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Cleavage
is the tendency of some crystals
to split along definite planes. Crystals with cleavage tend to break parallel
to certain atomic planes, creating relatively smooth surfaces.
The presence of cleavage indicates a weaker bonding
across the planes of a crystal or a greater spacing
between the planes. Some crystals are without cleavage.
The
terms which describe cleavage are quality, ease
of production and crystallographic form.
Quality is expressed as perfect, distinct, good, imperfect
and poor. Ease of production refers to how easily a
crystal splits along its cleavage planes and is
expressed as easy, difficult, etc. The crystallographic
form describes the crystal form to which the cleavage
plans are parallel. Cleavage in Calcite
is perfect, easy, rhombohedral; indicating that perfect
is the quality, easy is the ease of production
and rhombohedral is the crystallographic form.
The cleavage of a diamond is perfect, octahedral. Cleavage
in Beryl is poor, pinacoidal.
Cleavage
is of particular importance in the cutting of gemstones.
Gems with perfect cleavage must be faceted, set and
worn carefully because a sharp impact to the gem along
a cleavage direction may easily split the gem. Gem cutters
must be very aware of cleavages because it is virtually
impossible to polish a gemstone surface that is parallel
to a cleavage plane.
Cleavage,
along with fracture,
can be of important diagnostic value in determining
a type of unknown mineral.
Fracture
is the way a mineral breaks other than along cleavage
planes. Please see the Fracture
page for more information.
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