Cleavage is how a mineral breaks. Some minerals, such as mica, split into thin layers. Calcite, however, splits in a different way. It splits into three planes to make a rhombohedron, or diamond shape. There are many different ways minerals can split. When you do a cleavage experiment, you look at the rock that has been broken and you describe the shape that it is split into. The first thing you ask is if the mineral shows cleavage or fracture. Cleavage splits flatly. Fracture is uneven. Haltite, pictured below, has a cubic cleavage pattern.

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