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What is Mineral Cleavage?

man looking through a loupe at a small gemstone that he's holding in tweezers

Mineral Cleavage Simply Explained

Cleavage is the tendency for a crystalline solid to break along smooth, even flat surfaces.  These are known as planes of weakness.

Cleavage planes are the lines of weakness in the structure of a crystal.  In other words the building blocks of the crystal have natural planes of weakness.

The cause of the weakness can be traced to the structure of the atoms that make up the crystal.

A crystalline solid is a mineral.  A naturally occurring solid that's non crystalline meaning it's not made up of crystals is known as a mineraloid or an amorphous solid. Examples include obsidian, shungite and opal.

When a mineral breaks along a plane of structural weakness it leaves a smooth flat face.  The new crystal face created at the point of the break is known as a cleavage plane.  Cleavage planes form along the weakest section of a mineral.
A mineral can have one, two, three, four or six cleavage planes.  This refers to different sides of the crystal that cleave.  Minerals that don't exhibit a smooth flat surface on any one side generally have no cleavage planes.  The bond of these materials is equally strong in all directions.

A mineral with one cleavage plane breaks into thin flat sheets.  Those with two break into a cube that's slightly more elongated and those with three cleavage planes break into an almost perfect cube.

Fluorite breaks along four cleavage planes.  This forms an octahedral shape which is similar to two pyramids stuck base to base.

Although diamond is the hardest material on Earth so can't be scratched by any other mineral, if you can find a plane of weakness it can be cleaved.  Due to its hardness it wouldn't be quite as easy as splitting a softer mineral along a cleavage plane.  Like fluorite, diamond also cleaves along four cleavage planes.    

Minerals described as having a "perfect cleavage" cleave with an exceptionally clean break.  Both edges and the new crystal faces that are created as a result of the break are smooth and flat. Cleavage surfaces always reflect light.

The term "good cleavage" describes a mineral that breaks along planes of weakness but leaves a slightly rougher edge.   Poor cleavage means it breaks in a more random manner and with rougher edges.

The surface of minerals whose cleavage is good or poor will still reflect light from the same direction. 

Non crystalline solids have no crystal structure because they're not made up of crystals.  As such they cannot have cleavage planes.  Instead of cleaving these materials fracture.

Minerals with cleavage planes can also fracture.  Mineral cleavage and fracture are not the same.  When a mineral fractures it breaks randomly instead of breaking along specific planes of weakness.  Once broken, no two pieces will be the same.  Think of a piece of a glass after it's been dropped.

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