What is Mineral Cleavage?

Understanding Mineral Cleavage

Cleavage is the tendency for a crystalline solid to break along smooth, even flat surfaces, known as cleavage planes.

These planes represent lines of weakness within the crystal structure, where the crystal breaks more easily.

The cause of the weakness is related to the structure of the atoms in the crystal lattice. The atomic structure creates natural planes of weakness, where bonds between atoms are weaker, leading to cleavage patterns.

A crystalline solid is a mineral, while a naturally occurring solid that lacks a crystalline structure is called a mineraloid.

When a mineral breaks along a plane of structural weakness, it leaves a smooth, flat surface known as a cleavage plane. Cleavage planes form along the weakest directions in the mineral's structure, causing these flat surfaces.
 
A mineral can have one, two, three, four or six cleavage planes. This refers to different sides of the crystal that cleave. Minerals that don't exhibit a smooth, flat surface on any one side generally have no cleavage planes. The bond of these materials is equally strong in all directions.

A mineral with one cleavage plane breaks into thin, flat sheets. Those with two breaks into a slightly more elongated cube, and those with three cleavage planes break into an almost perfect cube.

Fluorite breaks along four cleavage planes, forming an octahedral shape, which is similar to two pyramids stuck base to base.

Although diamond is the hardest material on Earth and can't be scratched by any other mineral, if you can find a plane of weakness, it can be cleaved. However, due to its hardness, it wouldn't be quite as easy as splitting a softer mineral along a cleavage plane. Like fluorite, diamond also cleaves along four cleavage planes.    

Minerals described as having a 'perfect cleavage' cleave with an exceptionally clean break. Both edges and the new crystal faces created as a result of the break are smooth and flat. Cleavage surfaces typically reflect light due to their flatness.

The term 'good cleavage' describes a mineral that breaks along planes of weakness, leaving a smooth surface but with a slightly rougher edge compared to 'perfect cleavage.' Poor cleavage means it breaks more randomly and with a rougher, less defined edge.

Non-crystalline solids lack a crystal structure because they are not composed of crystals, so they do not have cleavage planes. Instead, these materials fracture when broken.

Minerals with cleavage planes can also fracture. Mineral cleavage and fracture are not the same.

When a mineral fractures, it breaks in a more irregular and unpredictable manner rather than along specific planes of weakness. After fracturing, no two pieces will be the same, similar to how glass shatters when dropped.