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:

• 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.

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: 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.
• Examples: Epidote group and 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: 3:1.
• Examples: 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.
• Cyclosilicates: Form ring structures.

Mineralogical Structures

Inosilicates or Chain Silicates

1. Single Chain Silicates (SiO3)²:

• This type of structure forms open-ended chains by sharing oxygen atoms.
• Examples: Pyroxene group of minerals.

2. Double Chain Silicates (Si4O11):

• These structures consist of continuous double chains of tetrahedra.
• Examples: Amphibole group of minerals, such as hornblende.

3. Phyllosilicates or Sheet Silicates [(Si2O5)2]

• Sheet silicates involve tetrahedra sharing three oxygen atoms with adjacent tetrahedra, forming stacked sheets.
• Examples: Micas and clay minerals.

4. Tectosilicates or Framework Silicates [(SiO2)]

• These silicates form a three-dimensional framework where each tetrahedron shares all its oxygens.
• Examples: Feldspars, quartz, feldspathoids, and zeolites.

Structural Classification of Silicates

• Nesosilicates (Orthosilicates or Island Silicates): Consist of independent or isolated tetrahedral groups.
• Sorosilicates: Feature double tetrahedral groups (Si₂O₇)⁶.
• Cyclosilicates (Ring Silicates): Have closed ring structures formed by tetrahedra sharing oxygen atoms.
• Inosilicates (Chain Silicates): Include single or double chain structures.
• Phyllosilicates (Sheet Silicates): Known for sheet structures where tetrahedra share three oxygen atoms.
• Tectosilicates (Framework Silicates): Comprise a three-dimensional 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.