Cleavage (geology)

Cleavage, in geology and related disciplines, describes the tendency of a mineral or rock to break along preferred planes of weakness.

Cleavage in mineralogy

Minerals cleave along particular crystallographic planes along which bonds are comparitively weak. The number of cleavage planes and the angles between them are a characteristic of a particular mineral. Minerals can have from one up to five cleavage planes e.g. micas have a single plane of cleavage while galena has three cleavages at 90° to each other. The quality of the cleavage is also important in identifying minerals and is described both in terms relative to a perfectly planar cleavage (perfect, imperfect, distinct, good, fair, and poor) and the difficulty of getting the mineral to cleave (easy, hard and difficult).

Some minerals do not have any cleavage and break unevenly. This is called fracture e.g. quartz exhibits a conchoidal fracture.

Cleavage in structural geology

Rocks deformed under very low to low metamorphic grade often develop planes along which the rock can easily be split. Slates are an example of a rock with a penetrative cleavage caused partly by the realignement of phyllosilicate minerals with increasing flattening strain.

With increasing metamorphic grade and growth of new minerals, penetrative cleavage grades into a foliation.

See also

Foliation (geology)