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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q8(b)(ii). Classification of silicates; SiO₄⁴⁻, Si₂O₇⁶⁻ and Si₃O₉⁶⁻

  • 10 marks
  • Answered
Question

How are silicates classified? Predict the names and structures of the following silicates:
(1) SiO₄⁴⁻ ion, (2) Si₂O₇⁶⁻ ion, (3) Si₃O₉⁶⁻ ion

Answer in brief

By the number of O corners each SiO₄ tetrahedron shares with others (0 to 4). (1) SiO₄⁴⁻: orthosilicate, one discrete tetrahedron; (2) Si₂O₇⁶⁻: pyrosilicate, two tetrahedra sharing one O; (3) Si₃O₉⁶⁻: cyclic trisilicate, a ring of three tetrahedra each sharing two O.

Key points

  • SiO₄⁴⁻, orthosilicate (nesosilicate). One discrete tetrahedron, Si at the centre, four terminal O each carrying -1: charge 4(-1)=-4.
  • Si₂O₇⁶⁻, pyrosilicate (sorosilicate, disilicate). Two tetrahedra share one corner, O₃Si-O-SiO₃. Six terminal O: charge -6. Check: 2(+4)+7(-2)=-6.
  • Si₃O₉⁶⁻, cyclic trisilicate (cyclosilicate). Three tetrahedra each share two corners and close a ring of three tetrahedra (an Si₃O₃ ring of alternating Si and O). Each Si keeps two terminal O, six in all: charge -6. Check:…

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Principle: silicon is always four-coordinate. Each terminal O contributes -1 to the charge and each shared O contributes 0, so sharing more corners lowers both the O: Si ratio and the charge per Si.

ClassO shared per SiAnionExamples
Ortho (neso)0SiO₄⁴⁻peridot (olivine) Mg₂SiO₄, phenakite Be₂SiO₄, garnet Mg₃Al₂(SiO₄)₃
Pyro (soro)1Si₂O₇⁶⁻the disilicate ion [O₃Si-O-SiO₃]⁶⁻
Cyclic (cyclo)2(SiO₃)ₙ²ⁿ⁻: Si₃O₉⁶⁻, Si₆O₁₈¹²⁻beryl Be₃Al₂Si₆O₁₈ (emerald when Cr³⁺ replaces some Al³⁺)
Single chain (ino)2(SiO₃)ₙ²ⁿ⁻pyroxenes; jadeite NaAl(SiO₃)₂
Double chain (ino)2 and 3 alternately(Si₄O₁₁)ₙ⁶ⁿ⁻amphiboles; tremolite Ca₂(OH)₂Mg₅(Si₄O₁₁)₂
Sheet (phyllo)3(Si₂O₅)ₙ²ⁿ⁻talc Mg₃(OH)₂Si₄O₁₀, kaolinite Al₂(OH)₄Si₂O₅, micas
Framework (tecto)4SiO₂ (Al for Si gives a negative framework)quartz; feldspars; zeolites

The three ions

  1. SiO₄⁴⁻, orthosilicate (nesosilicate). One discrete tetrahedron, Si at the centre, four terminal O each carrying -1: charge 4(-1)=-4.
  2. Si₂O₇⁶⁻, pyrosilicate (sorosilicate, disilicate). Two tetrahedra share one corner, O₃Si-O-SiO₃. Six terminal O: charge -6. Check: 2(+4)+7(-2)=-6.
  3. Si₃O₉⁶⁻, cyclic trisilicate (cyclosilicate). Three tetrahedra each share two corners and close a ring of three tetrahedra (an Si₃O₃ ring of alternating Si and O). Each Si keeps two terminal O, six in all: charge -6. Check: 3(+4)+9(-2)=-6.

Why cyclic and not a chain: Si₃O₉⁶⁻ has the same ratio, SiO₃²⁻, as a pyroxene chain. A discrete ion with a fixed formula must close on itself, so it is the ring of three tetrahedra.

Application. silicate structure decides use: chain and double-chain minerals are fibrous (asbestos), sheet silicates are soft and cleave (talc, mica, china clay), and Al-substituted frameworks with cages are the zeolites used as molecular sieves, ion exchangers and cracking catalysts.

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Model answer to Q8(b)(ii), 2026 Paper I: solution page 51
Solution page 51 of 55
Read the whole 2026 solution2026 question paper

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