Knife blade
Strong bond
Weak
bond
hardness of 2, can be easily scratched with a
fingernail. On the other hand, the mineral
calcite, which has a hardness of 3, will scratch
a fingernail but will not scratch glass. Quartz,
one of the hardest common minerals, will
easily scratch glass. Diamonds, hardest of all,
scratch anything, including other diamonds.
CLEAVAGE. In the crystal structure of many minerals, some atomic bonds
are weaker than others. It is along these weak bonds that minerals tend to
break when they are stressed. Cleavage
is the tendency
of a mineral to break (cleave) along planes of weak bonding. Not all minerals have cleavage, but those that do can be identified by the relatively
smooth, flat surfaces that are produced when the mineral is broken.
The simplest type of cleavage is exhibited by the micas (FIGURE 2.15).
Because these minerals have very weak bonds in one direction, they cleave
to form thin, flat sheets. Some minerals
have excellent cleavage in one, two,
three, or more directions, whereas others
exhibit fair or poor cleavage, and still
others have no cleavage at all. When
minerals break evenly in more than one
direction, cleavage is described by the
number of cleavage directions and the
angle(s) at which they meet (FIGURE 2.16).
Each cleavage surface that has a different orientation is counted as a different direction of cleavage. For example,
some minerals cleave to form six-sided
cubes. Because cubes are defined by
three different sets of parallel planes that
intersect at 90-degree angles, cleavage is
described as three directions of cleavage
that meet at 90 degrees.
Do not confuse cleavage with crystal
shape. When a mineral exhibits cleavage,
it will break into pieces that all have the
same geometry. By contrast, the smoothsided quartz crystals shown in Figure 2.1
(p. 38) do not have cleavage. If broken,
they fracture into shapes that do not
resemble one another or the original
crystals.
FRACTURE. Minerals having chemical
bonds that are equally, or nearly equally,
strong in all directions exhibit a property called fracture. When minerals fracture, most produce uneven surfaces and
are described as exhibiting irregular fracture. However, some minerals, such as
carve)
(Kleiben =
FIGURE 2.15 The thin sheets
shown here were produced by
splitting a mica (muscovite) crystal
parallel to its perfect cleavage.
(Photo by Chip Clark)
Number of
Cleavage
Directions
Shape
Sketch
1
2 at 90˚
2 not at 90˚
3 at 90˚
3 not at 90˚
4
Flat sheets
Elongated form
with rectangle
cross section (prism)
Cube
Rhombohedron
Octahedron
Elongated form
with parallelogram
cross section (prism)
Directions
of Cleavage
Sample
Muscovite
Feldspar
Hornblende
Halite
Calcite
Fluorite
FIGURE 2.16 Common cleavage directions
exhibited by minerals. (Photos by E. J. Tarbuck
and Dennis Tasa)
Strong bond
Weak
bond
hardness of 2, can be easily scratched with a
fingernail. On the other hand, the mineral
calcite, which has a hardness of 3, will scratch
a fingernail but will not scratch glass. Quartz,
one of the hardest common minerals, will
easily scratch glass. Diamonds, hardest of all,
scratch anything, including other diamonds.
CLEAVAGE. In the crystal structure of many minerals, some atomic bonds
are weaker than others. It is along these weak bonds that minerals tend to
break when they are stressed. Cleavage
is the tendency
of a mineral to break (cleave) along planes of weak bonding. Not all minerals have cleavage, but those that do can be identified by the relatively
smooth, flat surfaces that are produced when the mineral is broken.
The simplest type of cleavage is exhibited by the micas (FIGURE 2.15).
Because these minerals have very weak bonds in one direction, they cleave
to form thin, flat sheets. Some minerals
have excellent cleavage in one, two,
three, or more directions, whereas others
exhibit fair or poor cleavage, and still
others have no cleavage at all. When
minerals break evenly in more than one
direction, cleavage is described by the
number of cleavage directions and the
angle(s) at which they meet (FIGURE 2.16).
Each cleavage surface that has a different orientation is counted as a different direction of cleavage. For example,
some minerals cleave to form six-sided
cubes. Because cubes are defined by
three different sets of parallel planes that
intersect at 90-degree angles, cleavage is
described as three directions of cleavage
that meet at 90 degrees.
Do not confuse cleavage with crystal
shape. When a mineral exhibits cleavage,
it will break into pieces that all have the
same geometry. By contrast, the smoothsided quartz crystals shown in Figure 2.1
(p. 38) do not have cleavage. If broken,
they fracture into shapes that do not
resemble one another or the original
crystals.
FRACTURE. Minerals having chemical
bonds that are equally, or nearly equally,
strong in all directions exhibit a property called fracture. When minerals fracture, most produce uneven surfaces and
are described as exhibiting irregular fracture. However, some minerals, such as
carve)
(Kleiben =
FIGURE 2.15 The thin sheets
shown here were produced by
splitting a mica (muscovite) crystal
parallel to its perfect cleavage.
(Photo by Chip Clark)
Number of
Cleavage
Directions
Shape
Sketch
1
2 at 90˚
2 not at 90˚
3 at 90˚
3 not at 90˚
4
Flat sheets
Elongated form
with rectangle
cross section (prism)
Cube
Rhombohedron
Octahedron
Elongated form
with parallelogram
cross section (prism)
Directions
of Cleavage
Sample
Muscovite
Feldspar
Hornblende
Halite
Calcite
Fluorite
FIGURE 2.16 Common cleavage directions
exhibited by minerals. (Photos by E. J. Tarbuck
and Dennis Tasa)