Question

Classification of silicates.

Answer 1

Based on the way SiO₄^4- tetrahedral units are linked.

(a) Orthosilicates – Contain single discrete unit of SiO₄^4- unit eg. Zircon ZrSiO₄ 

(b) Pyrosilicates – Contain two units of SiO₄^4- joined along a corner containing oxygen atom.

Pyrosilicate ion is Si₂O₇^6-

(c) Cyclic structure – Cyclic or ring silicates have general formula (SiO₃^2-)_n or (SiO₃)_n^2n- . These are formed when two oxygen atoms of each SiO₄^4- tetrahedron are shared with others. Structures and examples of cyclic silicates containing SiO₉^6- and Si₆O₁₈¹² ions are given below.

(d) Chain silicates – If two oxygen atoms per tetrahedron are shared such that a linear single strand chain of the general formula (SiO₃^2-) or (SiO₃)_n^2- is formed, then the silicates containing these anions are called chain silicates.

(e) Sheet silicates – Three oxygen atoms of a tetrahedral SiO₄^4- are shared.

(f) Three dimensional sheet silicates – These silicates involve all four oxygen atoms in sharing with adjacent SiO₄^4- tetrahedral. Since, all the oxygen atoms are shared, the 4 silicates are neutral. The common examples are quartz, tridymite and cristobalite (forms of silica).