What is Crystallisation?
Crystallisation Simply Explained
Crystallisation is the process by which atoms or molecules arrange themselves into an ordered, repeating structure to form a solid.
One common way crystallisation occurs is from a homogeneous solution. A homogeneous solution is uniform throughout, meaning it has the same composition wherever it's tested. The word comes from the Greek for 'same kind'.
A simple example of crystallisation is water freezing to form ice or snow. When atoms come together in a highly ordered repeating arrangement, they form a crystal.
Crystals can also form through precipitation, a process in which dissolved minerals come out of a solution. A simple example is salt crystallising after being dissolved in water.
In nature, crystallisation can occur quickly or over very long periods of time. Some crystals that form deep within Earth’s crust take millions of years to grow, while crystals grown at home may form in a matter of hours or days.
To grow crystals at home, dissolve three cups of sugar or salt in one cup of boiling water. Pour the solution into a jar and add a piece of string that has been roughened with a knife, as crystals grow more easily on rough surfaces.
As the water slowly evaporates, the solution becomes saturated and crystals begin to form. This crystallisation occurs through precipitation, leaving behind a solid mass of crystals.
If a saturated solution cools quickly, the result will be smaller crystals. If it cools slowly, the crystals will be larger because they have more time to grow.
The same process occurs in nature. Granite has large crystals because the magma from which it formed cooled and crystallised very slowly over millions of years. Obsidian, which is volcanic glass, has no significant crystal structure because the lava cooled so rapidly that crystals had no time to grow.
A naturally occurring substance that does not have an internal crystalline structure is not a mineral. Minerals are generally crystalline, while non-crystalline substances are correctly known as mineraloids.
The fastest way to see the effects of crystallisation is to place some water in the freezer. Within a few hours, you'll have ice, which is crystallised water.
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Porphyritic Granite: Courtesy of Stan Celestian. Porphyritic granite is granite with large, visible crystals surrounded by much smaller crystals, showing that the rock did not cool at a single, even rate.
