Cleavage refers to secondary foliation defined by a preferred orientation of inequant fabric elements. The name is usually restricted to fine-grained rocks such as slates and phyllites, but is also used by some authors as a general name for any secondary foliation except coarse layering. A mineral is said to possess cleavage if when it breaks it yields definite plane surfaces. Cleavage can be perfect as in micas or, in some minerals, completely lacking. Cleavage is always parallel to crystal faces.
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