Class 11 Chapter 6 Geometrical Shape Cleavage Plane Habit of Crystal Crystal Growth
By Tariq Pathan | Published: June 25, 2025
Table of Contents
- Introduction
- Geometrical Shape of Crystals
- Cleavage Plane
- Habit of Crystal
- Crystal Growth
- 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 Type | Description | Example |
|---|---|---|
| Prismatic | Elongated prisms | Beryl, tourmaline |
| Tabular | Flat, plate-like | Graphite |
| Acicular | Needle-like | Natrolite |
| Dendritic | Tree-like branching | Native silver |
| Botryoidal | Globular clusters | Hematite |
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:
- Nucleation: Formation of a stable microscopic cluster.
- Growth: Addition of building units to facets, edges, and corners.
- 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.
