Crystallization is a natural process which occurs as materials solidify from a liquid, or as they precipitate out of a liquid or gas. This can be caused by a physical change, such as a temperature change, or a chemical change such as acidity. Crystallization is a process directed by the size and shapes of the molecules involved, and their chemical properties. Crystals can be formed out of a single species of atom, different species of ions, or even large molecules like proteins. Some large molecules have a harder time undergoing the crystallization process, because their internal chemistry is not very symmetrical or interacts with itself to avoid crystallization.
The smallest unit of a crystal is called the unit cell. This is the base formation of atoms or molecules upon which additional units can be attached. You can think of this as a children’s building block, to which other blocks can be attached. Crystallization proceeds as if you were attaching these blocks in all directions. Some materials form different shaped crystals, which accounts for the great variation in shape, size, and color of various crystals.
The first step in the crystallization process is nucleation. The first atoms in the mass to form a crystal structure become a center, and more atoms organize around this nucleus. As this happens, more unit cells assemble around the nucleus, a small seed crystal is formed. The process of nucleation is extremely important in crystallization, is the nucleus of a crystal will determine the structure of the entire crystal. Imperfections in the nucleus and seed crystal can lead to drastic rearrangements as the crystal continues to form. Nucleation happens in a supercooled liquid or a supersatured solvent.
A supercooled liquid is any liquid on the verge of becoming a solid. In order for that to happen, an initial nucleus must form. It is around this nucleus that the process of crystallization will continue. In a cooling liquid, the nucleus will form when atoms or molecules no longer have the kinetic energy to bounce off of each other. Instead, they begin to interact with each other and form stable crystal formations. Pure elements typically form a crystal structure, while larger molecules may be hard to crystalize at normal temperatures and pressures.
In a supersaturated solution, the solvent carrying the desired crystal is at capacity. As the temperature cools, or the acidity changes, the solubility of the atoms or molecules in the solution changes, and the solvent can hold less of them. As such, they “fall out” of the solution, colliding into each other. This too causes nucleation, and subsequent crystallization.
As other molecules and atoms surround the nucleus, they branch of from the symmetry which has already been set up, adding to the seed crystal. This process can happen very quickly, or very slowly, depending on the conditions. Water can crystalize into ice in a matter of minutes, while it takes millennia to form “typical” geological crystals like quartz and diamonds. The basic formation set up around the nucleus determines the entirety of the crystal structure. This difference in formation accounts for the differences in crystals, from the uniqueness of a snowflake to the clarity of a diamond.
There are only a handful of geometric shapes that crystals can take. These are determined by the bonds and interactions of the molecules involved. The different shapes are caused by the different bond angles of atoms, based on the original nucleus. An impurities in the solution or material will lead to diversion from the typical pattern. As seen in snowflakes, even tiny impurities in the nucleus lead to completely new and unique designs.
Laboratory Uses of Crystallization
Crystallization is a common and useful laboratory technique. It can be used to purify substances, and can be combined with advanced imaging techniques to understand the nature of the substances crystallized. In laboratory crystallization, a substance can be dissolved into an appropriate solvent. Heat and changes in acidity can help the material dissolve. When these conditions are reversed, the materials within the solution precipitate out at different rates. If the conditions are controlled properly, pure crystals of a desired substance can be obtained.
An advanced imaging technique, called crystallography, x-rays or other high-energy beams and particles can be shot through the crystal structure of a pure substance. While this doesn’t create a visible image, the rays and particles are diffracted in specific patterns. These patterns can be detected by special developing paper or electronic detectors. The pattern can then be analyzed by mathematics and computers, and a model of the crystal can be formed. The diffraction patterns are created when particles or beams are redirected by dense electron-clouds within the crystal structure. These dense areas represent the atoms and bonds present in the crystal, formed during crystallization. Using this method, scientists can recognize almost any substance based on its crystal form.
Crystals can take an enormous amount of time to form, or they can form quickly. Scientists were able to study crystallization, because there are many events in nature in which crystallization takes place quickly. As already discussed, ice and snowflakes are great examples of the crystallization of water. Another interesting example is the crystallization of honey. When bees regurgitate honey into the honeycomb, it is a liquid. Over time, sugar molecules within the honey begin to form crystals, through the process of crystallization described above. If you have an old bottle of honey, look inside. There will likely be little crystals of sugar within the liquid. If you want to speed the process up, put the honey in the refrigerator. The cooling of the liquid decreases the solubility of the sugar within the liquid, and it will rapidly form crystals.
Geological time scale
While the process is similar, the time it takes to form things like quartz, ruby, and granite are much longer. These crystals are formed under extremely high pressures within the crust and magma of the Earth. While the process of crystallization is the same, it takes a long time for the conditions and atoms to unite in just the right way to crystallize. These processes can be replicated in the laboratory, in shorter times, by creating ideal conditions for the crystallization to occur. Laboratories can also grow seed crystals, which can be introduced to greatly speed the production of large batches of crystal at once.
On a slightly shorter timescale, mineral buildups like stalactites and stalagmites are also formed through the crystallization process. As small drops of water are dropped onto these crystals, the minerals within are integrated into the crystal structure already present, and the water drains off.