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UPSC MainsGEOLOGY-PAPER-II201720 Marks
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Q6.

Describe the structural classification of silicates with neat sketches and mineralogical examples.

How to Approach

This question requires a detailed understanding of silicate structural classification, a core concept in mineralogy. The answer should begin with a definition of silicates and their importance. Then, systematically describe each structural class – Nesosilicates, Sorosilicates, Cyclosilicates, Inosilicates (single & double chains), and Phyllosilicates – with neat sketches illustrating their basic building blocks and provide relevant mineralogical examples for each. A clear, organized presentation with diagrams is crucial for scoring well.

Model Answer

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Introduction

Silicates are the most abundant group of minerals in the Earth’s crust, constituting approximately 90% of its mass. Their fundamental building block is the silicon-oxygen tetrahedron (SiO₄)⁴⁻. These tetrahedra can be arranged in various ways, leading to a diverse range of silicate structures and, consequently, a wide variety of silicate minerals. The classification of silicates is based on the arrangement of these tetrahedra – whether they are isolated, linked, or share oxygen atoms. Understanding this structural classification is fundamental to comprehending the physical and chemical properties of silicate minerals and their geological significance.

Structural Classification of Silicates

Silicates are classified based on how the SiO₄ tetrahedra are linked together. This linkage determines the overall structure and properties of the silicate mineral.

1. Nesosilicates (Isolated Tetrahedra)

In nesosilicates, the SiO₄ tetrahedra are isolated and linked by interstitial cations. They do not share any oxygen atoms. This results in a relatively open structure.

Nesosilicate Structure
  • Structure: Isolated SiO₄ tetrahedra linked by cations.
  • Formula: SiO₄
  • Mineral Examples: Olivine ((Mg,Fe)₂SiO₄), Garnet (X₃Y₂(SiO₄)₃ where X and Y are cations).

2. Sorosilicates (Double Tetrahedra)

Sorosilicates consist of two SiO₄ tetrahedra sharing one oxygen atom, forming a Si₂O₇⁶⁻ group. These groups are linked by cations.

Sorosilicate Structure
  • Structure: Two tetrahedra sharing one oxygen.
  • Formula: Si₂O₇
  • Mineral Examples: Epidote (Ca₂(Al,Fe)₃(SiO₄)₃(OH)), Hemimorphite (Zn₄Si₂O₇(OH)₂·H₂O).

3. Cyclosilicates (Ring Silicates)

Cyclosilicates are characterized by SiO₄ tetrahedra linked in rings. Common ring sizes include three, four, or six tetrahedra.

Cyclosilicate Structure
  • Structure: Tetrahedra linked in rings.
  • Formula: (SiₓO₃ₓ)ⁿ⁻
  • Mineral Examples: Beryl (Be₃Al₂Si₆O₁₈), Tourmaline ((Na,Ca)(Al,Li,Mg)₃(Al,Fe,Mn)₆(BO₃)₃Si₆O₁₈(OH)₄).

4. Inosilicates (Chain Silicates)

Inosilicates are formed by SiO₄ tetrahedra linked in chains. There are two main types:

  • Single Chain Inosilicates: Tetrahedra linked in a single chain, sharing two oxygen atoms.
    • Single Chain Inosilicate Structure
  • Formula: (Si,Al)₂O₆
  • Mineral Examples: Pyroxenes (e.g., Augite ((Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆)).
  • Double Chain Inosilicates: Two single chains linked together, sharing two or three oxygen atoms.
    • Double Chain Inosilicate Structure
  • Formula: (Si,Al)₄O₁₁
  • Mineral Examples: Amphiboles (e.g., Hornblende ((Ca,Na)₂(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂)).

5. Phyllosilicates (Sheet Silicates)

Phyllosilicates are characterized by SiO₄ tetrahedra linked in sheets, sharing three oxygen atoms. These sheets are weakly bonded together, resulting in a layered structure.

Phyllosilicate Structure
  • Structure: Tetrahedra linked in sheets.
  • Formula: (Si,Al)₂O₅
  • Mineral Examples: Micas (e.g., Muscovite (KAl₂(AlSi₃O₁₀)(OH)₂), Biotite (K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂)), Clay minerals (e.g., Kaolinite (Al₂Si₂O₅(OH)₄)).

Conclusion

The structural classification of silicates provides a fundamental framework for understanding the diversity of silicate minerals. The arrangement of SiO₄ tetrahedra dictates the physical and chemical properties of these minerals, influencing their behavior in geological processes. From isolated tetrahedra in nesosilicates to extensive sheets in phyllosilicates, each structural class exhibits unique characteristics that are crucial for interpreting Earth’s composition and history. Further research into silicate structures continues to refine our understanding of mineral formation and evolution.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Polymorphism
The ability of a solid chemical compound to exist in more than one crystal structure. Silicates often exhibit polymorphism, meaning the same chemical composition can result in different mineral forms due to varying pressure and temperature conditions.
Cation Substitution
The replacement of one cation by another of similar size and charge within the silicate structure. This is a common phenomenon that leads to variations in mineral composition and properties.

Key Statistics

Approximately 90% of the Earth’s crust is composed of silicate minerals.

Source: Winter, J. D. (2014). Fundamentals of Igneous and Metamorphic Petrology. Pearson Education.

Feldspars, a group of aluminosilicate minerals, constitute approximately 60% of the Earth’s crust.

Source: Deer, W. A., Howie, R. A., & Zussman, J. (1992). An Introduction to the Rock-Forming Minerals. Longman Scientific & Technical.

Examples

Asbestos

Asbestos is a group of naturally occurring fibrous silicate minerals belonging to the phyllosilicate group. Historically used for its heat resistance and insulating properties, its use has been significantly restricted due to health concerns related to asbestos exposure.

Frequently Asked Questions

▶How does the silica content affect the structure of silicates?

Higher silica content generally leads to more complex silicate structures with greater polymerization (more shared oxygen atoms). This results in stronger, more rigid frameworks, as seen in the transition from nesosilicates to phyllosilicates.

Topics Covered

GeologyMineralogyGeochemistryMineral structuresSilicate chemistryMineral properties
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