Cleavage (mineral property)
All minerals are solids consisting of atoms organized in a crystal lattice that repeats itself infinitely in all directions. The mechanical properties of minerals depends on how atoms are organized in their crystal lattice and, in particular, on the strength of chemical bonds in the lattice. The type of chemical bond and its strength may indeed vary greatly within a crystal lattice and, if weak bonds are oriented along a specific crystal plane, this will result in a plane of weakness, along which the mineral breaks preferentially at the macroscopic scale. For this reason, minerals that consists of chemical bonds with similar strength lack well-defined cleavage planes (examples: quarzo, diamond). On the other hand, minerals with complex chain or layered structures containing different elements bound together by chemical bounds with different strength tend to show one or more cleavage systems (e.g. anfibolo, pyroxene). Cleavage planes in minerals are always oriented along specific crystal directions and, for this reason, they can be named using Miller indices (e.g. {001} basal cleavage in white mica). Cleavage differs from fracture, which is any irregular surface that breaks the continuity of the crystal lattice (and that, by definition, cannot be defined using Miller indices) and from parting, which is a plane of weakness related to the presence of impurities or inclusions within a mineral.
Cleavage systems are oriented parallel to specific lattice planes and can, therefore, be classified with respect to their orientation relative with respect to crystal faces and crystallographic axes.
Basal cleavage: parallel to the basal face (or base) of a crystal (perpendicular to the c-axis). Very common in platy, lamellar minerals like micas, breaking as ‘sheets’.
Cubic cleavage: a cubic cleavage is characterized by three cleavage planes that intersect at 90°, producing small cubes. Considering the cubic symmetry of the minerals that show this kind of cleavage, the cubic cleavage is not produced by three different cleavage system but only by one, which is repeated by the symmetry of the mineral. The same happens for other three-dimensional cleavage systems (e.g. rhombohedral, octahedral, dodecahedral…). Cubic cleavage is common in cubic minerals like galena or halite.
Rhombohedral cleavage: similar to the cubic cleavage, but the angle between the three cleavage planes is different than 90°. Typical of trigonal and hexagonal minerals like calcite e dolomite.
Octahedral cleavage: it occurs in cubic minerals that crystallize as octahedrons. Four cleavage planes, repeated by the symmetry of the mineral, intersect producing small octahedrons. The classic example is fluorite.
Prismatic cleavage: parallel to the long axis (c-axis) of prismatic minerals. Examples are anfiboli e pirosseni.
Dodecahedral cleavage: some high symmetry minerals may have six cleavage planes that intersect in space separating small dodecahedrons (e.g. sphalerite).
Pinacoidal cleavage: it occurs in some prismatic minerals like barite, whose planes of weakness are oblique to both the basal and prismatic planes (i.e. pinacoidal plane).
Cleavage systems may be well or poorly developed, depending on how ‘weaker’ they are compared to the rest of the crystal. Cleavage systems developed on very weak planes tend to be well developed in comparison with those developed along planes that are only slightly weaker with respect to the surrounding crystal. The quality of cleavage planes can be estimated, using the following scheme:
Perfect: smooth cleavage surfaces.
Good: smooth cleavage surfaces with some rough asperities.
Poor: the cleavage surface is rough.
Indiscernible/indistinct: a cleavage that is so poor that is barely recognizable.
None: a mineral lacking cleavage.
Examples of cleavage in minerals
Pyroxenes
Pyroxenes show two sets of prismatic cleavage planes that intersect at nearly 90° on basal faces. The intersection is also visible as small orthogonal steps on broken prismatic section (but appear parallel in polished sections, e.g. thin sections).
Above: cleavage planes of pyroxene in the field. Castiglioncello, Livorno, Italy.
Anfibolo
Amphiboles are also characterized by prismatic cleavage planes that are similar to those of pyroxenes, except for their angle of intersection in the basal section, which is 60/120°.
Feldspars
Feldspars show two systems of prismatic cleavage that intersect at about 90°.
Above: intersecting cleavage planes in a twinned orthoclase phenocryst. Sant’Andrea, Elba, Italy.
Calcite
Calcite shows well-developed rhombohedral cleavage.
Above: traces of rhombohedral cleavage in a large calcite crystal. Moriglion di Penna, Monti Pisani, Italy.
Mica
Micas (like muscovite o biotite) are lamellar minerals with a flat habit and a tendency to break parallel to their basal faces (basal cleavage).
NX
N//
Above: Biotite grains with well-developed basal cleavage in a schist. Width: 1.2 mm. Calamita Schists (Elba, Italy).
NX
N//
Above: Large muscovite grain with prominent basal cleavage. Width: 3 mm. Calamita Schists (Elba, Italy).
Risorse
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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