Physical Properties of Crystals
The physical properties of crystals like hardness, cleavage, optical properties, heat and electrical conductivity differ from crystal to crystal. Crystallographers deem it necessary to learn more about these properties so that they can determine what the crystals can be used for.
Crystallography is the scientific study of crystals where the arrangement of atoms and molecules in solid matter are determined. With the developments made in this field of science, crystallographers have found that crystals have numerous physical properties. It is important to know that all crystals do not possess the same properties and hence crystals have been classified into different classes and groups. Crystals have hence been divided into 32 different classes to make studying about them easier. Some of the important physical properties of crystals are discussed below.
The color in crystals appears as some wavelengths of light are absorbed by the solid matter. Some metals like chrome and iron and some others have color as they are able to absorb these particular wavelengths of light. Color is not a very good property to use for identification of crystals as many different kinds of matter are of the same or similar color and can hence cause confusion.
Refraction of light
There is a change in the refraction of light through materials. The velocity at which light passes through a given material is inversely related to its index of refraction. For a vacuum, n = 1.0. It is found that in most of the minerals the range of the index of refraction is between 1.4 and 3.2.
This property is when there are two different refractive indices of light that are noticed, which cross-polarized when it enters matter. It is noticed that if one ray enters matter, there are two rays that emerge, these are called the ordinary and extraordinary rays. Should the material be rotated, the ordinary ray will remain still and the extraordinary ray makes a circle around the ordinary ray. This effect is easily seen in Calcite and Sodium Nitrate. The extraordinary rays which exit always display polarization at right angles.
This is the property when the refracted indices of light spread through a material in such a manner that it causes a variation in the wavelength from red to violet. When the dispersion of light in the material is larger, there is a greater amount of white light separated when the rays are exiting from the particular material. A good example is Diamond, where the dispersion of light is extremely great, hence one can see so many colors in this crystal.
This property is of two types, One type of color change is Pleochroism in which the material seems to possess many different colors when it is looked at from different directions. When two colors are exhibited the effect is know as Dichroism. When three colors are exhibited the effect is called Trichorism. The mineral Iolite (cordierite) changes colors from dark blue to colorless, hence it is an example of dichroism.
The other type of change in color is noticed by the existence of artificial light, natural light, fluorescent light or even incandescent light. This type of change in color is noticed in nickel sulfate making it look like Alexandrite, which is a priceless gemstone.
Polarization of light
This property can be seen in some materials. This property is found naturally in tourmaline. As it is believed to be placed between what is called a “polarization sandwich,” sodium chlorate allows different colors through it, since there are changes in the angles between the polarizers.
This property refers to the ability of a crystal to break along certain specific planes with a lot more ease when compared with the other directions. Naturally crystals can break either perfectly or imperfectly. There is a marked difference between these two forms of cleavage. A perfect cleavage is one that will always break on the cleavage plane, which is found in fluorite, calcite and diamond. This property of the diamond makes cutting the gem very challenging as the planes the person is cutting along may not really be the cleavage planes. On the other hand imperfect cleavage is one in which it can break along any plane. Quartz and beryl are good examples. When quartz breaks, some of the broken pieces look very similar to glass.
This is that physical property found in a compressed crystal, which causes a flow of charge and a drop in voltage across the opposite poles in the crystal. These materials are easily used in communication equipment. This effect is easily seen in both Rochelle salt as well as natural quartz, which are known to be able to supply voltage when mechanical force is applied. Materials like germanium, silicon, galena and silicon carbide are used as semi conductors as they carry current unequally in different directions.
Of the 32 classes of crystals 20 are piezoelectric. Crystals of the piezoelectric classes lack a center of symmetry. When an electric field is applied to any material it develops a dielectric polarization. Materials that naturally have a charge separation are called polar materials. The structure of the crystal determines whether it is polar or not. Of the 32 classes only 10 have polar crystals. Polar crystals are all pyroelectric and hence these ten classes are generally referred to as the pyroelectric classes.
Some crystal structures display ferroelectric behavior. Ferromagnetism, which is the similar to ferroelectric behavior, is a property where due to the electric field being absent, polarization is not displayed by the ferroelectric crystal. In the presence of an electric field the ferroelectric crystal displays permanent polarization. With the application of a large counter charge this polarization can be reversed just like it is reversed in a ferromagnetic. It is important to note here that though this effect is called ferroelectric, there is no presence of the ferrous metal and this effect is produced by the structure of the crystal.
The physical properties of crystals have been spelt out in detail in the above paragraphs. Crystallographers have made the study of crystals simple and convenient based on these properties.