The Columbia Encyclopedia, Sixth Edition. 2001-07.
cleavage
tendency of many minerals to split along definite smooth planar surfaces determined by their crystal structure. The directions of these surfaces are related to weaknesses in the atomic structure of the mineral and are always parallel to a possible crystal face. The property of cleavage is useful in identifying a mineral species. The tendency for certain varieties of metamorphic and sedimentary rock to split along more or less smooth surfaces is sometimes referred to as rock cleavage. Flagstone, slate, and schist are noted for this property, which arises from the parallel alignment of fine, platy mineral grains, themselves displaying cleavage.
