Formation of crystals.
Crystallization, that is, the formation of crystals may occur in nature in various ways: 1) from the gaseous state, 2) from alloys, 3) from solutions.
Crystallization from the gaseous state can occur either as a result of sublimation, or reactions occurring between gases or vapors An example of crystal formation by vapor crystallization is the separation of fine sulfur crystals, sometimes covering the interior of volcanic craters with a delicate tarnish. Snow crystals are produced in a similar way - by cooling the water vapor.
The process of crystallization from alloys, which can be compared with the solidification processes of metals in metallurgy, many minerals owe their formation. The simplest example of this process is sulfur crystallization. Powdered when heated; sulfur, placed on a porcelain bowl, can be said, that at 119 ° C it changes from solid to liquid.
This is the melting point of sulfur. Stopping further heating is concluded, that after a while it starts to solid again. This happens first in the coldest places, i.e.. in the outer parts, and only then gradually becomes more and more the inner parts of the alloy, finally, all of the sulfur from the liquid state turns into a crystalline solid, creating lots of needle-shaped crystals.
Crystallization from solutions also plays a great role in the formation of crystals. It can take place in various ways, however, crystallization due to solvent evaporation or crystallization due to temperature drop plays the greatest role. By evaporating the common salt solution, the salt can be obtained back in crystalline form. So that the pairing takes place fairly quickly, just put the solution in a flat dish. After some time, you will notice the release of tiny crystals of solid material.
The solvent that plays the most important role in the processes taking place in nature is water. Solutions, from which minerals are released, are aqueous solutions. Solution, which, under the given conditions of temperature and pressure, contains the greatest possible amount of solute, is called a saturated solution. If, due to evaporation or temperature drop, the amount of the substance contained in the solution exceeds the saturation limit, it is the saturated solution that becomes supersaturated and the excess solute separates out in a crystalline form. The solubility of solids depends on temperature and pressure. The solubility of most solids increases with increasing temperature. By cooling the saturated solution it is thus made supersaturated, which will result in the separation of crystals. The first crystal seeds that appear already have the form of polyhedrons.
By observing the separation of a crystalline solid from the solution can be seen, that the angles between the faces of even the smallest crystal do not change with its further growth. The new crystal faces do not have any position in space. They are arranged like this, as it is determined by the strict crystallographic laws and the characteristic structure of a given substance. From the form of a crystal of a given substance separated from a solution, even if it only consists of four planes, can be predicted, what will be the further faces of the crystal and determine their inclination towards each other.
The size of the crystals varies greatly and depends both on the type of substance, and the conditions of crystallization. Minerals usually form millimeter-sized crystals, less often greater than 1 cm. However, there are also crystals with a size of several dozen centimeters, and exceptionally over 1 m. Crystals of this size, e.g.. giant quartz crystals, feldspar or beryl, they are formed under conditions of slow and quiet crystallization with a sufficient supply of substances, from which they are made.