Mineral Cleavage
Cleavage is the tendency of a mineral to break along specific crystallographic planes where chemical bonding is weaker than elsewhere in the structure. These planes of weakness are called cleavage planes. Because cleavage is controlled by crystal structure and symmetry, cleavage planes are always crystallographic planes and can be identified by their Miller indices - a cleavage parallel to a (100) face, for instance, is described as a (100) cleavage. [1]
Because cleavage planes are planes of weakness, it is theoretically possible to cleave a mineral into layers as thin as a single unit cell - only a few angstroms. In practice this cannot be done by hand, and the resulting sheets would be invisible to the naked eye. The important consequence is that when a mineral does cleave, the resulting surfaces are crystallographically controlled flat surfaces that reflect light and define the geometry of the broken piece.
Cleavage Forms
Cleavages are described by the cleavage form - the collection of equivalent cleavage planes related by the mineral’s point group symmetry. In isometric minerals, {001} cleavage means three equivalent cleavage planes parallel to the faces of a {001} cube - that is, the (100), (010), and (001) planes. In monoclinic minerals, {010} cleavage means a single plane parallel to the {010} side pinacoid. The symmetry of any cleavage must be consistent with the mineral’s symmetry - a monoclinic mineral cannot have cubic cleavage. [1]
Cleavage surfaces need not be parallel to the crystal’s external growth faces. Fluorite is the standard example: it grows as {001} cubes but cleaves on {111} octahedral planes, so the fragments produced by breaking fluorite are octahedra, not cubes. The growth habit and the cleavage form are independent expressions of the same underlying structure.
Cleavage Quality
Beyond citing the Miller index form, cleavage is always described by its quality - both the ease with which the mineral cleaves and the perfection of the resulting surface. [1]
- Perfect: Breaks easily to produce continuous, flat surfaces that reflect light uniformly like a mirror. [1]
- Good: Relatively easy to produce, but surfaces are interrupted by other fractures and are less continuous. [1]
- Fair: Between good and poor in quality. [1]
- Distinct, Indistinct, Poor: Progressively less well-developed surfaces.
The range from perfect to poor is a continuum. At the poor end, cleavage grades imperceptibly into fracture - there is no sharp boundary between the two properties.
Tenacity
Tenacity describes how a mineral responds when it is deformed or broken - a different question from whether it cleaves. A mineral can be brittle yet cleave perfectly, or it may be flexible without having any cleavage at all. The common tenacity terms are: [1]
| Term | Description |
|---|---|
| Brittle | Breaks or powders easily. Most ionic- and covalent-bonded minerals are brittle. |
| Malleable | Can be pounded into thin sheets. Metallic-bonded minerals may be malleable. |
| Ductile | Can be drawn into a wire. Malleable materials may also be ductile. |
| Sectile | Can be cut smoothly with a knife. Few minerals are sectile. |
| Elastic | If bent, springs back to its original shape when stress is released. |
| Flexible | If bent, stays bent - does not spring back. |
The dominant bond type controls tenacity. Ionic and covalent bonds are directionally strong but resist bending poorly, so these minerals are typically brittle. Metallic bonds are non-directional and allow atoms to slide past one another, producing malleability and ductility in minerals like native gold and copper. [1]
Fracture
Fracture is breakage without crystallographic control - surfaces that do not correspond to cleavage planes. No sharp boundary separates indistinct cleavage from fracture; the two properties form a continuum, with cleavage at one end and entirely non-planar fracture at the other. Even though fracture surfaces lack the flat geometry of cleavage, their texture is often characteristic enough to be diagnostic for certain minerals. [1]
Fracture surfaces are described in order of increasing topographic relief: [1]
| Term | Description |
|---|---|
| Conchoidal | Smoothly curved, like broken plate glass. Characteristic of quartz. |
| Irregular or uneven | A rough surface with random irregularities. |
| Hackly | A surface with sharp-edged irregularities. |
| Splintery | Resembles the broken end of a piece of wood. |
Conchoidal fracture, the smoothest type, is produced when breakage propagates through a uniformly bonded structure with no preferred planes of weakness - quartz is the classic example and was exploited prehistorically for making flint tools because of the predictably curved edges it produces.
[1]References
- Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.
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References & Citations
- 1.Introduction to Mineralogy Nesse, W. D.
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