Cleavage is a fundamental property of minerals, describing their tendency to break along specific, smooth, flat planes of weakness. This characteristic is directly determined by the mineral’s internal atomic arrangement, known as the crystal lattice. When a mineral breaks, the resulting flat surface, known as a cleavage plane, is a repeatable feature that reflects the symmetry of the mineral’s structure. Cleavage is a physical manifestation of atomic organization within the material.
Why Minerals Break Along Flat Surfaces
The reason a mineral breaks along a flat surface is rooted in the varying strength of the chemical bonds holding the atoms together within the crystal lattice. Every mineral is composed of atoms or ions arranged in a precise, three-dimensional pattern. This repeating structure means that chemical bonds are not uniformly strong in every direction.
Cleavage planes form precisely where the bonds between layers of atoms are the weakest, offering the least resistance to applied stress. When a mineral is struck, the break occurs preferentially along these planes of weaker bonding. For example, mica can be easily peeled into thin sheets because it has extremely weak bonds in one direction.
Classifying Cleavage: Quality and Plane Directions
Geologists classify a mineral’s cleavage based on two main criteria: the quality of the break and the number of distinct plane directions. Cleavage quality describes how smooth and even the broken surface is. Categories range from “perfect,” which yields an exceptionally smooth surface, to “good,” “poor,” and “indistinct,” where the surface is rougher.
The number of cleavage directions refers to how many sets of parallel planes of weakness exist within the crystal structure. A mineral may exhibit cleavage in one, two, three, four, or six directions. Mica has one perfect cleavage direction, allowing it to split into thin, flexible sheets. Calcite displays rhombohedral cleavage (three directions intersecting at non-90 degree angles), while halite exhibits cubic cleavage (three directions meeting at 90-degree angles), producing cube-shaped fragments.
The Difference Between Cleavage and Fracture
Cleavage and fracture represent different ways a mineral breaks. Cleavage is a smooth, predictable break that occurs repeatedly along specific internal planes of weakness, resulting in flat, reflective surfaces that correlate with the crystal’s atomic structure.
Fracture, conversely, is an irregular or random break that occurs when the mineral’s bond strength is relatively equal in all directions, meaning there are no distinct planes of weakness. The broken surface does not follow any consistent crystallographic direction. Common types of fracture include conchoidal (a smooth, curved, shell-like surface seen in quartz) and irregular (a rough and jagged surface). Minerals without cleavage, such as quartz, always break by fracturing, though minerals with cleavage can fracture if struck improperly.
Cleavage as a Key Tool for Mineral Identification
Cleavage is an intrinsic property tied directly to the unique atomic structure of a mineral, making it a reliable diagnostic tool for identification. Geologists use the observation of cleavage to narrow down possibilities when examining an unknown mineral sample. The presence or absence of cleavage is often the first distinguishing factor.
Observing the number of cleavage directions, the quality of the surfaces, and the precise angles at which multiple planes intersect provides specific structural information. For example, the angle between the two cleavage planes in pyroxene is around 88 and 92 degrees, while in amphiboles, the angle is closer to 56 and 124 degrees. This difference in angle is sufficient to distinguish between these two large mineral groups, revealing the internal geometry of the crystal.