Certain rocks, such as slate, will split along well-defined, smooth parallel surfaces, commonly called cleavage planes . This splitting property has long been an advantage in making roofing and writing slates. Cleavage planes in most slates are not parallel to bedding and were evidently formed after the rock was. Most cleaved rocks are metamorphic, even though metamorphism may, in some cases, be very weak. In all cases, cleavage is in systematic association with strain. In particular, cleavage planes are axial planes of associated folds, though fanning about the axial hinge surface is common (Fig. 1). Texturally, cleavage is due predominantly to the preferred orientation of platy minerals of the mica family, but other minerals, such as elongated or platy grains of calcite or quartz, may impart cleavage.
Axial plane cleavage (spaced) showing fanning in competent layer, and cleavage refraction.
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Dennis, J.G. (1987). Cleavage and schistosity . In: Structural Geology and Tectonics. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-31080-0_12
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