Cleavage Habit
Different habits of cleavage exist on different minerals, depending on their mode of crystallization. These forms of cleavage are:
Basal cleavage:
Cleavage exhibited on a horizontal plane of the mineral by way of its base. Minerals with basal cleavage can sometimes be "peeled".
An example of basal cleavage are the mica minerals.
Cubic cleavage:
Cleavage exhibited on minerals of the isometric crystal system that are crystallized as cubes. In this method of cleavage, small cubes evenly break off of an existing cube.
An example is Galena.
Octahedral cleavage:
Cleavage exhibited on minerals of the isometric crystal system that are crystallized as octahedrons. In this method of cleavage, flat, triangular "wedges" peel off of an existing octahedron.
An example is Fluorite.
Prismatic cleavage:
Cleavage exhibited on some prismatic minerals in which a crystal cleaves as thin, vertical, prismatic crystals off of the original prism.
An example is Aegirine.
Pinicoidal cleavage:
Cleavage exhibited on some prismatic and tabular minerals in which a crystal cleaves on the pinacoidal plane, which is the third dimension aside from the basal and prismatic sides.
An example is Barite.
Rhombohedral cleavage
Cleavage exhibited on minerals crystallizing in the hexagonal crystal system as rhombohedrons, in which small rhombohedrons break off of the existing rhombohedron.
An example is Calcite.
| Parting: |
Parting is characteristically similar to cleavage. It is easily confused with cleavage, and it may be present on minerals that do not exhibit any cleavage. There are two causes of parting:
- Two separate pressures pushed toward the center of a crystal after its formation, causing the crystal interior to evenly dislodge on a flat, smooth plane.
- Twinned crystals that separated from one another, leaving a flat, smooth plane.
With enough perception, a distinction can be made between parting and cleavage. If fracture marks are present on a crystal in addition to a cleaved plane, the "cleaved" surface is usually the result of parting, not cleavage. An outline of a crystal etched in a mineral is also the result of parting, in the form of twinned crystals that separated.
In general, one need not worry about confusing parting with cleavage. Parting is uncommon, and it can usually be determined by the distinguishing characteristics mentioned above.
| Fracture: |
Fracture is the characteristic mark left when a mineral chips or breaks. Cleavage and fracture differ in that cleavage is the break of a crystal face where a new face (resulting in a smooth plane) is formed, whereas fracture is the "chipping" shape of a mineral. All minerals exhibit a fracture, even those that exhibit cleavage. If a mineral with cleavage is chipped a certain way, it will fracture rather than cleave.
How to use cleavage, parting, and fracture as an identification mark
A specimen need not be broken to check its fracture habit or cleavage. Rather, it should be checked for areas of stress where it could have broken or chipped. Fracture marks are rarely present on minerals with good or excellent cleavage. Minerals with poor cleavage will fracture more often than those with good or perfect cleavage.
How to testing using cleavage, parting, and fracture
Observe the mineral to see if it has any cleaved surfaces or fractured edges. If it has cleaved surfaces, the quality of the smoothness on the surface should be noted. If there are no visible cleaved surfaces, it does not mean the mineral does not exhibit cleavage. It is possible that particular specimen did not cleave. On such a specimen, it is only possible to check cleavage by chipping off a piece. This should be done gently and carefully in an area which will not degrade its value. If there is a noticeable fracture on the mineral, it is an likely indication that the mineral probably has poor cleavage or none at all.
Careful observation and experience should also be able to distinguish between a cleaved crystal and a crystal that parted.
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