Silicate minerals are rock-formingminerals made up ofsilicate groups. They are the largest and most important class of minerals and make up approximately 90 percent ofEarth's crust.[1][2][3]
Inmineralogy, the crystalline forms ofsilica (silicon dioxide,SiO2) are usually considered to betectosilicates, and they are classified as such in the Dana system (75.1). However, the Nickel-Strunz system classifies them asoxide minerals (4.DA). Silica is found in nature as the mineralquartz, and itspolymorphs.
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 partialmelting,crystallization,fractionation,metamorphism,weathering, anddiagenesis.
Diatomaceous earth, a biogenic form of silica as viewed under a microscope. The imaged region measures approximately 1.13 by 0.69 mm.
A silicate mineral is generally aninorganic 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. Thecoordination 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 extracations. For example, in the mineralorthoclase[KAlSi 3O 8] 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[SiO 2] n. Replacement of one in every four silicon atoms by analuminum atom results in the anion[AlSi 3O− 8] n, whose charge is neutralized by thepotassium cationsK+ .
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.
Orthosilicate anionSiO4− 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
Sorosilicates (from Greekσωρόςsōros 'heap, mound') have isolatedpyrosilicate anionsSi 2O6− 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:
Cyclosilicate specimens at the Museum of Geology, South DakotaPezzottaiteBazzite
Cyclosilicates (from Greekκύκλοςkýklos '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:
Inosilicates (from Greekἴςis [genitive:ἰνόςinos] 'fibre'), or chain silicates, have interlocking chains ofsilicatetetrahedra with eitherSiO3, 1:3 ratio, for single chains orSi4O11, 4:11 ratio, for double chains. The Nickel–Strunz classification is 09.D – examples include:
Phyllosilicates (from Greekφύλλονphýllon 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedra withSi2O5 or a 2:5 ratio. The Nickel–Strunz classification is 09.E. All phyllosilicate minerals arehydrated, with eitherwater orhydroxyl groups attached.
Tectosilicates, or "framework silicates," have a three-dimensional framework of silicatetetrahedra withSiO2 in a 1:2 ratio. This group comprises nearly 75% of thecrust of theEarth.[13] Tectosilicates, with the exception of the quartz group, arealuminosilicates. The Nickel–Strunz classifications are 9.F (tectosilicates without zeoliticH2O), 9.G (tectosilicates with zeoliticH2O), and 4.DA (quartz/silica group). Below is a list of tectosilicate minerals and their chemical formulas, organized by groups and series:
^Deer, W.A.; Howie, R.A., & Zussman, J. (1992).An introduction to the rock forming minerals (2nd edition ed.). London: LongmanISBN0-582-30094-0
^Hurlbut, Cornelius S.; Klein, Cornelis ||1985).Manual of Mineralogy, Wiley, (20th edition ed.).ISBN0-471-80580-7
^"Mica Group".mindat.org. Hudson Institute of Mineralogy. Retrieved24 February 2025.
^"Brittle Mica".mindat.org. Hudson Institute of Mineralogy. Retrieved24 February 2025.
^"Muscovite".mindat.org. Hudson Institute of Mineralogy. Retrieved24 February 2025.
^"Fuchsite".mindat.org. Hudson Institute of Mineralogy. Retrieved24 February 2025.
^"Illite".mindat.org. Hudson Institute of Mineralogy. Retrieved24 February 2025.
^"Mariposite".mindat.org. Hudson Institute of Mineralogy. Retrieved24 February 2025.
^"Phengite".mindat.org. Hudson Institute of Mineralogy. Retrieved16 March 2025.
^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.
^"Feldspar Group".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Oligoclase".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Andesine".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Labradorite".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Bytownite".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Hyalophane".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Rubicline".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Feldspathoid".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Danalite".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Tugtupite".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
^"Scapolite".mindat.org. Hudson Institute of Mineralogy. Retrieved23 February 2025.
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