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Cleavage describes the preferred direction or shape in which a mineral breaks under mechanical stress. The mineral may exhibit flat, smooth cleavage planes or break into regular, geometric shapes. For example, galena breaks into small cubes, and calcite into rhombohedral fragments. In the mineral's description, "very perfect" or "very perfect" is indicated after {100}, the cube, or after {1011}, the rhombohedron. Cleavage can vary in degree from "very perfect" or "perfect" through "good" and "barely perceptible" to "not perceptible." Some minerals do not exhibit cleavage at all. Whether a surface of a mineral is a crystal or a cleavage plane can often be determined by its luster or by certain crystallographic features (e.g., striations).
The cleavage angle, i.e., the angle at which the cleavage planes intersect on the surface of a mineral, is often important for identification. For example, pyroxenes have a cleavage angle of approximately 90° and thus differ from the very similar amphiboles, which have a cleavage angle of 120° (see image).
Figure 3: Image of an amphibole mineral grain under a polarizing microscope.
The cleavage surfaces, which are at an angle of 120° to each other, are clearly visible.