As minerals are broken (such as with a rock hammer, for example), some may cleave, or break, along smooth flat planes known as cleavage. These flat surfaces are parallel to directions of weakness within the crystal. All the bonds among the atoms within a mineral may not be of the same strength so that when a mineral is broken, it breaks along these zones of weakness. This results in flat cleavage planes. Minerals with perfect cleavage break along a smooth, flat plane, while those with poor cleavage break in a more irregular fashion. Some minerals do not contain zones of weakness either because all of the bonds are the same strength or the weaker bonds are not aligned within a plane. If this is the case it will not have cleavage, but rather breaks in a random and irregular fashion. Make sure to distinguish cleavage from crystal form. Crystal form occurs as a mineral grows, while cleavage only forms as a mineral breaks. See Figure 7.10 for the main types of cleavage and an example of each.
|# of Cleavages & Direction||Cleavage Name||Example|
|0 (none) – mineral fractures||No cleavage planes|
|1||Basal cleavage – flat sheets|
|2 – cleavages at or near 90°||Prismatic cleavage – rectangular cross-sections|
|2 – cleavages not at 90°||Prismatic cleavage – parallelogram cross-sections|
|3 – cleavages at 90°||Cubic cleavage – cubes|
|3 – cleavages not at 90°||Rhombohedral cleavage – rhombs|
A mineral may have one or more cleavage planes. Planes that are parallel are considered to be in the same direction of cleavage and should only count as one. One direction of cleavage is termed basal cleavage. Minerals that display this cleavage will break off in flat sheets. Two directions of cleavage is termed prismatic, while three directions of cleavage at 90° is referred to as cubic. A mineral with four directions of cleavage is termed octahedral. With 2 or more cleavage planes present, it is important to pay attention to the angle of the cleavage planes. To determine the angle of cleavage, look at the intersection of cleavage planes. Commonly, cleavage planes will intersect at 60°, 90° (right angles), or 120°.
Be cautious when you see a flat surface on a mineral – not every flat surface is a cleavage plane. Crystal faces can be flat, but remember they form as a mineral grows, while cleavage forms as a mineral breaks. The crystal form of quartz is a hexagonal prism, with nice flat sides. But when quartz is hit with a rock hammer, it breaks in an irregular fashion and does not exhibit cleavage. Also, use caution when trying to distinguish the minerals pyroxene and amphibole. Both minerals are black or greenish-black, with similar hardness, making them difficult to tell apart. You must observe the cleavage angles to tell them apart. Cleavage angles in pyroxene are near 90°, so expect it to look boxy and form right angles. Cleavage angles in amphibole are 60° and 120°, so expect a more bladed or pyramid-like appearance (Figure 7.11).
|Amphibole – cleavage angles at 60° and 120°||Pyroxene – cleavage angles near 90°|