Cleavage

Image credit: Scott Brande

The Bottom Line on Cleavage - Here on Top!

The geometric shape of broken fragments of minerals is an important clue for identification. However, you may not be allowed to break samples you study, or the study sample may be an aggregate of small or microscopic crystals which make it difficult (or impossible) for you to determine cleavage directions and angles. Therefore, information on cleavage in some minerals is more difficult to evaluate than other properties such as hardness, streak, reaction to acid, and magnetism.   

Bottom Line: When well-developed, cleavage is an important property that may help you to identify a mineral by name, especially in combination with other properties. In finely crystalline (microscopic) samples, cleavage surfaces will likely not be apparent.

Errors/Suggestions: Contact Scott Brande (see footer).

Cleavage in Broken Mineral Fragments

Because chemical bonds are arranged in a repeating geometric fashion throughout a single crystal, hammer blows will tend to break the repetitive chemical bonds that lie at right angles across a plane, or flat surface. When the mineral exhibits the property of cleavage, fragments "cleaved" from the larger crystal exhibit the same geometry of flat surfaces and the angles at which they meet (if a mineral exhibits more than one cleavage direction). 

This model shows cleavage, the flat (planar) surfaces that result when a larger sample is fragmented.

Image credit: Crystal structure of calcite, Nurettin_Sezer,  https://www.researchgate.net/figure/-25_fig3_331530017 

How Do We Recognize Cleavage Surfaces and Angles?

Note the products of breakage

• straight edges

• sharp corners

• flat, shiny (reflecting) surfaces 

• overall geometric shape

To better understand that flat surfaces you can observe on a mineral sample, there's no substitute for seeing this surface appear in a video. Examples are available below

Angle Between Cleavage Planes

Geometric Models

Cleavage means the separation by breakage of a mineral into fragments across planes of weakness in chemical bonds. Because the orientation of ions and molecules in a mineral is geometrical and repetitive, minerals fragment into the same geometric shape - for example, bigger cubes and smaller cubes. Two or more cleavages produce flat (planar) surfaces oriented in different directions, and that means that these flat surfaces intersect at an angle. So we refer to the "angle of cleavage" between any two planes. 

Here are some models. Note each pair of planes (flat surfaces) intersect at an angle, and the intersection is marked by a line. The cleavage angle measures the angular separation between the intersecting planes.

Cleavage in a Mineral Fragment

These fragments are broken from larger (bulk) samples. Note the planar (flat) surfaces. Also prominent are lines/surfaces seen in the interior of the transparent crystal.

Examples: Number of Cleavages

0 (Zero)

Breakage fragments exhibit irregular masses without repetitive flat (planar) surfaces.

1 Cleavage

Breakage fragments split apart into sheet-like forms that stack one on another in parallel planes.

2 Cleavage Directions

An example of a mineral that split apart into a blocky form. The planar (flat) surfaces that result from the fragmentation meet along a line at a corner of the block. Each flat surface oriented differently represents a different direction of cleavage.

3 Cleavage Directions

Breakage fragments split apart into three dimensional forms with geometrically shaped faces. Faces may be shaped as a triangle (with three sides), as a quadrilateral (with four sides), etc.

4 Cleavage Directions

Breakage fragments split apart into a pyramid-like three dimensional form. This pyramid has four surfaces (faces), each triangular in shape (with three sides).

Instructions: How To Observe Cleavage Breakage Pattern

Background - A defining property of minerals is "crystallinity", a property of ions and molecules geometrically and repetitively arranged throughout the solid. When the strength of the various types of chemical bonds differs, some directions through the solid lie on flat surfaces between weaker bonds, and some on surfaces of stronger bonds. The planes across weaker bonds are more susceptible to breakage, and the resulting flat surfaces are called "cleavage" planes. When well-developed and easily observed, the degree of perfection, the number, and the angles between cleavage surfaces are properties that help to identify a mineral.

• More information on cleavage in mineral identification

Caution

• Cleavage appears when chemical bonds are broken across a plane through a crystal. 

• If crystals in a sample are microscopic, then it may be very difficult to observe microscopic reflections on the inch-sized sample.

• The bottom line is that the lack of observable cleavage doesn't necessarily mean that the sample lacks cleavage. It may simply be that any cleavage may not be observable because of these other reasons.

Materials for test.

• mineral samples broken from larger fragments

• the best samples are single crystals rather than aggregates of smaller crystals

Procedure for the test.

• In a typical geology laboratory course, we don't break mineral samples with a hammer. 

• Rather, we observe mineral samples that have already been broken from larger fragments, and we simply observe these smaller broken samples for evidence of cleavage surfaces. If present, we count the number of any different cleavage directions, and eyeball estimate the angle between any two cleavage planes.

Possible test results and interpretation.

Positive result

• As the sample is turned against a light, one or more flat surfaces are observed flashing a bright reflection, as in the video.

• Count each different flat surface flashing light as a different cleavage plane. 

• If two or more cleavage planes are observed, note the approximate angle along the line of intersection where each pair of planes meets. For our purposes, noting simply that the angle is either close to 90 degrees, or different from 90 degrees, may be sufficient to help identify the mineral

Negative result

• No flat surfaces that strongly flash light are observed. If we're sure the sample is composed of a single crystal, then we may conclude the sample exhibits no cleavage. In this case, the few of the common minerals that lack cleavage may exhibit in the reflection a particular pattern called "conchoidal" fracture that aids in the identification of quartz and garnet. For details, check out the section on fracture.