Structural Classification of Silicates | Geology Optional Notes for UPSC PDF Download

Structural Classification of Silicates

• Silicates are minerals that are abundant in Earth's crust.
• They consist of oxygen (O) and silicon (Si) atoms, which are the most common elements found in combination with other cations.
• The basic structural unit of silicates is the SiO₄ tetrahedron, which has a four-sided pyramidal form.
• Each silicon atom in the tetrahedron is tetravalent and is surrounded by four oxygen atoms at the corners of a regular tetrahedron.

SiO₄ Tetrahedron Structure

• The SiO₄ tetrahedron is composed of a silicon atom at the center bonded to four oxygen atoms.
• Each oxygen atom forms a corner of the tetrahedron.
• This structure results in the silicon atom carrying a net charge of +4.

Neutralizing Charge in Silicates

• For the SiO₄ tetrahedron to be electrically neutral, the +4 charge on silicon must be balanced.
• This balancing can be achieved in two ways:
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  • The silicon ion can bond with cations such as sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), or iron (Fe²⁺).
  • The silicon ion can share oxygen atoms with other SiO₄ tetrahedra.

Composition of Silicate Minerals

• All silicate minerals are constructed from silicon-oxygen tetrahedra as the fundamental unit, linked in various combinations as described above.

Unit 5: Classification of Minerals

Introduction

• Silicate minerals are categorized based on the arrangement of silica tetrahedra in their structures.

Nesosilicates (Island Silicates)

• These minerals consist of independent tetrahedral groups that are connected by cations.
• The tetrahedra are isolated from each other by cations, with an oxygen to silica ratio of 4:1.
• Examples include olivine group, garnet group, and zircon.

Sorosilicates (Double Tetrahedral Group)

• In these silicates, two tetrahedra are linked by a single oxygen atom, with an oxygen to silica ratio of 2:7 or 3.5:1.
• An example of this structure is seen in the epidote group and the melilite group.

Cyclosilicates (Ring Silicates)

• Tetrahedra in cyclosilicates share two oxygen atoms and form closed rings.
• These rings can consist of different numbers of tetrahedra sharing oxygen atoms.
• Oxygen to silica ratio is 3:1 in this type of silicate structure.
• Examples include minerals like benitoite, axinite, and beryl.

Summary

• Silicate minerals are classified based on the structure of silica tetrahedra.
• Nesosilicates have isolated tetrahedral groups, sorosilicates feature double tetrahedral groups, and cyclosilicates form ring structures.
• Understanding these classifications helps in identifying different types of silicate minerals.

Mineralogical Structures Summary

• Inosilicates or Chain Silicates

  • Single Chain Silicates (SiO3)²

    This type of structure forms open-ended chains by sharing oxygen atoms. Examples include the pyroxene group of minerals.

  • Double Chain Silicates (Si4O11)

    These structures consist of continuous double chains of tetrahedra. The amphibole group of minerals, such as hornblende, exhibits this structure.

• Phyllosilicates or Sheet Silicates [(Si2O5)2]

  Sheet silicates involve tetrahedra sharing three oxygen atoms with adjacent tetrahedra, forming stacked sheets. Common examples are micas and clay minerals.

• Tectosilicates or Framework Silicates [(SiO2)]

  These silicates form a three-dimensional framework where each tetrahedron shares all its oxygens. Important minerals in this group include feldspars, quartz, feldspathoids, and zeolites.

Structural Classification of Silicates

• Nesosilicates (orthosilicates or island silicates)
• Sorosilicates
• Cyclosilicates (or ring silicates)
• Inosilicates (or Chain silicates)
• Phyllosilicates (or Sheet silicates)
• Tectosilicates (or Framework silicates)

Summary of Structural Classification of Silicates

• Nesosilicates: These silicates consist of independent or isolated tetrahedral groups.
• Sorosilicates: Characterized by closed ring structures.
• Cyclosilicates: Have double tetrahedral (Si₂O₇)-6 groups.
• Inosilicates: Feature single or double chain structures.
• Phyllosilicates: Known for sheet structures (SiO₄)-4 or -12.
• Tectosilicates: Have a framework structure (SiO₂)⁰ with varying oxygen to silica ratios.

Examples of Silicate Structures

• Nesosilicates: Olivine group, garnet group, zircon, benitoite, axinite, beryl.
• Cyclosilicates: Pyroxene group.
• Inosilicates: Amphibole group.
• Phyllosilicates: Micas and clay minerals.
• Tectosilicates: Feldspars, quartz, feldspathoids, zeolite group.