Double Refraction in Minerals and Pyroelectricity
Double Refraction in Minerals
All minerals except those belonging to the Isometric System show in general a double refraction of light. That is, when a ray of light enters such a mineral it is broken up into two rays, each of light enters such a mineral it is broken up into two rays, each of which travels with a different in each case and the paths of the two rays will be divergent. In other words, the light has undertone double refraction. In the majority of cases the amount of this double refraction is small, and the fact that it exists can only be demonstrated by special and delicate instrument. Calcite, however, shows such a strong double refraction that it can be easily observed. Take a cleavage block of clear calcite (Iceland spar), for instance, and place it over an image marked on paper. The image will appear double.
The amount of double refraction, or in other words the amount of divergence of the two rays, shown by any mineral depends, first, upon the refracting power of the mineral, or its strength of birefringence, as it is called; second, upon the thickness of the block of the mineral; and lastly, upon the crystallographic direction in which the light is traveling in the mineral. In the case of tetragonal and hexagonal minerals, there is one direction (that of the vertical crystallographic axis) in which no double refraction takes place. As soon as a ray of light in the mineral diverges from this direction it is doubly refracted, and the amount of double refraction increases as the path of the light becomes more oblique, and attains its maximum when it is at right angles to the vertical axis. Such minerals belong to the optical class known as uniaxial. In the case of orthorhombic, monoclinic and triclinic minerals, there are two directions similar to the one described above, in which no double refraction takes place, and the minerals of these systems are therefore spoken of as optically biaxial.
In addition to doubly refracting light, all minerals except those of the Isometric System polarize it as well. Ordinary light is conceived as made up of vibrations taking place in all planes. Light is polarized when it vibrates in a single plane. In the case of both uniaxial and biaxial crystals, each of the two rays into which a beam of light is refracted is polarized and in planes which are perpendicular to each other. For a fuller consideration of the optical properties of minerals, the reader must be referred to sites of a more detailed character.
Pyroelectricity
Crystals of certain mineral, on cooling after being heated to about 100° C., will develop upon different portions a positive and a negative electric charge. This can be proved by the power that such minerals show under these conditions to attract and hold to themselves small pieces of paper, etc. Minerals which are hemimorphic in their crystallographic character, like calamine, tourmaline, etc., exhibit this property.
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