Crystals often contain planes of atoms along which the bonding between the atoms is weaker than along other planes.  In such a case, if the mineral is struck with a hard object, it will tend to break along these planes.  This property of breaking along specific planes is termed cleavage.  Because cleavage occurs along planes in the crystal lattice, it can be described in the same manner that crystal forms are described.  For example if a mineral has cleavage along {100} it will break easily along planes parallel to the (100) crystal face, and any other planes that are related to it by symmetry.  Thus, if the mineral belongs to the tetragonal crystal system it should also cleave along faces parallel to (010), because (100) and (010) are symmetrically related by the 4-fold rotation axis.  The mineral will be said to have two directions of cleavage.  [Note that in the tetragonal system, the form {100} has four faces: (100), ( 00), (010), and (0 0).  But if we are referring to cleavage directions, the mineral only has two, because the cleavage planes (0 0) and ( 00) are parallel to, and thus in the same direction as (010) and (100).]  

Note: Please do not attempt to cleave the minerals in the laboratory.  Many of the specimens you examine cannot be readily replaced.  Cleavage is  usually induced in the mineral when it is extracted from the rock when it is found, and can usually be seen as planes running through the mineral.  Therefore, you do not have to break the mineral in order to see its cleavage.

Cleavage can also be described by general forms names, for example if the mineral breaks into rectangular shaped pieces it is said to have cubic cleavage, if it breaks into prismatic shapes, it is said to have prismatic cleavage, or if it breaks along basal pinacoids it is said to have pinacoidal cleavage.  For examples, see figure 6.4 on page 253 of your text.

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