Cleavage, being related to structure, is important at times in the correct identification of a mineral's
Remember, cleavage must obey the symmetry of the mineral and must
be parallel to a possible crystal face. A mineral of the isometric
symmetry class can either have no cleavage or at least three
directions of identical cleavage that form a closed three dimensional polygon.
A mineral of a uniaxial class
The angle between cleavages is also important to note and may be diagnostic. The pyroxene and amphibole groups of minerals are distinguished primarily by cleavage angle with the pyroxenes having a more acute angle. The angle may also help identify the type of cleavage. Three identical directions of cleavage in one mineral can only be either cubic cleavage, rhombic cleavage or prismatic (forming six sided prisms). If the angle between cleavage faces is 90 degrees, then the cleavage is cubic. If the angle is 60 degrees, then the cleavage is prismatic. Also, if the angle is something else and there are three identical cleavages, then the cleavage is rhombic.
The phyllosilicates are a group of minerals whose structure is based upon stacked layers. A natural cleavage plane is produced between these layers. Other minerals may have cleavage planes that are more related to bond strength. Weak bonds that all lie in a plane will produce a cleavage direction.
To identify cleavage in a mineral remember that it is always parallel to a possible crystal face, it is reproducible over and over again and that it may be seen as internal reflection planes. Cleavage can be observed without the specimen being cleaved all the way through as pictured above. Minerals with perfect cleavage will sometimes have a stairstep look around a broken section. Twinning may break a minerals cleavage at the twin plane and this should be kept in mind. Knowledge of a mineral's cleavage can be important in determining if a given specimen has been broken or not (a key characteristic in a mineral specimen's value). The related property of parting is thought by many to just be an example of poor cleavage. Many minerals lack any cleavage at all and will only show fractures.