Definitions. Cleavage-The tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice. These two-dimensional surfaces are known as cleavage planes and are caused by the alignment of weaker bonds between atoms in the crystal lattice.leavage and fracture describe different ways in which a mineral can break. Cleavage-The tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice.