CLEAVAGE
The way a mineral breaks is determined by
the arrangement of its atoms and the strength of the chemical bonds holding
them together.
As these properties are unique to the mineral,
careful observation can aid in mineral identification.
Cleavage: mineral breaks along
parallel, flat surfaces (cleavage planes).
Example:
Halite – three directions of cleavage, 90˚
to each other. The number of cleavage angles and the angles between them
reflect the atomic architecture that defines each mineral.
Cleavage Planes: individual planes may extend across the whole mineral or,
more commonly, they may be slightly offset from each other.
Even though they are offset, they work as tiny mirrors that create
the single flash seen here.
Cleavage is described as:
Perfect: minerals
break easily along flat surfaces and are easy to spot.
Good: minerals
do not have such well-defined cleavage planes and reflect less light.
Poor: these are
the toughest to recognize, but can be spotted by small flashes of light in
certain positions.
Minerals have characteristic numbers of cleavages
– determined by counting the number of cleavage surfaces that are NOT parallel.
One cleavage direction (often called “basal” cleavage);
Two cleavage directions:
may define an elongate prism and are said to have prismatic cleavage. When only two
cleavages are present, note the angle between them (it can be diagnostic).
Three cleavage
directions: if they intersect at 90˚ = cubic cleavage; if the angles are
not 90˚ = rhombohedral.
Minerals with 4 or 6 cleavages are not common.
Four cleavage
planes can form an 8-sided shape = octahedral cleavage (e.g., fluorite).
Six cleavage planes
can form a 12-sided shape = dodecahedral cleavage (e.g., sphalerite).
When counting cleavage planes, count on only ONE crystal.