Cleavage, Parting and Fracture

Cleavage.
If a mineral, when the proper force is applied, breaks so that it shows definite plane surfaces, it is said to possess a cleavage. These cleavage surfaces resemble natural crystal faces. They are always parallel to some possible crystal face, and usually to one having simple relations to the crystallographic axes. They may be perfect, as in the cases of the micas, calcite, gypsum, etc., or they may be more or less obscure. Cleavage is due to the fact that in the mineral structure there is a certain plane or planes perpendicular to which the molecular cohesion is weaker than in other directions. All minerals do not show cleavage, and only a comparatively few show it in an eminent degree. The quality of the cleavage and its crystallographic direction are often important aids in the identification of a mineral. The cleavage of a mineral is described according to the crystal face to which it is parallel, as cubic cleavage (galena, halite) (see Fig. 192), octahedral cleavage (fluorite), dodecahedral cleavage (fluorite), dodecahedral cleavage (shplerite), rhombohedral cleavage (calcite), prismatic cleavage (amphibole), basal cleavage (topaz), pinacoidal cleavage (stibnite), etc.

Parting
Certain minerals when subjected to a strain or pressure develop planes of molecular weakness along which they may subsequently be broken. Crystals may also contain twinning lameleae along whose surfaces the crystal may break. When plane surfaces are produced on a mineral in this way it is said to have a parting. This phenomenon resembles cleavage, but is to be distinguished from it by the facts that not every specimen of a certain mineral will exhibit it, but only those specimens which have been subjected to the proper pressure, and that even in these specimens there are only certain planes in the given direction along which the mineral will break. In the case of cleavage, every specimen of the mineral will in general show it, and it can be produced in a given direction in all parts of a crystal. Familiar examples of parting are the cases of the octahedral parting of magnetite, the basal parting of pyroxene and the rhombohedral parting of corundum.

Fracture
By the fracture of a mineral is meant the way in which it breaks when it does not show plane surfaces as in cleavage or parting. The following terms are commonly used to designate different sort of fracture:

• a. Conchoidal. When the fracture has smooth, curved surfaces like the interior surface of a shell it is said to be conchoidal. This is most commonly observed in such substance as glass, quartz, etc.
• b. Fibrous or Splintery. When the mineral breaks showing splinters or fibers.
• c. Hackly. When the mineral breaks with a jagged, irregular surfaces with sharp edges.
• d. Uneven or Irregular. When the mineral breaks into rough and irregular surfaces.