How Do Crystals Break?
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Atomic Structure Influences Breakage
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Crystals are formed by certain atoms that come together to form three-dimensional patterns. In essence, the seed of the future crystal sets up a pattern out of particular atom "ingredients," then more of the same atom ingredients attach to the outside of the fledgling crystal structure, repeating the pattern. When crystals have room to grow, they will become large and their geometric forms and faces are often easy to see--these faces arise from the internal atomic structure of the crystal. The atomic structure is important in understanding how crystals break, because in places where structural defects exist or where there are fewer atomic bonds or weaker atomic bonds, the crystal will break if enough force is applied. The resistance a mineral has to breaking is known as tenacity.
Cleavage
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Since crystals form three-dimensional patterns, the atoms have joined to one another in certain directions to form planes. You can see some of these flat surfaces looking at a well-grown crystal. In some directions, the bonds are weaker or fewer than in other directions. Graphite, for instance, creates plates that stack upon one another. Within the plates, the bond is strong, but between them, the bonds are weak. Graphite splits between the sheets. This tendency of crystals to break parallel to the atomic planes they form is called cleavage. Thus, by understanding the internal atomic structure of a crystal, you can understand cleavage directions the crystal might have. Galena, which is the main ore of lead, is quite cubic, and breaking a face will end up creating small steps in the shape of cubes.
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Parting
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When a crystal breaks in places where there is a structural weakness, the crystal is said to have parting. The parting might look like a cleavage break, but it's not. Parting won't be shown by all crystals of the same type, only by those that have been put under enough pressure to cause the part or those that are twinned. Twinning occurs when two of more growing crystals intersect with one another.
Fracture
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Some crystals neither part nor cleave because the strength of their atomic bonds are about the same in all directions. These crystals fracture instead. Some, like quartz, fracture in a curved, smooth, shell-like way, as glass might. This kind of fracture is called conchoidal. Other fractures types are fibrous and splintery; hackly, which is when the fractures produce jagged, sharp edges; and uneven or irregular, where the fractures create rough surfaces.
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