Minerals are crystalline solids that are fundamentally identified by their unique set of physical properties. Among the most telling of these characteristics are the ways a mineral breaks when subjected to stress, which are broadly categorized as either cleavage or fracture. These breakage patterns offer direct insights into the mineral’s internal atomic structure and bond strength. Observing how a mineral splits or cracks is a primary method used by geologists to classify and distinguish one mineral from another. Both properties describe the separation of a mineral, but the resulting surfaces and the underlying mechanisms that cause them are distinctly different.
The Nature of Cleavage
Cleavage is the consistent tendency of a mineral to break along smooth, flat surfaces called cleavage planes. This predictable splitting occurs because of zones of weakness within the mineral’s repeating, ordered crystal structure. The arrangement of atoms in a crystal lattice is not uniform in all directions; instead, some planes contain weaker chemical bonds compared to others. When external stress is applied, the mineral preferentially breaks along these planes where the atomic bonds are least strong.
The quality of cleavage is described based on how easily and perfectly the mineral breaks along these planes, ranging from perfect, good, poor, or indistinct. Cleavage is further categorized by the number of distinct planes and the angles at which they intersect. Basal cleavage occurs in one direction (e.g., mica), while prismatic cleavage involves two planes (e.g., pyroxene, meeting at approximately 90 degrees). Octahedral cleavage is defined by four planes (e.g., fluorite), and rhombohedral cleavage (e.g., calcite) involves three planes that do not meet at right angles. Halite demonstrates perfect cubic cleavage, resulting in three planes that intersect at 90-degree angles, causing it to break into smaller cubes.
The Nature of Fracture
Fracture is the breakage of a mineral along surfaces that are irregular, rough, and not parallel to any internal plane of weakness. This type of breakage occurs in minerals where the strength of the atomic bonds is relatively equal in all directions throughout the crystal structure. When stress is applied to these minerals, there is no pre-determined path of least resistance for the break to follow. Consequently, the break is random and produces an uneven surface texture.
The appearance of a fractured surface is used to classify the type of fracture exhibited by the mineral. Common types of fracture include:
- Conchoidal, which creates smooth, curved, shell-like surfaces with concentric ridges. Quartz is the most common mineral exhibiting this distinct pattern.
- Hackly, which is jagged and sharp, often seen in native metals like copper.
- Splintery, which produces sharp, elongated points and is characteristic of fibrous minerals such as chrysotile.
- Uneven, which describes a rough, irregular surface that lacks any specific pattern.
Fracture is a property that all minerals possess, but it is most useful for identification in minerals that lack consistent cleavage.
Practical Application and Identification
The distinct characteristics of cleavage and fracture make them powerful tools for mineral identification in the field and laboratory. Geologists use the number of cleavage planes, the quality of the cleavage, and the precise angles at which the planes intersect to narrow down the identity of a specimen. For instance, observing two cleavage planes at 90 degrees suggests a mineral like pyroxene, while two planes at 60 and 120 degrees points toward amphibole.
Cleavage surfaces are typically smooth and reflect light, whereas fractured surfaces appear duller and irregular, providing a quick visual distinction. The specific fracture type, such as conchoidal, can be a definitive identifying feature for minerals like quartz that lack consistent cleavage. Many minerals can exhibit both properties; they will cleave perfectly along their planes of weakness but fracture randomly when broken in any other direction.