When examining a mineral specimen, look for areas where the cleavage planes appear to be parallel. If the cleavage planes are parallel, the rock is likely to have perfect cleavage. If the cleavage planes appear to overlap, it is a sign of poor cleavage. Cleavage planes are not the same as mineral faces, and are sometimes mistaken for them. The difference between the two is that the face of a crystal is formed when the mineral grows, while the cleavage occurs when a mineral breaks.
When examining a mineral, it’s important to know the difference between a prismatic cleavage and a rhombohedral cleavage. The cleavage in a rock is visible on each face, so if you notice one face in particular, you should be able to tell if the other one is cleavaged. If there are more than two cleavage planes, the cleavage in the rock is rhombohedral.
Minerals often break along cleavage planes or fracture patterns. The cleavage planes are parallel planes within the crystal, which may appear as reflective surfaces on the surface of the mineral. They can also appear as jagged fractures. Whether a mineral has a cleavage plane or not is largely dependent on the mineral, but it can be helpful in identifying it.
Cleavage is a form of foliation that occurs in rocks with uneven mineral distribution. It can result in discontinuous layers of minerals. These are formed by multiple phases of deformation and folding. In some rocks, there are multiple cleavage domains, whereas others contain only a few. The degree of deformation and metamorphism determines which type of cleavage domains a rock has.
Cleavage surfaces can be easily detected with a loupe. The cleavage surfaces may ‘light up’ when rotated. Be sure to check the directions of cleavage to see whether they are parallel. Also, be aware that not all flat surfaces are cleavage planes, as some are the result of crystal growth.
A good example of a rock’s cleavage is the clinopyroxene, a mineral with multiple directions of cleavage. A perfect end view shows one cleavage, whereas a fractured rock may have multiple fractures.
Parting is a similar phenomenon to cleavage, but occurs in twinned minerals or those under pressure. However, parting surfaces are not as well developed and resemble poorer cleavage. Nonetheless, specimens of parting surfaces are hard to obtain. Some of the minerals that show parting surfaces include pyroxene, “specular” hematite, and corundum.