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Determining Fracture and Cleavage in Minerals

Last Updated: 9th Jan 2019

By Donald B Peck

Muscovite Basal cleavage
Hornblende Prismatic cleavage
Calcite Rhombic cleavage
Fluorite Octahedral cleavage
Muscovite Basal cleavage
Hornblende Prismatic cleavage
Calcite Rhombic cleavage
Fluorite Octahedral cleavage
Muscovite Basal cleavage
Hornblende Prismatic cleavage
Calcite Rhombic cleavage
Fluorite Octahedral cleavage

In 1611 Johannes Kepler, better known as an astronomer, published a paper in which he suggested that snow flakes owed their hexagonal shape to tiny building block particles from which they are formed. More than 150 years later, Abbe Rene Hauy wrote that crystals are built from very small polyhedral solid blocks. So the story goes, he was examining a damaged calcite crystal when he dropped it and it shattered into pieces that were rhomboid in shape with very smooth surfaces. It probably did not happen, but there is no question that he examined the way calcite fractured. Today we call those little building blocks unit-cells and stacked together they form planes of ions in three dimensional lattices, that sometimes come apart.. . . and that leads to the topics of this article.

This discussion concerns the way mineral crystals break. There are distinct differences, and the differences can be diagnostic. If the crystal splits cleanly and smoothly along a flat plane the break is called cleavage. If the break does not show flat planes, we simply call it fracture.


Cleavage is the tendency of a crystal to break along smooth, flat planes of structural weakness. There are minerals that exhibit one, two, three, or more cleavage planes. And there are minerals that exhibit none at all. A cleavage plane is always parallel to a crystal face, or a possible crystal face. It is a lattice plane of the crystal structure. When there are cleavage planes in multiple directions through a crystal, it is important to estimate the angles between them for both the number of different planes and the angles between them are diagnostic.

Click photos to enlarge them.

A Single Plane of Cleavage:

There is a large number of minerals that exhibit a single plane of cleavage, the best known of which are the micas. The plane on which mica splits easily is perpendicular to the c-axis and is the basal plane, thus it is a basal cleavage. Some references call it pinacoidal.
Basal cleavage can be found in all crystal systems, except the isometric system. If there are cleavage planes, isometric symmetry requires three, four, or six directions of cleavage.
Apophyllite, clinochlore, graphite, and muscovite are common minerals with a basal cleavage,

Two Cleavage Planes:

When a mineral crystal exhibits two cleavage planes they are often parallel to the c-axis and the mineral is said to have prismatic cleavage.
Two directions of cleavage are diagnostic of amphiboles and pyroxenes. In amphiboles the two planes intersect at 124o and 56o, giving amphibole fragments (and crystals) their characteristic rhombic cross-section. On the other hand, prismatic cleavage planes for pyroxenes have angles of 87o and 93o and appear almost square in cross-section. For a hand sample, this is often the only property that distinguishes one group from the other.
Feldspars have two directions of cleavage, also, with one better than the other and at 90o to each other.
Tetragonal minerals often have two directions of prismatic cleavage in either or both prisms (i.e. wernerite, rutile)

Three Cleavage Planes:
Cubic Cleavage: Not surprisingly, cubic cleavage is three directions of cleavage with planes mutually at 90o to each other. Galena, halite, and sylvite are examples. If one cleavage plane is observed parallel to a (100) in a cubic crystal, the symmetry of the isometric system requires that there be two more planes at right angles to the observed plane.
Rhombohedral Cleavage: Rhombohedral cleavage occurs when there are three cleavage planes intersecting at regular angles other than 90o. Calcite, rhodochrocite, siderite, and many of the carbonate minerals show this cleavage.
Prismatic Cleavage: Three directions of prismatic cleavage are found rarely in both the hexagonal and trigonal systems.

Four Cleavage Planes:

Four cleavage planes produce an octahedral cleavage block and so four directions of cleavage are known as octahedral cleavage. Fluorite and diamonds both have octahedral cleavage. Enlarge the thumbnail photo at the right (click on it) and note that the incipient cleavage faces in the block are parallel to the external cleavage faces. The symmetry of the isometric system requires that if there is one cleavage plane parallel to the (111) face, the other three directions of cleavage must be present also. Similar cleavage in the tetragonal or orthorhombic systems is pyramidal and uncommon (i.e. Scheelite )

Six Cleavage Planes:

Six directions of cleavage are known as dodecahedral cleavage. The prime example is sphalerite. Generally, it is difficult to identify all six directions, but several planes are usually noted. Observation of one dodecahedral cleavage plane in the isometric system means that there must be five more directions of cleavage.

Identifying Cleavage Planes:
Since cleavage planes are always parallel to crystal faces, or possible crystal faces, they are identified with Miller indices, in the same way as are crystal faces. If the cleavage plane is parallel to the (111) face, then it is the 111 cleavage plane.

The number of cleavage faces in a form depends upon the number of crystal faces in the form. The {110}, or dodecahedral form, in the isometric system has 12 faces and, if there are parallel cleavage faces, 6 cleavage faces. In the tetragonal and orthorhombic systems the form will have 2 planes (prismatic cleavage) while in the triclinic system there would be only one (pedial or pinacoidal).

Perfection of Cleavage Planes:
There are differences in the perfection of cleavage planes. Some are exceedingly smooth and others are rather rough. Terms such as perfect, good, distinct, or indistinct are used.

Perfect Cleavage: Smooth, lustrous, flat; breaks easily along a cleavage plane. difficult to break across a cleavage plane; like the micas, calcite, or apophyllite.

Good Cleavage: Smooth, flat perhaps with a step, usually bright; breaks easily along the cleavage plane but can be broken across the plane; like the feldspars.

Distinct Cleavage: break relatively easily along cleavages and in other directions; cleavage faces usually small; like scapolite.

Indistinct Cleavage: Random fracture is as easy (or difficult) as cleavage; cleavage planes difficult to see and characteristically small; like beryl.


Parting is the separation of lamellae in a crystallized mineral, not due to cleavage. It is often confused with cleavage, as a parting plane and a cleavege plane appear much the same. But the cause is different. Cleavage is due to a structural weakness between lattice planes in the crystal, and theoretically can be repeated down to the atomic level of layering. It occurs in all crystals of a cleavable mineral species, and in the same manner. That is not the case for parting. It, also, is due to a plane of weakness, but the parting planes are not so closely spaced, may or may not occur, and have different causes. Some are due to exsolution of planes in the crystal. Texts often indicate that others are related to the composition plane in twinning, but this has been questioned (See research of John S. White). Physical forces can cause dislocation of lattice planes, often noticed in garnet. Parting can occur in some crystals of a mineral species but not in others. The principal clue to differentiate parting and cleavage is repeatablility, both within a crystal and across many crystals of a species. Corundum often shows a basal parting plane, although it may have parting planes in three directions. It never has cleavage.


When a break is not cleavage or parting, it is fracture. There are only a few terms used to describe the nature of mineral fracture. They include conchoidal, hackley, splintery, even, uneven, earthy, or rough. Most are easily understood, but there are two that may need explanation: conchoidal and probably hackley.

Conchoidal Fracture:
Quartz: Conchoidal Fracture
Obsidian: Conchoidal Fracture

A chonchoidal fractue looks like the way glass chips. It is usually a rounded depression at or near the edge of a piece, often with slight ridges following the curvature of the depression. It may be described as shell like. Quartz, flint, and obsidian often show a conchoidal fracture.
Hackely Fracture:
Copper: Hackley fracture

A hackely fracture is jagged, like torn metal, the edges of torn copper or aluminum. Some of the serpentines are examples, also. The term comes from the resemblance to feathers laying on a rooster's neck.

A Little Theory:

We all know that minerals, by definition, have a crystal structure that is a three dimensional array of ions or molecules. It is a lattice with ions or molecules at its nodes. The spacing of the particles (nodes) is dependent on the nature of the particle (positive ion, negative ion, or molecule), the size of the particles, the shape of the electrical field around the particles, the strength of the field, and the type of bonding between particles. The resulting unit cell, which contains all the chemistry and symmetry of the mineral species, stacks to produce the crystal lattice. Thus, the way the ions or molecules settle into their equilibrium positions in the unit cell determines the symmetry of the cell, the lattice, and the properties (including symmetry) of the crystal.

X-ray diffraction has revealed that the lattice structure can be viewed as an array of stacked parallel planes in many directions. Those directions are dictated by the symmetry of the crystal class. The most prominent directions are parallel to the crystal faces.

If the bonding strengths between particles in parallel lattice planes is considerably stronger than the bonding strength between those planes, cleavage may be possible. In any case, a cleavage plane is parallel to a crystal face, or possible face. The greater the difference in bond strength within planes and between planes the more perfect the cleavage. The relative distances between ions in a plane is often referred to as point density. Planes of high point density are often cleavage planes. Similarly, they are planes of higher hardness for the crystal. For a diamond, the plane of highest point density is the octahedral plane. Not coincidentally, it is the hardest plane in the crystal and it is also the cleavage plane.

Links to the "Determining . . ." Series: How To
    What Is a Mineral? The Definition of a Mineral
  1. Determining Color and Streak
  2. Determining Lustre: For Beginning Collectors
  3. Determining the Hardness of a Mineral
  4. Determining the Specific Gravity of a Mineral
  5. Determining Symmetry of Crystals: An Introduction
  6. Determining Fracture and Cleavage in Minerals


Mason, Brian and Berry, L.G. (1968) Elements of Mineralogy. W. H. Freeman and Company, San Francisco. pp. 115-117, pp. 170-171
Dana, Edward Salisbury, Ford, William E. - Editor (1991) A Textbook of Mineralogy. John Wiley & Sons, Inc., New York. pp.208-209
https://www.minerals.net/resource/property/Cleavage_Fracture_Parting.aspx Excellent explanations.
https://pubs.geoscienceworld.org/msa/ammin/article-abstract/64/11-12/1300/41006/boehmite-exsolution-in-corundum?redirectedFrom=fulltext :John S. White; American Mineralogist: Vol 64, pgs 1300-1302, 1979. Parting, cause of.


Photo Credits:
Top of Page: Muscovite, Harold Moritz; Hornblende, P.Cristofono; Calcite; John H. Betts; Fluorite; David Von Bargen
Thumbnails: Muscovite: Harold Moritz; Hornblende, P.Cristofono; Aegirine: Christopher O'Neil Halite, Rob Lavinski; Calcite: E.. Smith; Fluorite: Jamison K Brizendine; Quartz Conchoidal Fracture: Daniel Levesque; Obsidian: Conchoidal Fracture: António Manuel Ináçio Martins; Copper: D. Mylius

Article has been viewed at least 1346 times.


Awesome article Don!! Clear and concise!!

Scott Rider
19th Dec 2018 7:42pm
Thank you, Scott. I started this project on 26 December 2017 and finished it (I think) about 14 December 2018. The 16 articles are about determining the properties of minerals and observing mineral crystals. Erin Delventhal and Al Ostrander has a very large role with input suggestions and critical analysis of what was written (actually keeping me out of trouble). The articles were fun to do, and our hope is that both neophyte collectors and moderately advanced students of the hobby and the science will find them useful.

Donald B Peck
19th Dec 2018 10:12pm
Great article once again, Don!
Mindat is truly a better place because of the knowledge you have shared with others.
Thank you!

Paul Brandes
28th Dec 2018 11:24pm
Paul, I don't consider myself to be that knowledgeable. But I do try to be careful, checking facts and researching that which I am writing. In addition, Al Ostrander and Erin Delventhal have added immensely. Al is a stellar morphological crystallographer and the crystallography series would not be near what it is without his input. Erin provided some input on content and layout, but her suggestion to add the rotating crystal models was, I think, a stroke of genious. And then she convinced Jolyon to do the coding so it could happen. I am truly lucky to have had their advice and help.

Donald B Peck
1st Jan 2019 7:38pm

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