Cleavage is a mineral’s tendency to split along flat, parallel surfaces. It occurs because chemical bonds are weaker in particular crystallographic directions, allowing a mineral to break repeatedly along those planes.
Geologists describe cleavage by its quality and direction. Mica commonly has one perfect cleavage direction, so it peels into thin elastic sheets. Halite has three cleavage directions at right angles, producing cube-shaped fragments, while calcite has three directions that create rhombohedral pieces. Cleavage directions are related to a mineral’s internal atomic arrangement, not simply to the shape of a hand-sized specimen.
Cleavage is different from fracture, which describes irregular or non-planar breakage. Quartz lacks cleavage and usually shows conchoidal fracture, producing curved, shell-like surfaces. Cleavage should also not be confused with crystal faces: crystal faces form during growth, whereas cleavage surfaces are produced when the mineral breaks.