Cleavage / fracture
Cleavage and fracture describe different ways in which a mineral can break.
Cleavage - The tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice. These two-dimensional surfaces are known as cleavage planes and are caused by the alignment of weaker bonds between atoms in the crystal lattice. Cleavage planes are distinguished from fracture by being smooth and often having reflective surfaces.
Fracture - The way a mineral breaks other than along a cleavage plane.
Cleavage terms (only use if cleavage planes can be recognised):
Perfect - Produces smooth surfaces (often seen as parallel sets of straight lines), e.g. mica;
Imperfect - Produces planes that are not smooth, e.g. pyroxene;
Poor - Less regular.
Fracture terms (use in all other cases):
Conchoidal - Fracture surface is a smooth curve, bowl-shaped (common in glass);
Hackly - Fracture surface has sharp, jagged edges;
Uneven - Fracture surface is rough and irregular;
Fibrous - Fracture surface shows fibres or splinters.
Notes: The number of cleavage planes can differ from mineral to mineral. Mica (e.g. biotite, chlorite or muscovite) has one cleavage plane, feldspar (e.g. orthoclase or plagioclase) has two which intersect at 90°, and amphibole (e.g. hornblende) has two which do not intersect at 90°. Calcite has three cleavage planes which do not intersect at 90°.
Specimens of plagioclase and calcite, and particularly good specimens of some other minerals, may show twinning. Unlike cleavage planes, across which there is no change in reflectance, twin planes will appear alternately reflective / dull / etc. Other minerals (e.g. pyrite, magnetite) may show striations that are very similar to cleavage in appearance.