Silicates: Structure, Types and Properties

Silicates constitute the most important mineral class, due to their abundance on Earth. Silicates account for around 25% of all known minerals and 40% of the most common ones.

• Igneous rocks, which make up more than 90% of the Earth's crust, are nearly all silicate. 
• The silicon-oxygen (SiO₄)^4- tetrahedron acts as the fundamental unit of all silicate structures.
• It consists of a central silicon cation (Si⁴⁺) connected to four oxygen atoms placed at the corners of a regular tetrahedron.
• The strong silicon-oxygen bonds of these tetrahedrons hold the terrestrial crust together.
• The bonds, which are approximately 50% ionic and 50% covalent, form as a result of the attraction of oppositely charged ions and the sharing of electrons.

Key Terms: Silicate, Silicate mineral, Oxygen, Silicon, Earth’s crust, Minerals, Silica, Oxide, Sulphide, Sulphates, Ionic bond, Electronegativity, Hybridization

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What is Silicate?

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The elements that are most abundant on the surface of the earth are oxygen, silicon, and aluminium; more than 80% of the atoms in the solid crust are oxygen or silicon, most of which are silicates.

• Silicates are minerals composed of silicon and oxygen in tetrahedral SiO₄^4- units that are bonded together in various ways.
• Approximately 95% of the earth's crust is made up of silicate minerals, aluminosilicate clays, or silica.
• Silicates are especially important since their chemical composition underlies the cement, ceramic, and glass industries.
• Metallurgical extraction methods usually produce silicates as waste products or slag, either because the minerals are silicates.

What are Silicate Minerals?

Silicate minerals are minerals composed of silicate groups.

• They have highly complex structures.
• They can be divided into four major groups: isolated tetrahedra, chains of tetrahedra, sheets, and framework.
• Non-silicate minerals are minerals that do not contain silicate groups.
• They have comparatively fewer complex structures.
• They can be found in six different forms namely oxides, sulphides, sulphates, halides, phosphates and carbonates.

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Structure of Silicates

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The basic structure of silicates is shown below

The X-ray crystallographic approach suggests a structural principle in silicate structures.

• The electronegativity difference between oxygen and silicon (3.5-1.8 =1.7) indicates that O-Si bonds are 50% ionic and 50% covalent in nature.
• The radius ratio of Si⁴⁺/O^2- = 0.29 indicates that silicon is four-coordinated and surrounded by four oxygen atoms at the corners of a tetrahedron.
• The SiO₄^4- tetrahedron can exist as isolated units or polymerize into larger units by sharing corners with oxygen atoms.
• The oxygen atoms make a closely packed structure with octahedral and tetrahedral voids.
• Most metal ions are the appropriate size to fill a specific type of void. Al can occupy both tetrahedral and octahedral voids. 

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Importance of Silicates

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The following are the importance of silicates

• It is found in abundance on Earth. 
• It is of economic use in the present-day society.
• It is present in 25 per cent of all the known minerals.
• Our earth’s crust is made up of 90 per cent silicates.
• It is composed of dominant elements in the earth’s crust ( O, si, Al, Fe, Mg, Ca, Na, K )
• It is used in the construction of buildings such as in bricks, stones, mortar, and glass.
• We use silicates in the latest technology for silicon chips.

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Chemical Properties of Silicates

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The following are the chemical properties of silicates

• Silicates contain salt anions of silicon (si) and also the anions of oxygen (O).
• Each Si forms a single covalent bond with 4 oxygens.
• It has sp³ hybridization.
• The Si-O bond length is too long to form a Si double bond O.

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Physical Properties of Silicates

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The physical properties of different silicates are given below

Olivine

The following are the physical properties of Olivine

• Olivine has a high density.
• It has a very high melting point of 100 – 120 ^oC
• The silica content in the silicate is low which is around 50 per cent.
• It has a viscosity of a very low range.

Quartz

The following are the physical properties of Quartz 

• It has a very low density.
• The melting point of the quartz is low in the range of 60 – 80 ^oC
• The amount of silica in quartz is high, around 70 per cent.
• It has high viscosity.

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Types of Silicates

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The different types of silicates are given below

Orthosilicates

Orthosilicates are minerals having only single SiO₄^4- units. For example, Be and Zn ions are bound tetrahedrally to the oxygen of silicate in these two minerals: phenacite, B and willemite, Zn₂SiO₄.

• In olivine, (Fe, Mg)₂SiO₄, the cations are either of Fe²⁺ or Mg²⁺.
• This formula suggests that this mineral is a mixed form of salt of iron and magnesium silicates.
• These cations are coordinated octahedrally with the oxygen atoms of the silicates.
• Pure salt Fe₂SiO₄ is called the fayalite, and Mg₂SiO₄ is called the forsterite.

Pyro silicates

When two of the SiO₄^4- units are combined together, then they form the pyrosilicates group, Si₂O₇⁶. For example, thortveitite, Sc₂Si₂O₇ is a pyro silicate.

Ring and chain silicates

Whenever the two oxygens of the SiO₄^4- units are shared with the other SiO₄^4- units, then the silicates form the ring or it will form an infinite chain.

• The stoichiometry of the silicates becomes (SiO₃)_n^2n-.
• Benitoite BaTi(SiO₃)₃ contains three silica rings, but these are relaxed 6-atom rings Single chain of silica is called pyroxenes.

Double chain silicates

Double chains can form when two simple chain silicates share oxygens. Amphiboles are well-known. The structural unit of amphiboles is (Si₄O₁₁)_n^6n-. Tremolite Ca₂Mg₅[(Si₄O₁₁)₂](OH)₂.

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Things to Remember

• Silicon belongs to the 14th group of the periodic table.
• Silicates are the compounds in which silicon is combined with the oxygen atoms.
• The atomic configuration of the silicon is [Ne] 3s² 3p².
• Silicates generally have a tetrahedral structure. The Si-O bond length is from 0.161 to 0.164 nm.
• These bonds have a dissociation energy of nearly 452 KJ/mol.
• The angle between the bonds is determined as 109 28’.
• Most of the rocks are made up of silicates, especially igneous rocks.

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