Let's look at the crystals of different substances. How to distinguish them from each other? By color? By shine? No, these signs are not reliable. For example, quartz crystals can be colorless, golden, brown, black, lilac, purple, and violet. Looking at the crystals more closely, it is easy to see their much more characteristic feature: crystals of different substances differ from each other in their forms.
Long gone are the times when they believed that crystals are only natural polyhedrons, and therefore thought that the crystals are rare, considered them a game of nature. Crystals do not need to be specially searched. On the contrary, you won't find such metals and you won't find any stones that wouldn't be crystalline. But most of the stones and metals are polycrystals, i.e. the aggregates of many small crystalline "grains", and in these aggregates the multifaceted forms of individual single crystals are already indistinguishable. And not only are they indistinguishable: these forms simply do not exist, and the monocrystal grain does not grow into a polyhedron because the same monocrystals crowd it from all sides. Therefore, there is no longer any trace of a polyhedron in the grain contours.
Often a polycrystal is formed so finely that neither the magnifying glass nor the microscope can be distinguished in it by individual crystals. How can one be sure of its crystalline structure? What is the most characteristic, the most important feature of a crystal?
The answer is that the most characteristic feature of a crystal is its atomic structure, correct, symmetrical, and natural arrangement of atoms.
Crystals are built correctly, strictly according to the law. And in them, too, atoms, ions, molecules are not at rest, but the particles do not collide with each other, because they are all located in the right order and each can oscillate about a certain position. These correct rows of particles in space, three dimensional lattices of atoms, form a crystalline structure.
Is the order of the atoms in different solids the same? Of course not. Nature is infinitely diverse and does not like repetitions. The structure of iron atoms does not resemble the construction of atoms in an ice crystal. Each substance has its own characteristic pattern and order of atoms arrangement. And the properties of the substance depend on what this order is. The same atoms of the same "variety", being located differently, form substances of very different properties.
Let's look, for example, at carbon a atoms. Soot or soot, a soft black powder collected at the bottom of a pot or in a chimney, is carbon. Coal, whether wood or stone, is also carbon. Graphite, a soft pencil rod, withstands very high temperatures, it is a crystal composed of carbon a atoms. There is another form of carbon a crystals - diamond, the most expensive and the most beautiful of the precious stones. Diamond is very hard, the hardest of all stones on Earth. They can be used to cut, polish and drill any hard stones and metals.
It is hard to believe that diamond and graphite are made up of the same carbon atoms. Graphite is soft, opaque, black. Diamond - hard, transparent, sparkling with all the colors of the rainbow. Graphite is refractory, the diamond burns easily.
The structure of the crystal determines the properties of the substance and its shape. And the correct multi-faceted form is a consequence of the atomic structure. Flat faces of the crystal correspond to flat grids of the crystal lattice, sharp straight edges correspond to rows of atoms in the lattice.
Each crystal substance can be distinguished from the other by its atomic structure. Some crystals have very simple grids, while others have complex grids. Different substances have different distances between the particles in the lattice. But all these distances are very small, they are one hundred millionths of a centimeter (angstrom).
In all crystalline substances atoms, ions, molecules form symmetrical rows, grids and grids. The correct repetitive arrangement of particles is mandatory for crystals, and it is their main feature that distinguishes them from noncrystals. When asked what crystals are, the answer is as follows: crystals are substances in which their constituent particles are arranged strictly periodically, forming a geometrically logical crystal structure.
Crystalline structure is found not only in natural polyhedrons of stones, crystalline rocks and metals, but also in many other bodies. Clay does not seem to resemble crystals, but it also consists of the smallest crystalline particles. Even in substances such as human bones, hair, wool fibers, silk, and other substances, the crystalline structure is found.
The vast majority of solids on Earth are crystalline. Only crystals are mostly not the beautiful polyhedrons we admire in museums, but tiny, sometimes invisible grains. However, the internal structure of these unsightly grains is as beautiful and surprisingly natural as the structure of wonderful large polyhedrons.