1. Mineral Cleavage and
Fracture

2. What is cleavage?
How minerals break under stress on a
particular plane
Determined by the atomic arrangement of
the mineral
Cleavage is often measured by three
factors:
1) Quality of Cleavage
2) Number of Sides Exhibiting Cleavage
3) Cleavage Habit

3. Quality of Cleavage
 Quality of cleavage can be categorized into five
qualities:
– Perfect – cleaves with no rough surfaces; perfectly
smooth break
– Good – cleaves smoothly, but with some rough surfaces
– Poor – smooth crystal edge is barely visible, because
the rough surface dominates
– Indiscernible (Indistinct) – the mineral exhibits
cleavage, but it is so poor that it’s unnoticeable
– None – fractured and rough surfaces when broken;
does not have cleavage;
 Not an official measurement used by
mineralogists
 May have other names like “excellent” or
“distinct”

4. Number of Sides Exhibiting
Cleavage
This is a measure of how many planes the mineral exhibits cleavage
on. They are labeled as:
 One direction
 Two directions
 Three directions
 All directions
Each direction includes the sides opposite of the 3-dimensional
figure (like a cube)
Cleavage directions can be combined with cleavage quality like this:
 Good Cleavage, Two Directions, means that the mineral has good
cleavage on four out of six sides (while the other two sides exhibit no
cleavage).
 Perfect Cleavage, One Direction; Poor Cleavage, Two Directions,
means that the mineral has perfect cleavage on two sides, and poor
cleavage on the other four.

5. Cleavage Habit
 The 3-dimensional shape of the
mineral after it breaks
 Depends on the crystal structure

6. Basal cleavage
 Cleavage exhibited on a horizontal plane
of the mineral by way of its base. Minerals
with basal cleavage can sometimes be
"peeled".
 An example of basal cleavage are the mica
minerals.

7. Cubic cleavage
 Cleavage exhibited on minerals of the
isometric crystal system that are
crystallized as cubes. In this method of
cleavage, small cubes evenly break off of
an existing cube.
 An example is Galena or Halite

8. Octahedral cleavage
 Cleavage exhibited on minerals of the
isometric crystal system that are
crystallized as octahedrons. In this method
of cleavage, flat, triangular "wedges" peel
off of an existing octahedron.
 An example is Fluorite.

9. Prismatic cleavage
 Cleavage exhibited on some prismatic minerals in
which a crystal cleaves as thin, vertical, prismatic
crystals off of the original prism.
 An example is Tremolite.

10. Rhombohedral cleavage
 Cleavage exhibited on minerals
crystallizing in the hexagonal crystal
system as rhombohedrons, in which small
rhombohedrons break off of the existing
rhombohedron.
 An example is Calcite.

11. Dodecahedral cleavage
 Breaks off in a direction parallel to the
direction of the rhombic faces of the
existing dodecahedron.
 An example is garnet.

12. Mineral Fracture
 There are several terms to describe the various
mineral fractures:
– Conchoidal - Fracture resembling a semicircular shell, with a
smooth, curved surface. An example of conchoidal fracture can
be seen in broken glass. (This fracture is also known as
"shelly" in some reference guides.)
– Uneven - Fracture that leaves a rough or irregular surface.
– Hackly - Fracture that resembles broken metal, with rough,
jagged, points. True metals exhibit this fracture. (This fracture
is also known as "jagged".)
– Splintery - Fracture that forms elongated splinters. All fibrous
minerals fall into this category.
– Earthy or crumbly - Fracture of minerals that crumble when
broken.
– Even or smooth - Fracture that forms a smooth surface.
– Subconchoidal - Fracture that falls somewhere between
conchoidal and even; smooth with irregular rounded corners.