Calcite is one of the most common minerals on Earth, forming the primary component of limestone and marble. Chemically, it is calcium carbonate (\(\text{CaCO}_3\)), a compound foundational to the shells and skeletons of many marine organisms. When geologists examine a mineral, one of the most reliable characteristics for identification is its tendency to break along specific, flat surfaces, a property known as cleavage. Understanding calcite’s specific cleavage pattern is key to distinguishing it from other minerals.
Understanding Mineral Cleavage
Mineral cleavage is the tendency of a crystalline solid to split along smooth, flat planes of structural weakness. This behavior results directly from the orderly, repeating arrangement of atoms in the mineral’s internal crystal lattice. Cleavage surfaces are flat, smooth, and often reflective, contrasting sharply with the irregular surfaces produced by a random break.
Cleavage is described by its quality and the number of directions in which the mineral breaks. Quality ranges from “perfect,” meaning the break produces consistently smooth, continuous planes, to “poor” or “indistinct.” The number of cleavage directions refers to the distinct sets of parallel planes along which the mineral separates. These properties are constant for a given mineral and serve as diagnostic tools for identification.
The Rhombohedral Cleavage of Calcite
Calcite has perfect cleavage in three distinct directions. When broken, it consistently separates along these three planes of weakness, resulting in fragments that maintain a characteristic geometric shape. This shape is known as a rhombohedron, which is a six-sided prism where all faces are diamond-shaped.
This rhombohedral cleavage is distinct because the three cleavage planes do not intersect at \(90^{\circ}\) angles, unlike the cubic cleavage seen in minerals like halite. Instead, the adjacent faces meet at two specific, oblique angles: approximately \(74^{\circ}\) (acute) and \(106^{\circ}\) (obtuse). The consistent production of these non-orthogonal angles, even in the smallest fragments, is the defining physical manifestation of calcite’s cleavage.
Crystal Structure: The Reason for Calcite’s Break
The reason calcite breaks into rhombohedrons lies entirely within its atomic structure, which belongs to the trigonal crystal system. Calcite’s crystal lattice is composed of alternating layers of positively charged calcium ions (\(\text{Ca}^{2+}\)) and negatively charged carbonate groups (\(\text{CO}_3^{2-}\)). The carbonate group itself is a tightly bonded, planar arrangement of one carbon atom and three oxygen atoms.
The chemical bonds holding the carbon and oxygen atoms together within the planar \(\text{CO}_3^{2-}\) group are strong. Conversely, the electrostatic bonds linking the calcium ions to the adjacent carbonate groups are comparatively weaker. These weaker linkages create naturally occurring planes of weakness throughout the crystal structure.
When mechanical stress is applied, the mineral preferentially breaks along these specific planes where the bonds are weakest. The orientation of these weak bonding planes, determined by the geometry of the \(\text{Ca}^{2+}\) and \(\text{CO}_3^{2-}\) layers, dictates the precise \(74^{\circ}\) and \(106^{\circ}\) angles of the rhombohedral cleavage. Calcite’s cleavage is thus a direct, macroscopic expression of its internal atomic geometry and differential bond strengths.
Cleavage Versus Fracture
Cleavage is often confused with fracture, but the two describe different ways a mineral breaks. Cleavage is a structure-controlled break, meaning the resulting surfaces are smooth, flat, and repeatable because they follow fixed internal planes of weakness. Fracture, in contrast, occurs when a mineral breaks randomly along a surface unrelated to any structural weakness in the atomic lattice.
Minerals that have bonds of roughly equal strength in all directions, such as quartz, do not exhibit cleavage and instead fracture. The broken surfaces produced by fracture are irregular and rough. A common type of fracture is conchoidal, which results in smooth, curved, shell-like surfaces, similar to broken glass. Observing whether a break yields smooth, angled faces or an irregular, rough surface is a reliable way to differentiate cleavage from fracture.