Calcite, a common carbonate mineral (\(CaCO_3\)) and the main component of limestone and marble, exhibits a highly characteristic form of breakage known as cleavage. This tendency to split along smooth, flat surfaces is one of its most recognizable physical traits. This property directly reflects the mineral’s internal atomic arrangement. This predictable breaking pattern is far more useful for identification than variable properties like color or luster.
What is Mineral Cleavage?
Mineral cleavage is the tendency of a mineral to break along predictable, flat planes of weakness within its crystal structure. This occurs because the bonds holding the atoms together are weaker along certain directions than others. When stressed, the mineral preferentially breaks where atomic forces are weakest, resulting in a smooth surface.
Cleavage is distinct from fracture, which is breakage along surfaces unrelated to the crystal structure. Fractured surfaces are typically irregular, rough, or curved, unlike the smooth, flat faces produced by cleavage. Calcite exhibits “perfect” cleavage, meaning the break is exceptionally smooth.
The Rhombohedral Cleavage of Calcite
Calcite has perfect cleavage in three distinct directions. When broken, it consistently splits along these three planes, producing a unique six-sided geometric shape called a rhombohedron. This shape resembles a squashed cube.
The three cleavage planes do not meet at 90-degree angles, distinguishing it from minerals with cubic cleavage, such as halite. Instead, the faces of the rhombohedron intersect at oblique angles, specifically 102 and 78 degrees. This consistent, non-perpendicular breakage is known as rhombohedral cleavage, and the resulting fragments display mirror-like faces.
Why Calcite Cleaves Along Three Planes
The specific cleavage pattern results directly from calcite’s internal trigonal crystal structure. Calcite is composed of calcium ions and carbonate groups arranged in a repeating lattice. The atoms are not bonded with equal strength in every direction, creating planes of relative weakness.
The bonds within the flat carbonate planes are stronger than the bonds connecting these planes to the calcium ions. When stress is applied, separation occurs along these weaker planes between the layers. Because of the trigonal system, these planes of weakness repeat in three directions, dictating the characteristic rhombohedral angle.
Cleavage as a Key Identification Tool
The distinct, perfect rhombohedral cleavage is a fundamental property used to reliably identify calcite. Since calcite appears in many colors and forms, relying on this consistent physical trait is especially useful. The three non-90-degree cleavage planes immediately distinguish calcite from other common minerals.
For example, this characteristic separates calcite from quartz, which fractures irregularly, and from feldspar, which has cleavage planes that meet at near right angles. The ability to produce small, consistent rhombs is often more diagnostic than other observable features. This property provides a quick and accurate means of mineral classification.