Mineral cleavage describes a mineral’s tendency to break along flat, smooth surfaces. This property results in repeatable, parallel planes, known as cleavage planes, which are a direct reflection of the mineral’s internal structure. Observing and characterizing this specific breakage pattern is a primary method used for mineral identification. The presence, number, and orientation of these planar surfaces serve as a unique fingerprint for a mineral species.
The Atomic Basis of Cleavage
Cleavage is a direct consequence of a mineral’s internal atomic arrangement, known as a crystal lattice. Atoms are held together by chemical bonds, but the strength of these bonds is not uniform in all directions. Some planes within the structure involve fewer or weaker bonds than others, creating inherent zones of structural weakness.
When a force is applied to the mineral, it preferentially breaks along these planes of weak bonding. Less energy is required to separate the atoms there than in other directions. The resulting cleavage plane is a flat, continuous surface that runs parallel to the atomic layers of weakness.
Minerals that possess bonds of roughly equal strength in all directions, such as quartz, do not exhibit true cleavage because there are no predetermined weak planes for the break to follow. Conversely, minerals with complex, layered or chained atomic structures, like the sheet silicates, often show pronounced cleavage. The characteristic angles and directions of cleavage are predetermined by the mineral’s unique chemical composition and crystal geometry.
Describing Cleavage Quality and Pattern
Geologists use specific terminology to describe cleavage based on two main factors: the quality of the break and the geometric pattern it creates. Cleavage quality is a measure of how easily and cleanly the flat surfaces are produced when the mineral breaks. The highest quality, termed “perfect” or “excellent,” results in exceptionally smooth, mirror-like surfaces that separate with little effort.
Cleavage surfaces described as “good” or “distinct” are still identifiable as flat planes, but they may appear slightly less smooth or require more force to produce. At the lower end of the spectrum, “poor” or “indistinct” cleavage is difficult to observe and often results in rougher, less defined surfaces. The quality of the cleavage is directly related to the difference between the strongest and weakest bonds within the mineral’s structure.
The pattern of cleavage is described by the number of directions in which the mineral breaks and the angles at which those planes intersect. For instance, the mineral mica exhibits basal cleavage, meaning it has one direction of cleavage that allows it to be peeled into thin, flexible sheets. Halite, or table salt, displays cubic cleavage, breaking in three directions at 90-degree angles to produce cube-shaped fragments. Calcite, on the other hand, also has three cleavage directions, but they intersect at angles other than 90 degrees, resulting in rhombohedral fragments.
Identifying Cleavage in Minerals
Identifying cleavage in a mineral sample requires careful visual observation of broken surfaces. A true cleavage surface appears flat and smooth, often resembling a polished face on a crystal. When the mineral specimen is rotated under a light source, these flat planes reflect the light in a continuous, uninterrupted flash or sheen.
Multiple parallel lines or steps across a flat surface also indicate the presence of cleavage. These lines represent the edges of successive, microscopic cleavage planes that have separated. If a mineral has more than one cleavage direction, the observer must measure the angle at which the flat surfaces intersect to determine the specific cleavage pattern.
It is important to remember that cleavage is a property of a broken surface, not a surface formed during crystal growth. While crystal faces can also be flat, they form as the mineral grows, whereas cleavage surfaces form only when the mineral is subjected to stress and breaks. The consistent angle and parallel nature of the broken surfaces are the definitive characteristics used to confirm the presence of cleavage.
Cleavage Versus Fracture
Cleavage and fracture are two distinct ways a mineral can break, and distinguishing between them is important for mineral identification. Cleavage is predictable, following the predetermined planes of weakness in the atomic structure. Fracture occurs when a mineral breaks randomly across bonds that are of relatively equal strength, or in a direction that does not follow a weak plane.
The physical appearance of a fractured surface is irregular, rough, or uneven, lacking the characteristic smoothness and reflectivity of a cleavage plane. A common type of fracture is conchoidal fracture, which produces a smooth, shell-like, curved surface. Fracture surfaces are not parallel and do not yield repeatable, geometric shapes.