Silicate mineral


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 the Earth's crust.
In mineralogy, silica SiO2 is usually considered a silicate 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.
Living organisms also contribute to this geologic cycle. For example, a type of plankton known as diatoms construct their exoskeletons from silica extracted from seawater. The frustules of dead diatoms are a major constituent of deep ocean sediment, and of diatomaceous earth.

General structure

A silicate mineral is generally an ionic compound whose anions consist predominantly of silicon and oxygen atoms.
In most minerals in the Earth's crust, each silicon atom is the center of an ideal tetrahedron, whose corners are four oxygen atoms covalently bound to it. Two adjacent tetrahedra may share a vertex, meaning that the oxygen atom is a bridge connecting the two silicon atoms. An unpaired vertex represents an ionized oxygen atom, covalently bound to a single silicon atom, that contributes one unit of negative charge to the anion.
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, 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. Replacement of one in every four silicon atoms by an aluminum atom results in the anion, whose charge is neutralized by the potassium cations.

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 tetrahedra4−olivine
Sorosilicatesdouble tetrahedra6−epidote, melilite group
Cyclosilicatesrings2n−tourmaline group
Inosilicatessingle chain2n−pyroxene group
Inosilicatesdouble chain6n−amphibole group
Phyllosilicatessheets2n−micas and clays
Tectosilicates3D frameworkx−quartz, feldspars, zeolites

Note that tectosilicates can only have additional cations if some of the silicon is replaced by an atom of lower valence such as aluminium. Al for Si substitution is common.

Nesosilicates or orthosilicates

Nesosilicates, or orthosilicates, have the orthosilicate ion, which constitute isolated 4− tetrahedra that are connected only by interstitial cations. The Nickel–Strunz classification is 09.A –examples include:
crystals
Sorosilicates have isolated pyrosilicate anions , 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, or ring silicates, have three or more tetrahedra linked in a ring. The general formula is 2x−, where one or more silicon atoms can be replaced by other 4-coordinated atom. The silicon:oxygen ratio is 1:3. Double rings have the formula 2x− or a 2:5 ratio. The Nickel–Strunz classification is 09.C. Possible ring sizes include:
Some example minerals are:
Note that 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, 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

Phyllosilicates, 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.
Examples include:
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 09.F and 09.G, 04.DA. Examples include: