CALCITE

WHAT IS
CRYSTAL CLEAVAGE?
MUSCOVITE
When a mineral breaks it does so either by fracturing or by cleaving. Crystal cleavage is a smooth break producing what appears to be a flat crystal face. Here are a few rules about cleavage. First cleavage is reproducible, meaning that a crystal can be broken along the same parallel plane over and over again. All cleavage must parallel a possible crystal face. This means that the crystal could have a crystal face parallel to its cleavage, but these faces are not always formed. All cleavage planes of a mineral must match that mineral's symmetry. And finally, the same mineral will always, always have the same cleavage.

The tendency for minerals to cleave or not and in which directions is very characteristic and therefore important to the identification of minerals. Cleavage is described in terms of how easy the cleavage is produced. From easiest to hardest to produce the terms are: perfect, imperfect, good, distinct, indistinct, and poor. Cleavage is said to be basal when it occurs perpendicular to the major axis of the mineral and prismatic when it occurs parallel to the major axis. Multiple cleavages that produce geometric polygons are referred to using the name of the geometric polygon, such as octahedral cleavage in the mineral fluorite, cubic cleavage in the mineral halite or rhombohedral cleavage in calcite (pictured). Cleavage occurs in minerals that have specific planes of weakness. These planes or directions are inherent in the structure of the mineral and form from a variety of factors.

Cleavage, being related to structure, is important at times in the correct identification of a mineral's symmetry. 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 (trigonal, tetragonal or hexagonal) will potentially have a cleavage perpendicular to the dominant axis and/or prismatic cleavage of either 3, 4 or 6 directions respectively, running parallel to the axis. Other cleavage directions are possible, but will always be controlled by the symmetry of the crystal. A biaxial mineral, those belonging to orthorhombic, monoclinic or triclinic classes, can not have more than two identical cleavage directions.

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.

 

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