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) or post on Discussion Forum.

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).

Image credit: Modified by Scott Brande from Chemistry LibreTexts (2019), Figure 12.1: Cleaving a Crystal of an Ionic Compound. Available at: https://chem.libretexts.org/@api/deki/files/16476/27cdfcab90e30285567bc779caab2540.jpg?revision=1 [Accessed 6 July 2019]/ CC BY-NC-SA 3.0

Image credit: Modified by Scott Brande from Chemistry LibreTexts (2019), Figure 12.1: Cleaving a Crystal of an Ionic Compound. Available at: https://chem.libretexts.org/@api/deki/files/16476/27cdfcab90e30285567bc779caab2540.jpg?revision=1 [Accessed 6 July 2019]. CC BY-NC-SA 3.0

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

Watch the video demonstrations below.

How to Observe Mineral Cleavage

Video of Mineral Exhibiting Three Perfect Cleavages

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.

Rygel, M.C. (https://commons.wikimedia.org/wiki/File:Mineral-cleavage.gif ), „Mineral-cleavage“, re-order models and added annotation by Scott Brande, https://creativecommons.org/licenses/by-sa/4.0/legalcode

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.

Crystal with 3 Cleavage Surfaces

All cleavage surfaces are planar. On this crystal, we see three flat surfaces - top, front (right), side (left). Each surface is the plane along which this fragment broke away from the larger crystal.

Image credit: Scott Brande

3 Cleavages Meet At 90 Degree Angles

The top surface is cleavage #1. The front (right) is cleavage #2. The side (left) surface is cleavage #3.

  • Cleavage surface #1 meets (intersects) cleavage surface #2 along the red line. The cleavage angle between #1 and #2 is about 90 degrees.
  • Cleavage surface #1 meets (intersects) cleavage surface #3 along the orange line. The cleavage angle between #1 and #3 is about 90 degrees.
  • Cleavage surface #2 meets (intersects) cleavage surface #3 along the green line. The cleavage angle between #2 and #3 is about 90 degrees.

Image credit: Scott Brande

<90 (acute) and >90 (obtuse) Angles

Note the orientation of the left side and bottom side of this crystal. These two sides meet at the lower left corner. The cleavage angle of the lower left corner is < 90 degrees (an acute angle). The cleavage angle of the upper left and lower right corners are > 90 degrees (obtuse ).

Image credit: Scott Brande

Angle Comparison

Differences in cleavage angles in these two crystals are best seen when compared side-by-side.

Image credit: Scott Brande

Examples: Number of Cleavages

0 (Zero)

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

0 Cleavage. Fragments extremely irregular. Only 1 single planar side on one fragment (lower center) - this may have resulted from a portion of a single crystal face of growth, not cleavage.End of paperclip (about 5 mm inside width) for scale.

Image credit: Scott Brande

0 Cleavage. Fragments extremely irregular. Although several fragments exhibit one or two planar (flat) sides, these are remnants of planar crystal faces, not cleavage (broken) surfaces.End of paperclip (about 5 mm inside width) for scale.

Image credit: Scott Brande

0 Cleavage. Fragments extremely irregular. No repetitive planar (flat) sides. End of paperclip (about 5 mm inside width) for scale.

Image credit: Scott Brande

1 Cleavage

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

1 (perfect) Cleavage. A mineral that fragments into very even flat flakes. Cleavage is termed "perfect". Notice that the perimeter of each flake is quite irregular, because of the lack of additional cleavage directions at other angles to the flat surface. End of paperclip (about 5 mm wide) for scale.

Image credit: Scott Brande

1 (perfect) Cleavage. Note extremely planar (flat) surface, and irregular perimeter, indicating this crystal exhibits only 1 perfect cleavage.

Image credit: Scott Brande

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.

2 Cleavages. A mineral that produces two differently oriented planar surfaces when fragmented. The first direction is labeled 1, the top of the block. The second direction is labeled 2, the left side of the block. The right side of the block is very uneven, not flat (planar) so cleavage is absent in the third direction, perpendicular to the first two surfaces.

Image credit: Scott Brande

2 Cleavages. A view of the front of the blocky fragment. Note extremely planar (flat) surfaces along 1 and 2, and the 90 degree angle at which they meet.

Image credit: Scott Brande

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.

3 Cleavages. Cleavage is a property of broken fragments. Here the fragments exhibit 3 planar surfaces (flat sides) that all intersect at angles of about 90 degrees. End of paperclip (about 5 mm wide) for scale.

Image credit: Scott Brande

Fragment Geometric Form with Three Cleavages. The sides (faces) of rectangular solids meet at 90 degrees. Forms include cubes with all sides the same length, and forms with rectangular sides (of different lengths).

Image credit: Scott Brande

3 Cleavages (1 perfect, 2 good). These fragments exhibit forms with corners of less than and greater than 90 degree angles. End of paperclip (about 5 mm wide) for scale.

Image credit: Scott Brande

Fragment Geometric Form. These forms exhibit corner angles not at 90 degrees. The name for these forms is a parallelopiped.

Image credit: Scott Brande

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).

4 Cleavages. These fragments resemble the shape of a four-sided pyramid with triangular faces. This overhead view of the small fragment does not provide the perspective necessary to appreciate the fourth side, the base on which the pyramid is resting.

See the model for the three dimensional form. End of paperclip (about 5 mm inside width) for scale.

Image credit: Scott Brande

Fragment Geometric Form. With four triangular faces, these forms exhibit corner angles less than 90 degrees. The name for these four-sided forms is a tetrahedron, commonly referred to as a (triangular) pyramid (as contrasted with a five-sided, Egyptian pyramid with a square base).

Image credit: Scott Brande

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.

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.