Silicate mineral

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Lithium aluminium silicate mineral spodumene 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. [1] [2] [3]

Contents

In mineralogy, the crystalline forms of silica (silicon dioxide, SiO2) 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.

Diatomaceous earth, a biogenic form of silica as viewed under a microscope. The imaged region measures approximately 1.13 by 0.69 mm. 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.[ 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 [KAlSi
3O
8]
n, the anion is a tridimensional network of tetrahedrums in which all oxygen corners are shared. If all tetrahedrums had silicon centers, the anion would be just neutral silica [SiO
2]
n. Replacement of one in every four silicon atoms by an aluminum atom results in the anion [AlSi
3O−
8]
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 groupStructureChemical formulaExample
Nesosilicates isolated silicon tetrahedrums[SiO4]4− olivine, garnet, zircon...
Sorosilicates double tetrahedrums[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 SiO 4. The grey ball represents the silicon atom, and the red balls are the oxygen atoms. Silicate-tetrahedron-3D-balls.png
Orthosilicate anion SiO
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 exhibit, Museum of Geology, South Dakota.jpg
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) [SiO4]4− tetrahedrums connected only by interstitial cations. The Nickel–Strunz classification is 09.A –examples include:

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

Sorosilicates

Pyrosilicate anion Si 2O 7 Silicate-double-tetrahedra-3D-balls.png
Pyrosilicate anion Si
2O
7
Sorosilicate exhibit at Museum of Geology in South Dakota Sorosilicates exhibit, Museum of Geology, South Dakota.jpg
Sorosilicate exhibit at Museum of Geology in South Dakota

Sorosilicates (from Greek σωρός sōros 'heap, mound') have isolated pyrosilicate anions Si
2O6−
7, consisting of double tetrahedrums 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 Cyclosilicate exhibit, Museum of Geology, South Dakota.jpg
Cyclosilicate specimens at the Museum of Geology, South Dakota
Pezzottaite Pezzottaite-256889.jpg
Pezzottaite
Bazzite Bazzite - Fibbia Ticino Switzerland.jpg
Bazzite

Cyclosilicates (from Greek κύκλος kýklos 'circle'), or ring silicates, have three or more tetrahedrums 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 tetrahedrums 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 tetrahedrums with either SiO3, 1:3 ratio, for single chains or Si4O11, 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 tetrahedrums with Si2O5 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 KaolinUSGOV.jpg
Kaolinite

Examples include:

Tectosilicates

Silica family (SiO2 3D network), b-quartz Beta-quartz-CM-2D-balls.png
Silica family (SiO2 3D network), β-quartz
Aluminosilicate family, the 3D model of synthetic zeolite ZSM-5 Zeolite-ZSM-5-vdW.png
Aluminosilicate family, the 3D model of synthetic zeolite ZSM-5
Quartz Quartz oisan.jpg
Quartz
Lunar ferroan anorthosite (plagioclase feldspar) collected by Apollo 16 astronauts from the Lunar Highlands near Descartes Crater 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 SiO2 in a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth. [12] Tectosilicates, with the exception of the quartz group, are aluminosilicates. The Nickel–Strunz classifications are 9.F (tectosilicates without zeolitic H2O), 9.G (tectosilicates with zeolitic H2O), and 4.DA (quartz/silica group). Below is a list of tectosilicate minerals and their chemical formulas, organized by groups and series:

See also

References

  1. ↑ "Mineral - Silicates". britannica.com. Archived from the original on 25 October 2017. Retrieved 8 May 2018.
  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.
  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. ↑ "Mica Group". mindat.org. Hudson Institute of Mineralogy. Retrieved 24 February 2025.
  7. ↑ "Brittle Mica". mindat.org. Hudson Institute of Mineralogy. Retrieved 24 February 2025.
  8. ↑ "Muscovite". mindat.org. Hudson Institute of Mineralogy. Retrieved 24 February 2025.
  9. ↑ "Fuchsite". mindat.org. Hudson Institute of Mineralogy. Retrieved 24 February 2025.
  10. ↑ "Illite". mindat.org. Hudson Institute of Mineralogy. Retrieved 24 February 2025.
  11. ↑ "Mariposite". mindat.org. Hudson Institute of Mineralogy. Retrieved 24 February 2025.
  12. ↑ 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.
  13. ↑ "Feldspar Group". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  14. ↑ "Oligoclase". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  15. ↑ "Andesine". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  16. ↑ "Labradorite". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  17. ↑ "Bytownite". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  18. ↑ "Hyalophane". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  19. ↑ "Rubicline". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  20. ↑ "Feldspathoid". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  21. ↑ "Danalite". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  22. ↑ "Tugtupite". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  23. ↑ "Scapolite". mindat.org. Hudson Institute of Mineralogy. Retrieved 23 February 2025.
  24. ↑ "Zeolite Group". www.mindat.org. Retrieved 22 February 2025.
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