DEFINITION OF CLEAVAGE

A structural property of certain rocks to split

along closely spaced secondary penetrative features such as

aligned microcracks and tectonic foliations. Cleavage is a

type of foliation defined by penetrative or zonal alignments

of mica and quartz grains and domains, or by the preferred

alignment of microcracks. It may form discrete zones,

domains, be defined by anastomosing dissolution surfaces, or

by microfractures, with the character of the cleavage being

determined by the original rock type and the conditions of

deformation and metamorphism. It is best-developed in micarich

rocks such as slate. Cleavage often shows a regular geometric

relationship with folds in regionally deformed rocks,

suggesting that the folds and cleavage both form during the

same regional contraction event. The cleavage is typically

axial planar to folds, being aligned to the fold axial surface.

In complexly deformed terrains there may be cleavages and

foliations of several generations, forming more complex

fold/cleavage relationships.

Minerals may exhibit a different type of cleavage. Mineral

cleavage is the property of minerals to split along

smooth, crystallographically defined planes that are parallel

to possible crystal faces in the mineral. The number of cleavage

faces in a crystal is determined by the crystallographic

symmetry of the crystal lattice. The cleavage in minerals

forms because atomic bonds are weaker across these planes

than in other directions, leading them to be planes of weakness

along which the mineral breaks. Cleavage faces in minerals

are typically smooth and glistening but may also have

striations on them.

See also DEFORMATION OF ROCKS; MINERALOGY; STRUCTURAL

GEOLOGY.

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