: The interpreter exited with status 1.
    • ErioniteLua error: Internal error: The interpreter exited with status 1.
    • ChabaziteLua error: Internal error: The interpreter exited with status 1.
    • HeulanditeLua error: Internal error: The interpreter exited with status 1.
    • StilbiteLua error: Internal error: The interpreter exited with status 1.
    • ScoleciteLua error: Internal error: The interpreter exited with status 1.
    • MordeniteLua error: Internal error: The interpreter exited with status 1.
    • AnalcimeLua error: Internal error: The interpreter exited with status 1.

See also

References

  1. "Mineral - Silicates". https://www.britannica.com/science/mineral-chemical-compound/Silicates. 
  2. Deer, W.A.; Howie, R.A.; Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). London: Longman. ISBN 0-582-30094-0. 
  3. Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). Wiley. ISBN 0-47180580-7. https://archive.org/details/manualofmineralo00klei. 
  4. Deer, W.A.; Howie, R.A., & Zussman, J. (1992). An introduction to the rock forming minerals (2nd edition ed.). London: Longman ISBN:0-582-30094-0
  5. Hurlbut, Cornelius S.; Klein, Cornelis ||1985). Manual of Mineralogy, Wiley, (20th edition ed.). ISBN:0-471-80580-7
  6. Deer, W.A.; Howie, R.A.; Wise, W.S.; Zussman, J. (2004). Rock-forming minerals. Volume 4B. Framework silicates: silica minerals. Feldspathoids and the zeolites (2nd ed.). London: Geological Society of London. p. 982 pp. 

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External links

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