Silicates

What is Silicate?

Oxygen, silicon, and aluminium are the most abundant elements in the surface of the earth; more than 80% of the atoms in the solid crust are oxygen or silicon, mostly in the form of silicates.

Silicates are the minerals containing silicon and oxygen in tetrahedral SiO₄^4- units, which are linked together in several patterns. About 95% of the earth’s crust is composed of silicate minerals, aluminosilicate clays or silica.

Silicates are extremely important because the cement, ceramic and glass industries are based on their chemical composition. Metallurgical extraction processes often produce silicates as waste products or slag either because the minerals are silicates.

Table of Contents

• Basic structure of Silicates
• Occurance of silicates in the earth crust
• Preparation of Silicates
• Classification of silicates
• Uses of Silicates
• FAQs

Basic structure of Silicates

The X-ray crystallographic method suggests the structural principle in silicate structure.

• The electronegativity difference between oxygen and silicon (3.5-1.8 =1.7), suggests that the bonds between O and Si are 50% ionic and 50% covalent in nature.
• The radius ratio of Si⁴⁺/ O^2- = 0.29, suggests that silicon is four coordinated and it is surrounded by four oxygen atoms at the corners of a tetrahedron.
• The SiO₄^4- tetrahedral may exist as discrete units or may polymerize into large units by sharing corners with oxygen atoms.
• The oxygen atoms form a close-packed structure where both have an octahedral and tetrahedral void. Most metal ions are the right size to fit one type of void. Whereas Al can occupy both tetrahedral and octahedral void.

Occurrence of silicates in the earth’s crust

• About 95% of the earth’s crust is composed of silicate minerals, aluminosilicate clays or silica.
• These are found in all kinds of rock, sand, clay, soil, granite, bricks, cement, ceramic and glass.
• Silicate minerals are rock-forming minerals made up of silicate groups. These are

Preparation of Silicates

Sicates can be prepared by fusing an alkali metal carbonate with sand in an electric furnace at about 1400⁰C.

Classification of silicates

Depending on the way the tetrahedral units are linked, the silicates are classified into the following types.

(i) Orthosilicates

(ii) Pyrosilicates

(iii) Cyclic silicates

(iv) Chain silicates

(v) Double chain silicates

(vi) Sheet silicates

(vii) Three-dimensional silicates

(i) Orthosilicates

Ortho silicates are the simplest silicates which contain discrete SiO₄^4-tetrahedral units, that is they share no corner oxygen. These are also known as Neso or Island silicates. Examples of Orthosilicates: Willemite (Zn₂SiO₄ ) , Forsterite (Mg₂SiO₄), Phenacites (Be₂SiO₄) etc.

The basic structural unit of the ortho silicate unit is shown below.

(ii) Pyrosilicates

In these types of silicates, two tetrahedral units are joined by sharing the O at one corner. The structural units of pyrosilicates are (Si₂O₇)^6-. This is the simplest of condensed silicate ions. These are also known as Soro silicate or disilicate. Examples of pyrosilicates are Thortveitite Sc₂[Si₂O₇], and Hemimorphite Zn₄(OH)₂[Si₂O₇]. H₂O. The structure of pyrosilicate is shown below:

(iii) Cyclic silicates

In these types of silicates, two oxygen atoms per tetrahedral unit are shared to form a ring type of structure. The general formula of cyclic silicate is (SiO₃)_n^2n-. Rings containing three, four, six and eight tetrahedral units are known. But three and six are more common. Examples of Cyclic silicates are : Wollastonite Ca₃[Si₃O₉], and Beryl Be₃Al₂[Si₆O₁₈]. The structure of cyclic silicate is shown below:

(iv) Chain silicates

Simple chain silicates are formed by sharing oxygen atoms on two corners of each tetrahedral.

The general formula of cyclic silicate is (SiO₃)_n^2n- . Example: Spodumene – LiAl(SiO₃)₂, Diopside – CaMg(SiO₃)₂ . The structure of Chain silicate is shown below:

(iv) Double chain silicates

Double chains can be formed when two simple chain silicates are joined together by shared oxygens. These are called amphiboles and they are well known. The structural unit of amphiboles are (Si₄O₁₁)_n^6n- . Example tremolite Ca₂Mg₅[(Si₄O₁₁)₂](OH)₂.

(vi) Sheet silicates

When SiO₄ units share three corners an infinite two-dimensional sheet is formed called Sheet silicates. The empirical formula is (Si₂O₅)_n^2n-. There are strong bonds within the Si-O sheet, but much weaker forces hold each sheet to the next one. Thus the minerals tend to cleave into thin sheets.

(vii) Three-dimensional silicates

In the structure of the three-dimensional silicate all the four corner oxygen atoms of SiO₄ units are shared with others. The three-dimensional lattice formula is SiO₂ (quartz, tridymite etc.)

Uses of Silicates

• Alkali silicates are used mainly as glue.
• Silicates are extremely important in the cement, ceramic and glass industries.
• Quartz is another example of silicates with an ideal capability to give a high rhythmic frequency vibration. These characteristics make quartz crystals to be used in radios, pressure gauges, and watches.
• Industries use silicates to treat wastewater as an iron flocculent and alum coagulant.