Cleavage

Cleavage is the property of a mineral to break along planes where atomic bonds in its structure are the weakest, under the influence of force (impact or pressure). Cleavage creates flat surfaces, cleavage planes, that always appear in the same directions for a particular mineral, and the symmetry of the cleavage corresponds to the symmetry of the crystal. Some minerals exhibit cleavage in several directions, while others, like quartz or pyrite, have very weak or no cleavage at all.

Minerals that have cleavage are those where attractive forces in different directions vary. The attractive forces are weakest, or the chemical bonds are weakest, perpendicular to the planes along which the crystal cleaves. For example, in layered structures, the bonds between layers are much weaker than the bonds within the layers, so minerals with layered structures, like micas, cleave parallel to the layers.

The surfaces created by cleavage are called cleavage planes, and the pieces of crystal formed by cleavage are called cleavage fragments.