How Are Crystals Formed on Earth?
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Crystals are minerals that grow. Their atoms bond to one another in regular, 3-dimensional patterns and use particular chemical compositions. A crystal such as a ruby--corundum--will use the same kinds of atoms to grow itself, although other elements can take the place of one or more of the components that would normally be part of the arrangement. This causes certain variations. In turquoise, for example, when iron atoms take the place of aluminum atoms, the turquoise ends up becoming greener. Some crystals form beneath the surface of the Earth-like diamonds and others form above ground, like salt crystals do as saltwater evaporates, allowing sodium (Na) and chloride (Cl) to make sea salt.
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Crystals form out of solutions, melts or vapors. Atoms in these states are distributed randomly. When the raw materials for crystals--the atoms of elements--are exposed to a change in temperature, pressure or concentration, they can begin joining to one another in patterns that build into crystals. Salt is an example of crystals formed out of a solution. Water freezing to ice is an example of crystal forming from what melted as the temperature decreases and the atoms cannot move around as freely. Snowflakes are formed from cooling water vapor.
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When conditions allow it, some crystals will grow into a regular geometric form or "morphology" that possesses faces. Crystals, however, can be considered any solid that has an ordered internal structure, even without faces. To avoid confusion, sometimes the word crystalline is used to describe those solids without faces. Sometimes crystalline substances are too fine-grained, occurring in aggregate, for their natures to be obvious without the aid of a microscope. When crystals grow like this, they are called microcrystalline. Even more fine-grained substances are called cryptocrystalline. The more traditional type of crystal can have many different types of shapes. Salt, for instance, forms into cubes. Sapphires form into barrels or pyramids and beryl (emerald and aquamarine) grows into hexagonal crystals.
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Well-formed crystals, complete with faces, need room to grow, so they usually are found in places where there had been an open space for them in rock. The would-be crystals start with a seed or nucleus. The initial ingredients come together to form the first structure from which the rest of the crystal can form. This stage is called nucleation. After the seed has come together, atoms, clumps of atoms or ions attach themselves to the outside of it and the crystal begins growing. The faster this occurs, the better, and the better the chance that the crystal will survive to turn into a larger crystal.
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