This is the tendency of minerals to split in a uniform fashion along a series of parallel planes.
These planes correspond to the most dense reticulated planes which are known as cleavage planes.
Under the microscope, the cleavage planes are translated into one or more surfaces of parallel straight lines, as we can see from the figure.
1 cleavage direction
Only one preferential direction is appreciable for a crystal which splits into very thin plates of the mineral. This is very common in the minerals which have a layered structure, such as phyllosilicates.
2 cleavage directions
Two plane surfaces appear. These surfaces split according to particular angles for each kind of mineral. The figure shows that these are perpendicular for a feldspar (1) or form a rhombus for the silicates whose structure is constituted by chains. In the case of the pyroxenes the angles are very close to 90º, whilst for amphiboles, the largest angle is close to 120º.
3/4 cleavage directions
In some cases three (1), or even four (2), cleavage directions can appear.
1 
Some minerals, such as quartz, have irregular fractures and no preferential directions are appreciable. They do not show cleavage.
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