cleavage

In geology and mineralogy, the tendency of a rock or mineral to split along defined, parallel planes related to its internal structure; the clean splitting of slate is an example. It is a useful distinguishing feature in rock and mineral identification. Cleavage occurs as a result of realignment of component minerals during deformation or metamorphism. It takes place where bonding between atoms is weakest, and cleavages may be perfect, good, or poor, depending on the bond strengths; a given rock or mineral may possess one, two, three, or more orientations along which it will cleave.

Some minerals have no cleavage, for example, quartz will fracture to give curved surfaces similar to those of broken glass. Some other minerals, such as apatite, have very poor cleavage that is sometimes known as a parting. Micas have one perfect cleavage and therefore split easily into very thin flakes. Pyroxenes have two good cleavages and break (less perfectly) into long prisms. Galena has three perfect cleavages parallel to the cube edges, and readily breaks into smaller and smaller cubes. Baryte has one perfect cleavage plus good cleavages in other orientations.