Cleavage in minerals

Cleavage refers to the specific planes of weakness along which some minerals split. These planes (or directions) are smooth, shiny and parallel to the zones of weak atomic bonding inherent in the structure of the mineral. 

Cleavage is an important “tool” used in the identification of minerals. Unlike crystal faces, which can be also flat and smooth, cleavage is only evident when a mineral is broken.

Some of the more common types of cleavage are shown below:

PINACOIDAL = one direction of cleavage (like a sheet of paper), resulting in thin, flat sheets which can be peeled apart (e.g., mica).

Two views (oblique and side) of a sheet of biotite mica (maximum dimension 26.5 cm). 

RHOMBOHEDRAL = three directions of cleavage with the cleavage planes forming angles other than 90° (e.g., calcite).

rhombohedral calcite, 3 cm high

CUBIC = three directions of cleavage with the cleavage planes forming 90° angles; (e.g., halite).

cubic halite, 7.5 cm high; the lower right side has a piece broken off

OCTAHEDRAL = when a mineral breaks in the form of a diamond, resulting in eight nearly equal faces (octahedron); (e.g., octahedral fluorite).

octahedral fluorite, 3 cm high

DODECAHEDRAL = when a mineral breaks in the form of a polyhedron with 12 faces; (e.g., garnet).

dodecahedral garnet, 4 cm high

Not all minerals have cleavage (e.g., quartz), but all minerals show fracture, which is the tendency of a mineral to break along an irregular surface. In glass and some minerals, like quartz, the broken surface is called conchoidal fracture, and it does not follow any planar surfaces (i.e., crystal faces).

conchoidal fracture in quartz, specimen is 3 cm high

quartz crystal showing natural crystal faces, specimen is  4.5 cm high