Crystals grow through a process called nucleation. Molecules are forced together in some way that they form structured chemical bonds. This can happen in two ways:
Homogeneous: A change in state, such as heat or pressure, causes the molecules to group and bond together. When water reaches freezing temperature, the molecules draw together and bond. However, the structure formed actually spreads out the water molecules further than they would be in a liquid state; this is why ice takes up more space than water. Most crystalline structures are denser than their components, and even different structures of the same compound can vary: diamonds, which are formed by heat and pressure acting on carbon, are about 60% denser than graphite.
Heterogeneous: A second material acts as a sort of scaffolding for the crystal material to gather around. Often, the material that forms the crystal is in a solvent, and as that solvent disappears, the material's molecules join together. For example, stalactites in a cave form from mineral molecules dissolved in water. As the water drips down, it evaporates, leaving the molecules behind.
This doesn't have to be a slow process. In the case of lava rock, magma beneath the earth's surface is under pressure, forcing it into a liquid state. When it reaches the surface, it cools quickly, releasing steam and gases. The leftover minerals form the rock.
If you want to see this in action at home, make some rock candy:
The sugar-covered stick serves as a surface for nucleation, where the sucrose molecules in the sugar syrup gather together to form crystals.
Posted 3480 day ago