To do it, chemists measure physical and electrical properties of each atom and use those measurements to describe the bond mathematically. One of the most important measurements is the distance between atoms. As a rule of thumb, the closer two atoms are to each other, the more tightly they bond together, all other factors being equal. In most crystal structures, atoms are closer together in some directions than in others. Thus, the bonds in those directions are stronger.
Cleavage takes place along planes that cut across the longest, weakest bonds in the structure.
Some minerals, such as the micas, cleave in only one direction. Others, in which more than one set of layers is weakly bonded, may cleave in two or more directions. Geologists can use the number of cleavage directions and the angles between them to distinguish one mineral from another. Some minerals, such as quartz, do not exhibit cleavage at all, because there is little difference in bond strength from one direction to another in their crystal structures. Such minerals break to produce a rough, non-planar surface called a fracture instead.