Class 11 Chapter 6 Geometrical Shape Cleavage Plane Habit of Crystal Crystal Growth

By Tariq Pathan | Published: June 25, 2025

Table of Contents

  1. Introduction
  2. Geometrical Shape of Crystals
  3. Cleavage Plane
  4. Habit of Crystal
  5. Crystal Growth
  6. Conclusion

Introduction

In Chapter 6 of Class 11 Chemistry, we explore the external morphology and internal features of crystalline solids. Key topics include the ideal geometrical shapes of crystals, their planes of weakness (cleavage), common habits, and the process by which crystals grow.

Geometrical Shape of Crystals

Crystals exhibit characteristic geometric forms determined by their internal lattice symmetry. Each crystal system has representative shapes:

  • Cubic: Cube, octahedron (e.g., halite, diamond)
  • Tetragonal: Square prism, bipyramid (e.g., zircon)
  • Orthorhombic: Rhombic prism, dipyramid (e.g., olivine)
  • Hexagonal/Trigonal: Hexagonal prism, rhombohedron (e.g., quartz, calcite)
  • Monoclinic/Triclinic: Slanted prisms, pinacoidal forms (e.g., gypsum, turquoise)

The ideal geometric shape is often modified by growth conditions, yielding imperfect but characteristic forms.

Cleavage Plane

Cleavage is the tendency of a crystal to break along specific crystallographic planes where atomic bonding is weakest.

Key Points:

  • Perfect Cleavage: Smooth, flat surfaces (e.g., mica splits into thin sheets).
  • Good Cleavage: Visible but less smooth planes (e.g., feldspar).
  • Poor Cleavage: Uneven breakage (e.g., quartz shows conchoidal fracture instead).
  • Directional: Described by Miller indices, e.g., {001}, {010}.

Habit of Crystal

Crystal habit refers to the common external appearance of a mineral specimen, influenced by growth environment and space constraints.

Habit TypeDescriptionExample
PrismaticElongated prismsBeryl, tourmaline
TabularFlat, plate-likeGraphite
AcicularNeedle-likeNatrolite
DendriticTree-like branchingNative silver
BotryoidalGlobular clustersHematite

Other habits include fibrous, massive, and granular forms, each reflecting growth kinetics and chemistry.

Crystal Growth

Crystal growth occurs by the addition of ions or molecules to active growth sites on a nucleus.

Stages of Growth:

  1. Nucleation: Formation of a stable microscopic cluster.
  2. Growth: Addition of building units to facets, edges, and corners.
  3. Termination: Growth slows as reactants are depleted or conditions change.

Factors Affecting Growth:

  • Supersaturation level of the solution or vapor
  • Temperature and pressure
  • Presence of impurities or inhibitors
  • Space and time available for growth

Conclusion

Understanding the geometrical shapes, cleavage planes, habits, and growth mechanisms of crystals provides insight into material properties and behaviors. These concepts are foundational in solid state chemistry and materials science.

Categories: Chemistry, Solid State Chemistry, Class 11 Notes

Tags: Geometrical Shape, Cleavage Plane, Crystal Habit, Crystal Growth, Chapter 6, Tariq Pathan

Introduction

Symmetry in crystalline solids underpins their classification into crystal systems and dictates many of their physical properties. In Chapter 6 of Class 11 Chemistry, you’ll learn how symmetry elements and operations define the repeating patterns in crystal lattices, and how these influence properties such as cleavage, optical behavior, and electrical conductivity.

Symmetry Elements & Operations

Rotation Axes (n-fold)

A rotation axis allows a crystal to be rotated by 360°/n and appear unchanged. Common axes are 2-, 3-, 4-, and 6-fold. For example, a 4-fold axis means a rotation of 90° maps the lattice onto itself.

Mirror Planes (σ)

A mirror plane reflects one half of the crystal onto the other. Planes can be vertical, horizontal, or diagonal relative to the unit cell axes.

Centre of Inversion (i)

An inversion center maps each point (x, y, z) to (–x, –y, –z). If present, the crystal is centrosymmetric, affecting properties like piezoelectricity (which requires non-centrosymmetry).

Rotation–Reflection Axes (Sn)

Also called improper axes, Sn combines rotation about an axis and reflection through a plane perpendicular to that axis. For instance, S4 rotates by 90° then reflects.

Seven Crystal Systems

System Axes & Angles Example
Cubic a = b = c; α = β = γ = 90° NaCl, Diamond
Tetragonal a = b ≠ c; α = β = γ = 90° Sn, TiO2 (rutile)
Orthorhombic a ≠ b ≠ c; α = β = γ = 90° S8, K2SO4
Hexagonal a = b ≠ c; α = β = 90°, γ = 120° Graphite, ZnO
Trigonal a = b = c; α = β = γ ≠ 90° Quartz (SiO2)
Monoclinic a ≠ b ≠ c; α = γ = 90°, β ≠ 90° Gypsum, Sugar
Triclinic a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90° CuSO4·5H2O

Physical Properties of Crystalline Solids

  • Cleavage & Fracture: Defined planes of weakness correspond to lattice planes.
  • Anisotropy: Direction-dependent properties, e.g., refractive index, conductivity.
  • Optical Behavior: Birefringence in uniaxial/biaxial crystals.
  • Mechanical Strength: Dependent on bond types and symmetry.
  • Electrical & Thermal Conductivity: Varies along different crystallographic axes.

Conclusion

Symmetry considerations form the foundation for classifying crystalline solids into the seven lattice systems and predicting their physical behavior. Mastery of these concepts aids in understanding material properties across chemistry, physics, and materials science.

 

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