Minerals/Silicates

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This is a naturally occurring collection of intergrown feldspar crystals. Credit: Dave Dyet.

Silicate minerals are those with more atomic percent silicon oxide than other constituent elements.

For example, kaolinite is Al2Si2O5(OH)4. If aluminum (Al) at two atoms per molecular unit were the most numerous element, kaolinite would be an aluminide.

As the most numerous element is oxygen at 9 for 52.9 at %, kaolin is an oxide. The crystal structure consists of a sheet of interconnected silica tetrahedra. So, kaolin is a phyllosilicate.

Metasilicates[edit | edit source]

Def. the "oxyanion of silicon SiO32- or any salt or mineral containing this ion"[1] is called a metasilicate.

Orthosilicates[edit | edit source]

This is Andradite Var: Demantoid garnet an orthosilicate. Credit: yaiba0390.

Def.

  1. "any salt or ester of orthosilicic acid, (M+)4SiO44− or Si(OR)4"[2] or
  2. "any silicate mineral, such as garnet or olivine, in which the SiO4 tetrahedra do not share oxygen atoms with each other"[2]

is called an orthosilicate.

On the right is an image of andradite variety demantoid. Andradite has the formula Ca3Fe3+3Si3O12.[3] Demantoid andradite is green.[4]

Nesosilicates[edit | edit source]

Although tephroite on the right, a nesosilicate, is gray, this specimen shows some brown color. Credit: Rob Lavinsky.
Sphene is widely distributed as an accessory mineral in intermediate and felsic plutonic rocks, pegmatites and alpine veins. Credit: Martine van der Westhuizen.

Def. "any simple silicate mineral in which the SiO4 tetrahedra are isolated and have metal ions as neighbours"[5] is called a neosilicate.

Def. a "type of silicate crystal structure characterized by the linking of SiO4 tetrahedra through other cations rather than the sharing of oxygens among SiO4 tetrahedra"[3] is called a nesosilicate.

The second nesosilicate image down on the right is of the mineral sphene. Its molecular formula is CaTiSiO5. Each crystallographic unit cell of sphene contains at least one molecular unit. The actual space group P 21/a has Z=4 (or four molecules per unit cell).

"Sphene is widely distributed as an accessory mineral in intermediate and felsic plutonic rocks, pegmatites and alpine veins, particularly in coarse-grained igneous rocks such as syenite, nepheline syenite, diorite and granodiorite. It occurs similarly in schists or gneisses and in some metamorphosed limestones."[6]

Sorosilicates[edit | edit source]

At the center of this image are yellowish crystals of the sorosilicate mineral leucophanite. Credit: Parent Géry.

Def. any group of silicates that have structurally isolated double tetrahedra (the dimeric anion Si2O76-)[7] is called a sorosilicate.

Cyclosilicates[edit | edit source]

Colorless beryl, a cyclosilicate, is called goshenite. Credit: Piotr Menducki

Def. any group of silicates that have a ring of linked tetrahedra is called a cyclosilicate.

"Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists ... Goshenite [a beryl clear to white cyclosilicate] is found to some extent in almost all beryl localities."[8]

Abenakiite-(Ce)[edit | edit source]

Abenakiite-(Ce) is a cyclosilicate. Credit: Salah Rashad Zaqzoq.{{free media}}

Abenakiite-(Ce) has the chemical formula Na
26
Ce
6
(SiO
3
)
6
(PO
4
)
6
(CO
3
)
6
(S4+
O
)O
. Abenakiite-(Ce) (IMA1991-054; IMA Symbol Abk-Ce[9]) is a mineral of sodium, cerium, neodymium, lanthanum, praseodymium, thorium, samarium, oxygen, sulfur, carbon, phosphorus, and silicon. The silicate groups may be given as the cyclic Si
6
O
18
grouping. Its Mohs scale rating is 4 to 5.[10]

Abenakiite-(Ce) was discovered in a sodalite syenite xenolith at Mont Saint-Hilaire, Québec, Canada, together with aegirine, eudialyte, manganoneptunite, polylithionite, serandite, and steenstrupine-(Ce).[10][11]

Combination of elements in abenakiite-(Ce) is unique. Somewhat chemically similar mineral is steenstrupine-(Ce).[11][12] The hyper-sodium abenakiite-(Ce) is also unique in supposed presence of sulfur dioxide ligand. With a single grain (originally) found, abenakiite-(Ce) is extremely rare.[10]

In the crystal structure, described as a hexagonal net, of abenakiite-(Ce) there are:[10]

  • chains of NaO
    7
    polyhedra, connected with PO
    4
    groups
  • columns with six-membered rings of NaO
    7
    , and NaO
    7
    -REEO
    6
    , and SiO
    4
    polyhedra (REE - rare earth elements)
  • CO
    3
    groups, NaO
    6
    octahedra, and disordered SO
    2
    ligands within the columns

Inosilicates[edit | edit source]

Def. "any silicate having interlocking chains of silicate tetrahedra"[13] is called an inosilicate.

To form these chains, each silica tetrahedron shares two oxygens with neighboring tetrahedra.

Single chain tetrahedra are the pyroxenes. Double chains of tetrahedra are the amphiboles.

Pyroxenes[edit | edit source]

Augite is a black, single-chain inosilicate mineral, a pyroxene. Credit: Didier Descouens.

Amphiboles[edit | edit source]

Anthophyllite (or asbestos) commonly occurs as a gray or white, double-chain inosilicate mineral. Credit: Aramgutang.
This image shows several amphibole crystals in a glass bowl. Credit: Karelj.

Anthophyllite is a double-chain inosilicate, or amphibole.

Phyllosilicates[edit | edit source]

Kaolin is a white phyllosilicate. Credit: USGS and the Minerals Information Institute.
Biotite is a black phyllosilicate mineral. Credit: United States Geological Survey and the Mineral Information Institute.

Def. any "silicate mineral having a crystal structure of parallel sheets of silicate tetrahedra"[14] is called a phyllosilicate.

Phyllosilicate tetrahedra share three oxygens with other silica tetrahedra to form two-dimensional sheets.

Clay minerals[edit | edit source]

View shows tetrahedral sheet structure of a clay mineral, where apical oxygen ions are tinted pink. Credit: Kent G. Budge.{{free media}}
This image depicts the structure of the dioctahedral sheet of a mica. Credit: Kent G. Budge.{{free media}}

Def. a "mineral substance made up of small crystals of silica and alumina,[15] that is ductile when moist"[16] is called a clay mineral.

Clay minerals can be classified as 1:1 or 2:1: a 1:1 clay consists of one tetrahedral sheet of corner-sharing SiO
4
tetrahedra, three vertex oxygen ions with other tetrahedra, the unshared vertex forms part of one side of the octahedral sheet, where the octahedral sheets form from small cations, such as aluminum or magnesium, and are coordinated by six oxygen atoms, all of the tetrahedra "point" in the same direction, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra, bonded to a hydrogen atom forming an OH group, and one octahedral sheet, of AlO
4
octahedra, where the sheet units have the chemical composition (Al,Si)
3
O
4
, examples are kaolinite and serpentinite; a 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite, and montmorillonite.[17]

Kaolinites[edit | edit source]

Kaolinite is from Twiggs County, Georgia, USA. Credit: James St. John.{{free media}}

Kaolinite has the chemical formula Al
2
Si
2
O
5
(OH)
4
.

Kaolinite is a clay mineral, a layered silicate mineral, with one tetrahedral sheet of silica (SiO
4
) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6
) octahedra.[18] Rocks that are rich in kaolinite are known as kaolin or porcelain (china) clay.[19]

The chemical formula for kaolinite as used in mineralogy is Al
2
Si
2
O
5
(OH)
4
,[20] however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al
2
O
3
*2SiO
2
*2H
2
O
.[21]

As the most numerous element is oxygen at 9 for 52.9 at %, kaolin is an oxide.

Tektosilicates[edit | edit source]

Def. a type "of silicate crystal structure characterized by the sharing of all SiO4 tetrahedral oxygens resulting in three-dimensional framework structures"[3] is called a tektosilicate.

Def. any "of various silicate minerals ... with a three-dimensional framework of silicate tetrahedra"[22] is called a tectosilicate.

Feldspars[edit | edit source]

This feldspar crystal is stark white showing excellent symmetry with appropriate faces. Credit: Rob Lavinsky.

Def. a group of "aluminum silicates [aluminosilicates] of the alkali metals sodium, potassium, calcium and barium"[23] are called feldspars, or feldspar.

"The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8)."[24]

"The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars due to the presence of alkali metals sodium and potassium."[24]

Feldspathoids[edit | edit source]

Def. any of a group of silicates "that did not contain enough silica to satisfy all the chemical bonds"[25] of the framework is called a feldspathoid.

Analcites[edit | edit source]

Colorless sharply formed undamaged crystals of analcime to 25 mm in diameter on a 78 mm x 65 mm x 53 mm matrix. Credit: Carles Millan.

The second image down on the right contains analcime, or analcite, as colorless sharply formed undamaged crystals to 25 mm in diameter on a 78 mm x 65 mm x 53 mm matrix. They are associated with numerous black prismatic terminated crystals of aegirine, as well as smaller colorless prismatic terminated crystals of natrolite, these from 3 mm to 10 mm in length. Aegirine is a pyroxene. Natrolite is another feldspathoid like analcime of the zeolite group.

Earth[edit | edit source]

"Feldspar is by far the most abundant group of minerals in the earth's crust, forming about 60% of terrestrial rocks."[24]

Hypotheses[edit | edit source]

  1. Most minerals on Earth are oxides.

See also[edit | edit source]

References[edit | edit source]

  1. "metasilicate". San Francisco, California: Wikimedia Foundation, Inc. June 20, 2013. Retrieved 2013-09-02.
  2. 2.0 2.1 "orthosilicate". San Francisco, California: Wikimedia Foundation, Inc. June 16, 2013. Retrieved 2013-09-02.
  3. 3.0 3.1 3.2 Willard Lincoln Roberts; George Robert Rapp Jr.; Julius Weber (1974). Encyclopedia of Minerals. New York, New York, USA: Van Nostrand Reinhold Company. pp. 693. ISBN 0-442-26820-3. 
  4. J.M. Duke; M. Bonardi (1982). "Chromian andradite from Reaume Township, Ontario". Canadian Mineralogist 20: 49-53. http://rruff.info/doclib/cm/vol20/CM20_49.pdf. Retrieved 2015-10-23. 
  5. "neosilicate". San Francisco, California: Wikimedia Foundation, Inc. June 15, 2013. Retrieved 2013-09-02.
  6. Martine van der Westhuizen (26 November 2010). "Mineral of the Month - Sphene or Titanite". Bothasig, Cape Town, South Africa: The Cape Town Gem & Mineral Club. Retrieved 2015-08-10.
  7. "sorosilicate". San Francisco, California: Wikimedia Foundation, Inc. June 19, 2013. Retrieved 2013-09-02.
  8. "Beryl". San Francisco, California: Wikimedia Foundation, Inc. April 15, 2013. Retrieved 2013-05-03.
  9. Warr, L.N. (2021). "IMA-CNMNC mineral symbols". Mineralogical Magazine 85: 291–320. http://cnmnc.main.jp/imacnmnc_approved_mineral_symbols.pdf. 
  10. 10.0 10.1 10.2 10.3 McDonald, A.M., Chao, G.Y., and Grice, J.D., 1994. Abenakiite-(Ce), a new silicophosphate carbonate mineral from Mont Saint-Hilaire, Quebec: Description and structure determination. The Canadian Mineralogist 32, 843-854
  11. 11.0 11.1 Mindat, Abenakiite-(Ce), Mindat.org
  12. "[International Mineralogical Association] : List of Minerals - IMA". Ima-mineralogy.org. Retrieved 2016-03-12.
  13. "inosilicate". San Francisco, California: Wikimedia Foundation, Inc. 21 June 2013. Retrieved 2013-09-02.
  14. "phyllosilicate". San Francisco, California: Wikimedia Foundation, Inc. June 17, 2013. Retrieved 2013-09-02.
  15. Paul G (21 September 2004). "clay mineral". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 26 April 2022.
  16. Goldenrowley (28 April 2007). "clay mineral". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 26 April 2022.
  17. Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. pp. 252–257. ISBN 9780195106916
  18. Deer WA, Howie RA, Zussman J (1992). An Introduction to the Rock-forming Minerals (2nd ed.). Harlow: Longman. ISBN 9780470218099. 
  19. Pohl, Walter L. (2011). Economic geology: principles and practice: metals, minerals, coal and hydrocarbons – introduction to formation and sustainable exploitation of mineral deposits. Chichester, West Sussex: Wiley-Blackwell. pp. 331. ISBN 9781444336627. https://books.google.com/books?id=Jq2rpN-6AccC. 
  20. Anthony JW, Bideaux RA, Bladh KW, Nichols MC, ed (1995). Kaolinite, In: Handbook of Mineralogy: Silica, silicates. Tucson, Arizona, USA: Mineral Data Publishing. ISBN 9780962209734. OCLC 928816381. http://www.handbookofmineralogy.org/pdfs/kaolinite.pdf. 
  21. Perry DL (2011). Handbook of Inorganic Compounds (2nd ed.). Boca Raton: Taylor & Francis. ISBN 9781439814611. OCLC 587104373. 
  22. "tectosilicate". San Francisco, California: Wikimedia Foundation, Inc. June 17, 2013. Retrieved 2013-09-02.
  23. Pinkfud (2 November 2004). "feldspar". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2015-07-30.
  24. 24.0 24.1 24.2 IMA-NA (30 July 2015). "What is Feldspar?". North America: Industrial Minerals Association - North America (IMA-NA). Retrieved 2015-07-30.
  25. Pinkfud (6 November 2004). "feldspathoid, In: Wiktionary". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2015-07-30.

External links[edit | edit source]