Silicates and its Classification

Silicates:

Silicates form an important class of minerals that constitute rocks and soil. Example- micas, clays, feldspars, asbestos etc. According to X-ray crystallographic studies, all silicates have SiO₄^4- units formed through SP³ hybridization and thus have tetrahedral structure as shown below-

The electronic structure of SiO₄^4- ion is written as-

This structure shows that the octet of Si is complete while each of the four O-atoms is short of one electron and have a tendency to get electrons from some other atoms and so acquire negative charges forming discrete SiO₄^4- ions. Example- Zircon (ZrSiO₄) and Wilemite (Zn₂SiO₄).

Classification of Silicates:

Silicates differ from one another in the manner in which SiO₄^4- tetrahedra are joined together. The silicates are classified into the following different types depending upon the number of corners (0, 1, 2, 3, or 4) of the SiO₄^4- tetrahedron shared with the other tetrahedra.

• Orthosilicates- These are simple silicates in which SiO₄^4- tetrahedra do not share corners (O-atoms) with one another and exist as discrete SiO₄^4- orthosilicate anions as shown in the figure. Example- Zircon (ZrSiO₄), Forestrite (Mg₂SiO₄), Zn₂SiO₄. In these compounds, SiO₄^4- ion is linked to M²⁺ ions through a coordinate bond.

• Pyrosilicates or Islands- When two SiO₄^4- tetrahedra join through a single common oxygen atom (share a corner), Island structures having the formula Si₂O₇^6- are obtained known as Pyrosilicates. Example- Thortveitite i.e. Sc₂(Si₂O₇), Hemimorphite i.e. Zn₃ (Si₂O₇) Zn(OH)₂.H₂O.

• Cyclic or Ring Silicates- These are those silicates in which two O-atoms of each tetrahedron are shared with others to form closed rings that starts with general formula (Si_nO_3n)^2n-. Example- Wollastonite (Ca₃Si₃O₉), Beryl (Be₃Al₂Si₆O₁₈).

• Chain Silicates- If two O-atoms per SiO₄^4- tetrahedron are shared such that a linear single strand chain of the general formula (Si_nO_3n)^2- is formed known as chain silicates. Thus, the primary unit of this chain structure is SiO₃^2-. Silicates of this type are called Pyroxenes or metasilicates. Example- Enstatite (MgSiO₃), Diopside [CaMg(SiO₃)₂], Spodumene [LiAl(SiO₃)₂]. Two chains may be linked together to form double chains, thereby having the formula (Si₄O₁₁)_n^6n- known as Amphiboles which are present in asbestos.

• Sheet Silicates- The sharing of three corners (i.e. three oxygen of each tetrahedron) result in an infinite two-dimensional sheet structure of the formula (Si₂O₅)_n^2n- known as Sheet Silicates. Example- Clay, Mica etc.

• Three-dimensional Silicates- These are those silicates in which all the four O-atoms of SiO₄^4- tetrahedron are shared with other tetrahedra resulting in a three-dimensional network structure. The empirical formula for this type of silicate is (SiO₂)_n. Example- Quartz (Silica).