Cleavage

Cleavage and fracture both refer to the way a mineral yields to mechanical stress, yet they are distinct. Every mineral will fracture when subjected to force, separating along an irregular boundary. However, only a subset of minerals possess cleavage, separating along smooth, flat planes that correspond to inherent weak points in the atomic lattice. ^[1] The distinction is visible in hand samples: a cleavage surface is flat and reflective, while a fracture surface is rough and irregular.

Cleavage Quality

Cleavage is graded as perfect, good, fair, or poor based on the minimal force required to initiate the break and the resulting smoothness and extent of the cleavage faces. ^[1] A mineral with perfect cleavage breaks with flat, mirror-like surfaces consistently; one with poor cleavage may produce a surface only occasionally and imperfectly. This range reflects differences in how much weaker the cleavage planes are relative to the rest of the structure.

Number of Cleavage Directions

Minerals with cleavage may have one, two, three, or more distinct cleavage directions. ^[1] The number of cleavage directions and the angles at which they meet are among the most reliable diagnostic observations available in hand-sample work.

Crystallographic Control

Cleavages are planes of weakness in a mineral’s crystal structure and therefore have direct crystallographic control - they are parallel to specific crystallographic planes identified with Miller index notation. ^[1] This is why cleavage directions are consistent from sample to sample of the same mineral: the atomic planes that are weakly bonded are always in the same orientation relative to the crystal axes. Understanding why a particular set of planes is weak - and therefore why cleavage occurs where it does - requires understanding crystal structure and chemical bonding.

Fracture Types

When a mineral breaks without cleavage, the fracture surface morphology provides some additional information. A conchoidal fracture - smooth, curved, like broken glass - is diagnostic for minerals without cleavage planes, such as quartz and obsidian. Irregular, hackly, and splintery fractures describe progressively rougher and more pointed surface textures. ^[1] These fracture types are less diagnostic than cleavage but still useful, particularly in distinguishing minerals that are otherwise similar.

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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