When crystals break, they can either split leaving a clean, flat face called a cleavage plane, or fracture leaving a more rough, uneven surface. We can find out more about a crystal by looking at the way it breaks.
Cleavage planes form along the weakest area of mineral's structure. If you break a mineral with a hammer it will always split along its weakest points. This is quite important, and gives some minerals a characteristic shape.
Mica, for example, has only one really good cleavage plane, it splits easily into very thin layers. Another example, calcite, will split along three cleavage planes giving a 'diamond' shape called a rhombohedron. You can see this in the example below .
About this resource
Science topic: Minerals
Key Stage: KS2, KS3
Keywords: minerals, mineral properties, mineral cleavage plane, mineral fracture
If a mineral's structure is equally strong in all directions it will not have any cleavage planes. Instead it will break unevenly, or fracture. There are different types of fracture. In the example below, quartz has a conchoidal (shell-shaped) fracture. Copper can have a jagged, hackly fracture.
Cleavage and fracture are important tools you can use to identify minerals, but you don't need to break your specimens to see this. Take a closer look, you should be able to see cleavage or fracture on any broken surface.