Los cristales están formados internamente por unidades idénticas de materia periódicamente ordenadas en el espacio, comopueden ver en esta representación de un cristal de pirita compuesto por átomos de hierro (naranja) y de azufre (amarillo).
“Build Your Crystal” allows you to see how, using simple cardboard boxes, a crystal is formed by stacking identical units of “integrant molecules” – thus called by René Haüy, who discovered this internal order upon noticing that when he broke a rhombohedron of calcite he obtained other smaller rhombohedrons, and from them other even smaller rhombohedrons… There is more information on the periodic ordering of units in crystals in the panel “Crystals: Ordered Matter”.
You can also see that the external shape of crystals is due to the prevailing order in their internal structure and that the same internal structure can create different morphologies. Try out how these cardboard cubes can create “crystals” in the shape of:
If you disorder all the cubes, you have created glass, an amorphous solid.
You can now understand that to create a crystal, more work, more energy, is needed than to create an amorphous. In an amorphous solid the units, the molecules, are disordered. In a crystal they must be ordered and positioned side by side. Someone has to take on the job of doing so. These are the bonds that join the units together.
In order to see how a crystal grows, you can create a crystalline surface with a layer of ordered cubes and a second that partially covers it, as seen in the figure.
Now imagine that this crystal is growing in a solution, like a salt crystal in a container with seawater. A new box arrives at the surface of the crystal and latches on to it with only one of its six faces. It is highly likely that the water molecules will pull at it to return it to the solution. But it can move over the surface and reach the step, and then it will latch on to the crystal with two of its six faces. It still has four faces that the water molecules will pull on to return it to the seawater. But if it continues moving and reaches the corner, then it will be bonded by three of its six faces. From there it will be difficult to shift it, and when another cube is placed alongside it, then there is no going back. This is how crystals grow. You can find more information on this subject in the panel “How Do Crystals Grow?”