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MINERAL PROPERTIES: CLEAVAGE
(Including Fracture and Parting)


Cleavage, fracture, and parting all have to do with the positioning of atoms in a mineral and how it breaks when put under stress. (These three properties are listed on the same page due to their comparability, but are each individually discussed).

  

Cleavage:

In mineral terms, cleavage describes how a crystal breaks when subject to stress on a particular plane. If part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off has no cleavage.

Cleavage is often measured by three factors:
1) Quality of Cleavage
2) Number of Sides Exhibiting Cleavage
3) Cleavage Habit


Quality of Cleavage:

Quality of cleavage can be categorized into five qualities:

  • Perfect
  • Good
  • Poor
  • Indiscernible (Indistinct)
  • None

Minerals with perfect cleavage will cleave without leaving any rough surfaces; a full, smooth plane is formed where the crystal broke. Minerals with good cleavage also leave smooth surfaces, but often leave over minor residual rough surfaces. On minerals with poor cleavage, the smooth crystal edge is not very visible, since the rough surface is dominant. If a mineral exhibits cleavage, but it so poor that it is hardly noticeable, it has "indiscernible" cleavage. Minerals with no cleavage never exhibit any cleavage, thus broken surfaces are fractured and rough.

Categorization of cleavage qualities is not scientifically affirmed. The above categorization is used by most mineral references, but some guides categorize cleavage in three or four groups, and may give them different names, such as "excellent" and "distinct".


Number of Sides Exhibiting Cleavage:

Many minerals exhibit cleavage only on one side, and some may exhibit different quality cleavage on different crystal sides. The following criteria may be expected when analyzing the cleavage of any particular mineral:

  • One Direction
  • Two Directions
  • Three Directions
  • All Directions

These identify how many "directions", or planes, the crystal is exhibiting the cleavage on. Each direction signifies the two opposite sides of a three-dimensional figure, (since opposite sides will always exhibit the same cleavage properties). If a mineral has cleavage in three directions, then every side of the mineral has cleavage (i.e. length, width, and height). If a mineral occurs in modified crystals with more than six sides (i.e. an octahedron) and exhibits cleavage on all the sides, than it has cleavage in "all directions".

Combining the cleavage level together with the number of sides will measure the cleavage of a mineral. For example, if a mineral has Good Cleavage, Two Directions, this means that it has good cleavage on four out of six sides (while the other two sides exhibit no cleavage). If a mineral has Perfect Cleavage, One Direction; Poor Cleavage, Two Directions, this means that the mineral has perfect cleavage on two sides, and poor cleavage on the other four.

In this guide, cleavage quality is measured in numbers, then the amount of sides, separated by a comma. 1 is perfect cleavage, 2 is good cleavage, and 3 is poor cleavage. If the cleavage of a mineral is written as 1,2 the mineral has perfect cleavage in two directions. If all sides of mineral have the same cleavage, and the mineral often occurs in modified crystals with more than six sides, than All Sides is written instead of a number. If a mineral exhibits different cleavage on different crystal planes, there will be two cleavage indicators separated by a semi-colon (;). For example, if the cleavage of a mineral is written as 1,2;3,1, than it has perfect cleavage in two directions, and poor cleavage in one other direction. If a mineral exhibits indistinct or no cleavage, Indiscernible or None is written in the cleavage field.


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:

  1. 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.
  2. 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.


There are several terms to describe the various mineral fractures:

Conchoidal - Fracture resembling a semicircular shell, with a smooth, curved surface. An example of conchoidal fracture can be seen in broken glass. (This fracture is also known as "shelly" in some reference guides.)

Uneven - Fracture that leaves a rough or irregular surface.

Hackly - Fracture that resembles broken metal, with rough, jagged, points. True metals exhibit this fracture. (This fracture is also known as "jagged".)

Splintery - Fracture that forms elongated splinters. All fibrous minerals fall into this category.

Earthy or crumbly - Fracture of minerals that crumble when broken.

Even or smooth - Fracture that forms a smooth surface.

Subconchoidal - Fracture that falls somewhere between conchoidal and even; smooth with irregular rounded corners.

Some references may describe additional fractures not mentioned above, but those terms are either synonymous or simply used as a verbal depiction of the authors inference.

Almost all minerals have a characteristic fracture. Some minerals of the same species may exhibit a different fracture, but this is rare.


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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