7.5: Cleavage - Geosciences LibreTexts

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As minerals are broken (such as with a rock hammer, for example), some may cleave, or break, along smooth flat planes known as cleavage. These flat surfaces are parallel to directions of weakness …
As minerals are broken (such as with a rock hammer, for example), some may cleave, or break, along smooth flat planes known as cleavage. These flat surfaces are parallel to directions of weakness within the crystal. All the bonds among the atoms within a mineral may not be of the same strength so that when a mineral is broken, it breaks along these zones of weakness. This results in flat cleavage planes.