Cleavage of Minerals: Types & Examples

Cleavage is The tendency of crystalline materials to split along definite crystallographic structural planes. This property is due to the alignment of weaker bonds between atoms in the crystal lattice or the presence of planes of atoms where the bonding is weaker than in other directions.

Cleavage occurs when minerals break along specific planes of weakness in their crystal structure. These planes of weakness correspond to the direction where the bonding forces between atoms are weakest. In other words, cleavage happens along planes where the bonds between atoms are less resistant to stress.

This tendency to break along planes is directly related to the atomic arrangement within the mineral’s crystal lattice. A crystal lattice is an orderly, repeating three-dimensional structure of atoms or ions in a mineral.

Cleavage is the result of weaker bond strengths or greater lattice spacing across the plane in question than in other directions within the crystal. Greater lattice spacing tends to accompany weaker bond strength across a plane, because such bonds are unable to maintain a close interatomic spacing.

Cleavage of Minerals

Both the positioning of crystal faces in a mineral and the property of cleavage are derived from the crystalline structure of the species. However, despite the fact that every mineral belongs to a specified crystal system, not every mineral exhibits cleavage. A mineral such as quartz may demonstrate beautiful, well-developed crystals and yet possess no distinct planes of cleavage.

Cleavage planes, if they exist, are always parallel to a potential crystal face. However, such planes are not necessarily parallel to the faces which the crystal actually displays. Fluorite, for example, has octahedral cleavage yet forms cubic crystals. Nonetheless, the property of cleavage, if it is present, can offer important information about the symmetry and inner structure of a crystal.

Mechanism of Cleavage

The bonds between atoms in a mineral’s structure can vary in strength depending on the direction. Along some planes in the crystal lattice, the bonding forces may be weaker, while along others, they are stronger. When a force is applied (like hitting the mineral with a hammer), the mineral tends to split or break along these planes of weaker bonds.

For example, in the mineral mica, there are strong bonds in two dimensions, but weak bonds in the third direction, allowing it to cleave easily into thin sheets.

Cleavage Quality

Cleavage can be described in terms of its quality, which depends on how easily and perfectly the mineral breaks along these planes:

Perfect cleavage: This is the most common type of cleavage, and it occurs when a mineral will break along one or more planes with a very smooth, almost mirror-like surface. Examples of minerals with perfect cleavage include mica, galena, and calcite.
Imperfect cleavage: This type of cleavage is not as smooth as perfect cleavage, but it can still be seen with the naked eye. Examples of minerals with imperfect cleavage include feldspar and hornblende.
Good cleavage: This type of cleavage is even less smooth than imperfect cleavage, but it can still be seen with a hand lens. Examples of minerals with good cleavage include pyroxene and olivine.
Poor Cleavage: The mineral shows a tendency to break along cleavage planes, but these planes are less pronounced, and the resulting surfaces are often rough or uneven. Example: Apatite has poor cleavage.
No Cleavage: Some minerals, such as quartz, do not exhibit cleavage. Instead, they fracture in an irregular or conchoidal (shell-like) pattern when broken.

A mineral which demonstrates 'perfect' cleavage breaks easily, exposing continuous, flat surfaces which reflect light. Fluorite, calcite, and barite are minerals whose cleavage is perfect.

'Distinct' cleavage implies that cleavage surfaces are present although they may be marred by fractures or imperfections. 'Difficult' or 'indistinct' cleavage produces surfaces which are neither smooth nor regular; samples possessing such cleavage tend to fracture rather than split.

Types of Cleavage Based on Planes: Cubic, octahedral, dodecahedral, prismatic, basal.

Types of Cleavage

Types of Cleavage Based on Planes. Cleavage is classified based on how many planes of cleavage a mineral has and the angles at which these planes intersect. Here are the main types:

Basal Cleavage (One Direction):

Occurs when the mineral splits easily along one flat plane. This type of cleavage is seen in minerals where the atomic bonds are weak in one direction but strong in the other two directions.
Example: Mica has basal cleavage, allowing it to split into thin, flexible sheets.

Prismatic Cleavage (Two Directions):

This type of cleavage occurs along two planes, usually at angles of 90° or less. The planes may intersect at a variety of angles.
Example: Feldspar has prismatic cleavage, with two planes that intersect at roughly 90°.

Cubic Cleavage (Three Directions at 90°):

The mineral cleaves along three planes, all of which intersect at 90°, creating cube-like fragments.
Example: Halite (rock salt) has cubic cleavage, and it can break into cubic-shaped pieces.

Rhombohedral Cleavage (Three Directions not at 90°):

This occurs when three cleavage planes intersect, but not at 90°, creating a rhombohedron shape.
Example: Calcite has rhombohedral cleavage.

Octahedral Cleavage (Four Directions):

Cleavage occurs along four planes, producing octahedron-shaped fragments.
Example: Fluorite exhibits octahedral cleavage.

Dodecahedral Cleavage (Six Directions):

Rare, this cleavage occurs along six planes, creating dodecahedron-like shapes.
Example: Sphalerite shows dodecahedral cleavage.

Factors Affecting Cleavage

Crystal Structure: The arrangement of atoms in the crystal lattice is the most significant factor determining cleavage. Minerals with different structures will have different cleavage planes.

Bonding Strength: In some minerals, the bonds between atoms are strong in all directions, making cleavage rare or non-existent (e.g., quartz). In others, the bonds are weak along certain planes, leading to prominent cleavage (e.g., mica).

Impurities and Defects: Impurities in a mineral’s structure can disrupt the regular atomic arrangement, affecting how the mineral cleaves.

Pressure and Temperature: Environmental conditions can also affect cleavage. For example, minerals under high pressure may not cleave as easily as those under normal conditions.

Key points about cleavage

Not all minerals have cleavage. Some minerals, such as quartz and opal, have no cleavage planes and will break in a more or less random pattern when struck.

The number of cleavage planes can vary. Some minerals, like mica, have only one cleavage plane, while others, like calcite, have three.

The angle between the cleavage planes can also vary. Some minerals, like feldspar, have cleavage planes that intersect at right angles, while others, like amphibole, have cleavage planes that intersect at other angles.

The quality of cleavage can vary. Some minerals, like mica, have perfect cleavage, meaning they break along their cleavage planes very easily and smoothly. Other minerals have poor cleavage, meaning they break along their cleavage planes with difficulty or not at all.

Industrial and Geological Applications

Cleavage is significant in various industrial applications. For example, mica's basal cleavage allows it to be split into thin sheets, making it useful as an insulator in electrical applications.

Cleavage also plays a role in gem cutting and mineral processing, where understanding how a mineral cleaves can influence how it is cut, shaped, or used in different products.

Cleavage is a critical concept in mineralogy, revealing essential details about a mineral’s atomic structure and aiding in its identification and practical use.