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Geology is the Way

Cleavage (mineral property)

Minerals are solid substances with atoms organized in a crystal lattice. The mechanical properties of minerals are the result of the strength of the chemical bonds of the atoms within the lattice, which varies significantly from one lattice plane to the other. Cleavage planes are evenly spaced sets of planar, parallel ‘fractures’, oriented along specific planes of weakness of the crystal structure, which can be identified with Miller indices (e.g. {001} basal cleavage in mica). Some minerals lack cleavage planes (like quartz), while others show well-developed cleavage (e.g. pyroxene). Cleavage is different from fracture, which is any irregular surface that breaks a mineral, and parting, planes of weakness similar to cleavage planes but related to the presence of impurities or inclusions.

Cleavage planes follow specific lattice planes and can be classified with respect to their orientation relative to the main crystallographic axes and the crystal symmetry of a mineral.

Basal cleavage: a cleavage plane 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 and 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: occurs in prismatic minerals, whose cleavage planes are parallel to the long axis of the prism (the c-axis). Examples are amphiboles and pyroxenes.
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).

How well cleavage planes are developed depends on the weakness of specific lattice planes relatively to the rest of the crystal. Some cleavages develop on planes that are extremely weak, while others along directions that are only slightly weaker compared to the rest of the crystal. As a result, cleavage planes range from well-developed to poorly-developed. Mineralogists developed a qualitative scheme to classify cleavage planes, based on the quality of cleavage:

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 (which can still fracture, though).

Examples of mineral cleavage

Pyroxenes
Pyroxenes have two sets of prismatic cleavage planes that intersect at nearly 90°. The intersection is visible only on basal faces. On prismatic sections, oriented parallel to the long axis of the mineral, the traces of cleavage planes appear parallel to each other. However, it is still possible to recognize that they intersect at 90° also on these sections, since they form a series of small ‘steps’ on broken surfaces.


Above: cleavage planes of pyroxene in the field. Castiglioncello, Livorno, Italy.

pyroxene in gabbro

Pyroxene (dark green). The reflective surface is a basal section, which shows cleavage planes that intersect at 90°. The surrounding pyroxene grains (darker) apparently show only one cleavage planes, since those are prismatic sections. Gabbro from Quercianella (Livorno, Italy). Width: 5 cm.

pyroxene cleavage planes

Basal section of pyroxene showing the typical cleavage planes intersecting at 90°. Vulsini, Italy. PPL. Width: 7 mm. Photo © Alessandro Da Mommio.

Amphibole
Amphiboles show prismatic cleavages that are similar to those of pyroxenes, except for the angle of intersection in the basal section, which in amphiboles is 60/120°.

hornblende basal section

Basal section of amphibole (hornblende) showing perfect cleavage planes that intersect at about 60/120°. Andesite from Dubník-Červenica (Slovakia). PPL. Width: 7 mm. Photo © Alessandro Da Mommio.

Feldspars
Alkali feldspar and plagioclase also have 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 has a well-developed rhombohedral cleavage. Its crystal break in small rhombohedrons, separated by cleavage planes.


Above: traces of rhombohedral cleavage in a large calcite crystal. Moriglion di Penna, Monti Pisani, Italy.

calcite crystal

Euhedral calcite crystals showing well-developed rhombohedral cleavage planes. Moriglion di Penna, Monte Pisano, Italy. [Blog post]

Mica
Micas (like muscovite or biotite) are lamellar minerals with a flat habit and the tendency to break parallel to their basal faces (basal cleavage).

CPL
CPL
CPL
PPL
PPL

Above: Biotite grains with well-developed basal cleavage in a schist. Width: 1.2 mm. Calamita Schists (Elba, Italy). 

CPL
CPL
CPL
PPL
PPL

Above: Large muscovite grain with prominent basal cleavage. Width: 3 mm. Calamita Schists (Elba, Italy). 

References

        

Free petrography websites:
Alexstrekeisen.it
Virtual Microscope

it_IT Italiano
Mineral Properties
Minerals

 

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