The Mineral Celestite
Cleavage & Fracture
Along with determining the hardness, the bonding and molecular structure of a given mineral will also determine the manner in which a mineral breaks. Surfaces held together by relatively weak bonds, such as those between repeated parallel layers of a crystal, will tend to break more easily than those held together by strong bonds. The tendency of a mineral to break along these flat parallel surfaces is known as cleavage. This can best be seen in the atomic model of the muscovite mica crystal below.
Crystal Structure of Muscovite
Notice the lack of bonds between the large yellow atoms (representing potassium) and the layers of highly bonded silica tetrahedra, aluminum, and hydroxide ions.
The number and strength of bonds between the silica, aluminum, and hydroxide ions make those layers much stronger. Therefore, muscovite mica is much more likely to break along the layers that only contain the weakly bonded potassium ions. This results in 1 excellent cleavage plane of mica. This cleavage is observed in the ability to peel sheets of mica.
Cleavage can be described as being excellent, good, poor, or absent. Excellent cleavage, such as that of muscovite mica, will result in smooth, flat, parallel surfaces. Good cleavage will often result in small, smooth, step-like flat surfaces.
In minerals that exhibit poor cleavage, it may be difficult to identify any cleavage surfaces. In poor cleavage, the cleavage surface is often mixed in with fractured surfaces. If a mineral exhibits no cleavage, cleavage is said to be absent; in these cases the mineral is said to have fracture.
Fracture can be described as either being irregular, as in the diagram at right, or conchoidal. Conchoidal fracture will result in the mineral breaking along smooth, curved surfaces. Typically, these surfaces resemble the inside of clam shells.
In order to describe the cleavage of a mineral, you must first count how many planes of cleavage exist. To do this, first rotate the mineral until you find a smooth flat surface. If the surface is a cleavage plane, there will be another smooth flat surface parallel to the first surface on the opposite side of the mineral. Two parallel smooth flat surfaces equal one cleavage plane. Therefore, if a mineral is in the shape of a cube, you should be able to count 3 cleavage planes.
When describing cleavage it is also necessary to describe the angle between the cleavage planes. If, for example, a mineral is in the shape of a cube the angle between the cleavage planes should be 90Â°. If, however, the angle is not 90Â° the mineral will no longer have a cubic shape. Below is a brief chart describing the shapes associated with certain cleavages.