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.