Cleavage and fracture are fundamental properties that describe how minerals break when subjected to stress. These characteristics provide important clues about a mineral's atomic structure and are essential for mineral identification in the field and laboratory.
Mineral Cleavage
What is Cleavage?
Cleavage is the tendency of a mineral to break along flat, smooth planes determined by its atomic structure. These planes of weakness occur where the chemical bonds between atoms are weaker than in other directions within the crystal lattice.
Types of Cleavage
Perfect Cleavage
Minerals with perfect cleavage break easily along smooth, flat planes, producing highly reflective surfaces. These planes can be repeated to produce multiple parallel surfaces.
- Examples: Mica (sheets), Calcite (rhombohedrons), Halite (cubes)
Good Cleavage
Minerals with good cleavage break along planes that are clearly visible but may not be perfectly smooth. The resulting surfaces are still recognizable as cleavage planes.
- Examples: Feldspar, Pyroxene
Poor Cleavage
Minerals with poor cleavage break along planes that are difficult to distinguish. The resulting surfaces are often irregular, making it challenging to identify the cleavage direction.
- Examples: Amphibole, Tourmaline
No Cleavage
Some minerals lack cleavage altogether, breaking instead with a characteristic fracture. These minerals have relatively uniform bond strength in all directions.
- Examples: Quartz, Obsidian
Cleavage Direction and Number
Minerals are classified by the number of cleavage directions and the angles between them:
| Cleavage Type | Description | Examples |
|---|---|---|
| Cubic | 3 directions at 90° angles | Halite, Galena |
| Octahedral | 4 directions | Fluorite |
| Rhombohedral | 3 directions not at 90° | Calcite, Dolomite |
| Prismatic | 2 directions at 90° | Pyroxene |
| Prismatic (Angled) | 2 directions not at 90° | Amphibole |
| Basal | 1 perfect direction (sheets) | Mica (Muscovite, Biotite) |
| Pinacoidal | 1 poor direction | Kyanite |
Cleavage Mechanism
The ability of a mineral to cleave depends on its internal atomic structure:
- Cleavage occurs along planes where bond strength is weakest
- These planes typically have the least dense atomic packing
- Some minerals have multiple cleavage directions corresponding to different planes of weakness
- The quality of cleavage is determined by how consistent the bond strength is along these planes
Mineral Fracture
What is Fracture?
Fracture refers to the way a mineral breaks when it does not follow a cleavage plane. Unlike cleavage, fracture produces irregular or non-planar surfaces and occurs when a mineral is broken in a direction where atomic bonds are relatively strong and uniform.
Common Fracture Types
- Conchoidal: Smooth, curved fracture surfaces resembling the interior of a seashell. Often seen in glassy or amorphous minerals (e.g., Quartz, Obsidian)
- Irregular/Uneven: Rough, random fracture surfaces with no characteristic pattern (e.g., Fluorite, Feldspar)
- Hackly: Sharp, jagged edges like broken metal (e.g., Native copper, Silver)
- Fibrous: Fracture produces stringy or splintery fragments (e.g., Asbestos, Chrysotile)
- Granular: Fracture results in grainy, sand-like fragments (e.g., Limestone, Sandstone)
Factors Affecting Fracture
The nature of a mineral's fracture is influenced by several factors:
- Atomic structure: Minerals with complex or irregular crystal structures tend to fracture unpredictably
- Bond strength: Uniform bond strength in all directions promotes fracture over cleavage
- Impurities: Inclusions can create stress points that influence fracture patterns
- Texture: Fine-grained minerals often have different fracture characteristics than coarse-grained ones
Practical Applications
Understanding cleavage and fracture is essential for various applications:
- Mineral identification: These properties are diagnostic for many mineral species
- Industrial uses: Cleavage properties determine how minerals can be processed and utilized
- Gemstone cutting: Knowledge of cleavage planes is crucial for cutting and polishing gemstones without damaging them
- Material science: Understanding fracture mechanics helps in developing stronger materials