Ploeochorism, Birefringence in Minerals |Geology Notes

Ploeochorism, Birefringence in Minerals |Geology Notes

The phenomenon of different colors appearing when certain crystals are viewed from different directions. The property possessed by some crystals of exhibiting different colors, especially three different colors, when viewed along different axes.

(1) Pleochroism:-

A crystal is said pleochoric if on rotating in polarized light it shows change in quality or quality of color. This change in color due to the change in absorption of light vibrating in different direction of the crystals is known as Ploeochorism.

(i) Unianial crystals:-Unianial crystals have two vibration direction. Such a crystals shows two different colour. e.g-Tourmaline

(ii) Bianial crystals:-There are three principal vibration directs X,Y,Z. Along there three colors appear. e.g of Ploeochorism:-Hornblende

X-pale yellow, Y-yellow green, Z-green

(2) Birefringence:-

Birefringence is formally defined as the double refraction of light in a transparent material.

The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material.

Crystals with non cubic crystal structure are often birefrimingent as are plastics under mechanical stress.

Birefringence is responsible for the phenomenon of double refraction whereby a ray of light when incident upon a birefrigent material is split by polarization into two rays taking slightly different paths.

(3) Pleochoric haloes:-

There are microscopic spherial shells of discoloration within the minerals such as biotite.

There shells are zones of radiating damage caused by the inclusion of minute radioactive crystals within host crystal structure. These inclusions are typically ziricon apatite or titanite.

(4) Dichroism:-

The property of some crystals and solutions of absorbing one or two plane-polarised components of transmitted light more strongly than the other.

In optics, a dichroic material is either one which causes visible light to split up into distance beams of different wavelenghts or one in which different light rays having different polarizations are absorbed by different amounts.

(5) Trichroism:-

It is pleochroism of a bianial crystal such that it exhibits three different colours when viewed from three different directions under transmitted light.

(6) Double Refraction:- This is an optical property in which a single ray of unporalized light entering an Isotropic medium is split into two rays, each travelling in different directions.One called extra ordinary ray is bent or refracted at an angle as it travels through a medium the other ray called the ordinary ray passes through the medium unchanged

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