Geology Reference
In-Depth Information
ample, metallic minerals can be hammered and shaped without breaking into pieces, so
they are called
malleable.
Some minerals like mica are
elastic:
They bend and then
bounce back to their original form. Many minerals (including pyrite, hornblende, and
olivine) are
brittle,
which means they break fairly easily into smaller pieces.
Cleavage and fracture
When you take your hammer and tap (or smash) a mineral, the way it breaks tells you
important information about its crystal structure and molecular bonds. The way a min-
eral breaks can be categorized in one of two categories: cleavage or fracture.
If a mineral has layers of weaker and stronger bonds, it will
cleave
(break) along
cleav-
age planes:
planes of weakness in the crystal structure. These cleavage planes produce
flat surfaces and angled geometry that are useful for identifying the mineral. The bonds
between atoms along the cleavage surfaces are weaker than other atomic bonds in the
minerals. Two minerals that are the same in other characteristics may have different
cleavage planes that allow you to accurately identify them.
A few examples of mineral cleavage planes are illustrated in Figure 6-3. Mineral cleavage
is described as the number of planes (flat surfaces) and the angle at which they inter-
sect.
In Figure 6-3, the sketch of muscovite has one cleavage plane and no angles of intersec-
tion, forming sheets. Feldspar has two planes of cleavage that intersect at a 90-degree
angle (a right angle). Calcite also has two cleavage planes, but they do not intersect at
right angles. And halite has three planes of cleavage intersecting at 90-degree angles,
which create cube shapes. There are also minerals with four cleavage planes (like fluor-
ite) and even five or six (such as the mineral sphelurite).
If all the bonds within the crystal are equally strong, the mineral will
fracture
instead of
cleave
.
A mineral that fractures often fractures
irregularly,
or with rough, uneven sur-
faces. However, some minerals fracture in a way that leaves a smooth, curved surface,
similar to a clamshell in appearance. This type of fracture, called a
conchoidal
fracture,
is illustrated in Figure 6-4.
Figure 6-3:
Cleav-
age planes of mus-
covite, feldspar,
halite, and calcite.
