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Full Definition of CLEAVAGE. 1. a: the quality of a crystallized substance or rock of splitting along definite planes; also: the occurrence of such splitting b: a fragment (as of a diamond) obtained by splitting. 2. : the action of cleaving: the state of being cleft. 3.
Cleavage is determined by the crystal structure of the mineral. Common Cleavage Descriptions. Cubic. When a mineral breaks in three directions and the cleavage planes form right angles (90 degrees to each other). Results in pieces in the shape of a cube.