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.