Cleavage of Minerals

Cleavage is the tendency of a mineral to break along flat, smooth planes due to the arrangement of its atoms. These planes of weakness are determined by the crystal structure of the mineral, and they can be used to help identify different minerals.

Cleavage is the tendency of minerals to break along definite crystallographic structural planes. Cleavage planes are like the grain boundaries in metals, but they are much more regular. Cleavage planes are caused by the way the atoms in the mineral are arranged.

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. 

The quality of a mineral's cleavage refers to both the ease with which the mineral cleaves and to the character of the exposed cleavage surface. The quality of a sample's cleavage is typically described by terms such as 'eminent,' 'perfect,' 'distinct,' 'difficult,' 'imperfect,' or 'indistinct.' 

'Eminent' cleavage describes the case in which cleavage always occurs readily and is in fact difficult to prevent from occurring. The mineral mica, for example, cleaves readily into thin, flat sheets.


Cleavage of Minerals

Types of Cleavage

There are several different types of cleavage, each with its own characteristics. Some common types of cleavage include:

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: This type of cleavage is very difficult to see, even with a microscope. Examples of minerals with poor cleavage include quartz and garnet.

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.

Cleavage may be determined by the examination of surfaces which have actually broken. It may also be determined by inspection of the interlacing systems of cracks which permeate the structure of certain specimens. These systems of cracks are beautifully apparent within transparent crystals such as fluorite or calcite.

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.

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