Structure and classification of Silicates

Introduction

In all silicates the silicon atoms are in four fold coordination with oxygen. This arrangement appears to be universal in these compounds and the bonds between silicon and oxygen are so strong that the four oxygen are always af the corners of a tetrahedron of nearly constant dimension and regular shape. The different silicate types arise from the various ways in which these silicon-oxygen tetrahedral are related to one another whatever the rest of the structure may be like.

The silicate classification is based on the following types of linkage -

1. Independent Tetrahedra Group (Nesosilicates)

In this group the silicon oxygen tetrahedra are present as separate entities. The resulting composition is SiO4. The typical mineral is forsterite (olivine). This division of silicate is known as Nesosilicate. Si:O = 1:4.

2. Double Tetrahedra Structure (Sorosilicate)

In this group the two silicon oxygen tetrahedra are linked by sharing of one oxygen between them. The resulting composition is Si2O7. The typical mineral is hemimorphite. These silicates are known as borosilicate.  Si:O = 2:7.

3. Ring Structure (Cyclosilicate)

In this type two of the oxygen of each tetrahedron are shared with neighbouring tetrahedra and angular position of the tetrahedra are such that closed unit of a ring like structure results. Rings of three, four and six tetrahedra are known as cyclosilicates. The typical examples are benitoit (3tetrahedra), axinite (4 tetrahedra), beryl (6 tetrahedra). Si:O = 1:3.

4. Chain Structure (Inosilicate)

In this structure tetrahedra are linked together to produce chain of indefinite extension. There are two types of chain structures.

A) Single chain tetrahedra

In this case Si:O= 1:3.

E.g. Pyroxene group of minerals.

B) Double chain tetrahedra

Here two parallel single chains are cross linked. These chains are indefinite in extension and are bounded each other by the metallic elements. Si:O=4:11.

E.g. Amphibole group of minerals.

5. Sheet structure (Phyllosilicates)

In this case three oxygen of each tetrahedron are shared with adjucent tetrahedra to form extended flat sheets. This is a double chain inosilicate structure extended indefinitely in two directions instead of only one. The ratio is Si:O= 2:5. E.g. Mica group of minerals.

6. Three dimensional network (Tectosilicates)

In this structure every SiO4 tetrahedron shared all its corners with other tetrahedra giving a three dimensional network in which Si:O = 1:2. All forms of silica have this structure. E.g. Quartz, Feldspar.