Mineral Cleavage Types to Know for Mineralogy

Mineral cleavage types describe how minerals break along specific planes, revealing their internal structure. Understanding these cleavage patterns helps identify minerals and their properties, connecting to broader concepts in mineralogy and enhancing our knowledge of Earth's materials.

1. Perfect cleavage

   • Minerals with perfect cleavage can split easily along specific planes.
   • The resulting surfaces are smooth and flat, reflecting light well.
   • Examples include mica and calcite, which exhibit this property prominently.

2. Imperfect cleavage

   • Imperfect cleavage allows for some splitting but may result in uneven surfaces.
   • The planes of cleavage are less distinct than in perfect cleavage.
   • Minerals like feldspar demonstrate imperfect cleavage characteristics.

3. Good cleavage

   • Good cleavage indicates that a mineral can be split along its planes with some effort.
   • The resulting fragments may not be as smooth as those from perfect cleavage.
   • Examples include amphibole and pyroxene, which show good cleavage but with some irregularities.

4. Poor cleavage

   • Poor cleavage means that a mineral may break along planes, but the breaks are often rough and irregular.
   • This type of cleavage is less pronounced and harder to identify.
   • Minerals such as quartz exhibit poor cleavage, often breaking in a conchoidal manner.

5. Basal cleavage

   • Basal cleavage refers to cleavage that occurs parallel to the base of the crystal.
   • This type of cleavage is common in sheet-like minerals, allowing them to peel off in thin layers.
   • Mica is a prime example, easily separating into thin sheets.

6. Cubic cleavage

   • Cubic cleavage occurs in three directions at right angles, forming cube-shaped fragments.
   • This type of cleavage is characteristic of minerals like halite and galena.
   • The resulting pieces are often cubic or rectangular in shape.

7. Rhombohedral cleavage

   • Rhombohedral cleavage involves cleavage in three directions, but the angles are not 90 degrees.
   • This results in rhomboid-shaped fragments, typical of minerals like calcite.
   • The cleavage planes are often less distinct than cubic cleavage.

8. Prismatic cleavage

   • Prismatic cleavage occurs along two directions, producing elongated, prism-like fragments.
   • This type of cleavage is common in minerals such as tourmaline and beryl.
   • The resulting pieces often have a characteristic elongated shape.

9. Octahedral cleavage

   • Octahedral cleavage occurs in four directions, forming octahedral shapes.
   • This type of cleavage is seen in minerals like fluorite and diamond.
   • The cleavage planes create distinct, symmetrical shapes when broken.

10. Dodecahedral cleavage

    • Dodecahedral cleavage involves cleavage in six directions, resulting in dodecahedron-shaped fragments.
    • This type of cleavage is less common and can be seen in minerals like garnet.
    • The resulting pieces have a complex, multi-faceted appearance.