Cleavage of Minerals

the ability of most minerals to break in certain directions yielding plane surfaces. Cleavage is a property related to characteristics of the mineral’s crystal structure. The cleavage planes are parallel to the lattice planes most densely packed with atoms, that is, planes with the greatest reticular density; the binding force between these planes is minimal. The type of chemical bond is also important; for example, with diamond and sphalerite, which have identical structures, the directions of the cleavage planes differ because of the chemical bonds, which are covalent in the former and ionic in the latter.

Depending on the ease with which a mineral breaks to yield cleavage planes, cleavage is described as highly perfect (mica, chlorite), perfect (calcite, galena), good (feldspars), or imperfect (apatite, cassiterite). It is also possible that cleavage may be virtually lacking (gold, corundum). Several types of cleavage can arise in a crystal corresponding to the crystal’s symmetry and structure. The ability of minerals to split in directions other than those of cleavage is called parting. Cleavage is an important diagnostic feature of minerals.