Cleavage
Definition
The tendency of a mineral to break along flat, smooth surfaces determined by its internal crystal structure. Cleavage planes are areas of weak atomic bonding.
Example
Mica has perfect cleavage in one direction, allowing it to split into thin, flexible sheets.
Understanding Cleavage in Mineralogy
Cleavage is one of the most critical diagnostic properties used by geologists to identify minerals. It refers to the tendency of a crystalline material to break along definite crystallographic structural planes. These planes correspond to weak atomic bonding in the crystal lattice. When a mineral is struck, it will split cleanly along these weak zones, leaving a flat, smooth, and reflective surface.
Types of Cleavage
Cleavage is described by the number of directions and the quality of the break:
- Basal Cleavage: Occurs in only one direction, creating thin sheets (e.g., Mica, Graphite).
- Cubic Cleavage: Occurs in three directions at 90 degrees (e.g., Halite, Galena).
- Rhombohedral Cleavage: Occurs in three directions not at 90 degrees (e.g., Calcite).
- Octahedral Cleavage: Occurs in four directions (e.g., Fluorite, Diamond).
Cleavage vs. Fracture
It is important to distinguish cleavage from fracture. Fracture describes how a mineral breaks when it does <strong>not</strong> follow a cleavage plane. For example, Quartz has no cleavage; it breaks with a conchoidal (shell-like) fracture. A mineral can exhibit both properties, but cleavage surfaces will flash light evenly, while fractures are usually irregular.