What is the term for the tendency of a mineral to break along weak planes?

The term that describes the tendency of a mineral to break along weak planes is cleavage. Cleavage refers specifically to the way in which certain minerals will split or break easily along specific planes of weakness in their crystal structure. These planes are due to the arrangement of atoms in the mineral, which creates zones of weakness where bonds are relatively weak compared to others. This can result in smooth, flat surfaces when the mineral breaks, which is a key characteristic of several minerals.

Fracture, in contrast, describes the way a mineral breaks when there are no defined planes of weakness; it often results in uneven or irregular surfaces. The terms breakage and splitting are more general and do not specifically refer to the atomic structure and the uniformity of the breaks as clearly as cleavage does. Thus, cleavage is the correct term for this phenomenon within mineralogy.