Cleavage is a key feature in identifying minerals. The angles between cleavage surfaces are important to determining the crystal form. Some minerals produce smooth, flat surfaces when they break. However, many do not. Therefore, it is important to evaluate the quality of the cleavage surfaces to determine the crystal form.
Cleavage planes form along weak areas of the mineral structure. For example, a hammer smashed into calcite will break the rock along the cleavage planes, causing the calcite crystal to fracture along the planes of weakness.
Cleavage surfaces may be found in many forms in rocks. Sometimes, they can be as distinct as rice terraces on a mountainside. They reflect sunlight at a specific angle, giving a distinctive appearance. For other types of minerals, cleavage surfaces are only weak and not visible.
Cleavage occurs when the bonds between atoms in a mineral break along certain planes. Minerals may have several cleavage planes, and some are parallel to the faces. If the cleavage planes are parallel to the faces, this is not a problem.
Cleavage planes are diagnostic for many minerals. Their angles and number of cleavage surfaces are usually used in the identification process. Learning to recognize these surfaces is an essential part of mineral study. There are many examples of cleavage planes, and it is easy to find the cleavage planes in a stone.
Cleavage planes are often difficult to distinguish from crystal faces, but if you understand how cleavage planes work, you can distinguish between them. They may look like multiple parallel cracks or flat surfaces, and sometimes they look like stairways or terraced rice paddies. The angles between cleavage planes represent the direction of weakening in the crystal.
Fractures are a common characteristic mark in rocks. The type of fracture depends on how the mineral is broken. Some minerals have jagged edges, while others have smooth, even surfaces. When broken, quartz will fracture. But not all minerals fracture this way.