Mineral cleavage is a fundamental physical property revealing the ordered, internal architecture of a mineral. It is the tendency of a crystalline solid to consistently break or split along smooth, flat surfaces called cleavage planes. This preferential breakage occurs because the chemical bonds holding the atoms together are naturally weaker along specific directions within the mineral’s repeating atomic structure. When subjected to stress, the mineral separates along these inherent planes of weakness, making cleavage an invaluable tool for identification.
Understanding the Difference Between Cleavage and Fracture
The distinction between cleavage and fracture lies in the arrangement and strength of the mineral’s atomic bonds. Cleavage is an organized break that follows planes of lower bond strength within the crystal lattice. This results in clean, parallel, and often highly reflective flat surfaces on the broken specimen. When multiple cleavage planes are present, they may intersect to form repeating, geometric shapes, sometimes appearing as a microscopic “stair-step” pattern across the surface.
Fracture, conversely, is the result of a mineral breaking randomly across atomic bonds that are essentially uniform in strength across all directions. Because the force required to break the bonds is similar everywhere, the mineral does not split along any predefined plane. This type of breakage produces irregular, rough, or jagged surfaces that do not follow a consistent pattern. The classic example of this is a conchoidal fracture, characterized by smooth, curved, shell-like surfaces, commonly seen in minerals like quartz or obsidian.
Techniques for Identifying Cleavage Planes
Identifying cleavage planes requires careful, systematic observation, often starting with existing broken surfaces. The most effective technique involves using a strong, focused light source and slowly rotating the mineral sample. As the sample turns, any present cleavage planes will momentarily reflect the light in unison, creating parallel flashes of light known as “sheeting.” These flashes confirm the presence of a single, continuous, flat surface.
For a clearer view, examine the broken edges of the mineral using a hand lens. Look specifically for tiny, flat, repeating surfaces that are parallel to one another, which is a strong indicator of cleavage. If an existing specimen does not have a fresh break, it may be necessary to gently tap the mineral with a small hammer to create a new surface for analysis. The goal is to confirm that the flat surfaces are parallel to one another and occur in the interior of the material, distinguishing them from random crystal faces.
Characterizing the Cleavage
Once cleavage has been identified, it is characterized using two primary criteria: its quality and its geometric configuration. Quality describes how cleanly and easily the mineral splits, based on the contrast in bond strength. Perfect or excellent cleavage, such as that found in mica, means the mineral splits effortlessly to produce mirror-smooth, continuous planes. Good or distinct cleavage yields smooth surfaces with minor irregularities, while poor or indistinct cleavage results in rougher surfaces that are difficult to discern.
The geometric configuration is described by the number of directions of cleavage and the characteristic angles at which these planes intersect. Muscovite mica exhibits basal cleavage (one direction), allowing it to peel into thin sheets. Halite displays cubic cleavage (three directions) that meet at 90° angles, producing perfect cube-shaped fragments. Calcite also has three directions of cleavage, but they meet at angles other than 90° (typically 78° and 102°), defining its rhombohedral cleavage. Minerals in the amphibole group are identified by two cleavage directions that intersect at approximately 60° and 120°.