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.

In mineralogy, the crystalline forms of silica (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.

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 [AlSi3O− 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:

Major groupStructureChemical formulaExample
Nesosilicatesisolated silicon tetrahedra[SiO4]4−olivine, garnet, zircon...
Sorosilicatesdouble tetrahedra[Si2O7]6−epidote, melilite group
Cyclosilicatesrings[SinO3n]2n−beryl group, tourmaline group
Inosilicatessingle chain[SinO3n]2n−pyroxene group
Inosilicatesdouble chain[Si4nO11n]6n−amphibole group
Phyllosilicatessheets[Si2nO5n]2n−micas and clays
Tectosilicates3D 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) [SiO4]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 Benitoite – BaTi(Si3O9)
  • 4-member single ring Papagoite – CaCuAlSi2O6(OH)3.
  • 6-member single ring Beryl – Be3Al2(Si6O18) Bazzite – Be3Sc2(Si6O18) Sugilite – KNa2(Fe,Mn,Al)2Li3Si12O30 Tourmaline – (Na,Ca)(Al,Li,Mg)3–(Al,Fe,Mn)6(Si6O18)(BO3)3(OH)4 Pezzottaite – Cs(Be2Li)Al2Si6O18 Osumilite – (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30 Cordierite – (Mg,Fe)2Al4Si5O18 Sekaninaite – (Fe+2,Mg)2Al4Si5O18
  • 9-member single ring Eudialyte – Na15Ca6(Fe,Mn)3Zr3SiO(O,OH,H2O)3(Si3O9)2(Si9O27)2(OH,Cl)2
  • 6-member double ring Milarite – K2Ca4Al2Be4(Si24O60)H2O

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 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

  • Inosilicate, pyroxene family, with 2-periodic single chain (Si2O6), diopside
  • Inosilicate, clinoamphibole, with 2-periodic double chains (Si4O11), tremolite
  • Inosilicate, unbranched 3-periodic single chain of wollastonite
  • Inosilicate with 5-periodic single chain, rhodonite
  • Inosilicate with cyclic branched 8-periodic chain, pellyite

Phyllosilicates

Phyllosilicates (from Greek φύλλον phýllon 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedra 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. Many phyllosilicates are clay-forming and may be further classified as 1:1 clay minerals (one tetrahedral sheet and one octahedral sheet) and 2:1 clay minerals (one octahedral sheet between two tetrahedral sheets). Below are some major phyllosilicate mineral species and their chemical formulas, with group and series names in italics:

Kaolinite
  • Phyllosilicate, mica group, muscovite (red: Si, blue: O)
  • Phyllosilicate, single net of tetrahedra with 4-membered rings, apophyllite group
  • Phyllosilicate, single tetrahedral nets of 6-membered rings, pyrosmalite-(Fe)-pyrosmalite-(Mn) series
  • Phyllosilicate, single tetrahedral nets of 6-membered rings, zeophyllite
  • Phyllosilicate, double nets with 4- and 6-membered rings, carletonite

Tectosilicates

Silica family (SiO2 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 SiO2 in a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth. 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 are some major tectosilicate mineral species and their chemical formulas, with group and series names in italics:

See also

External links