What is polarized light in mineralogy ?

Polarized light : definition

Polarization is a property that vector waves (waves which can oscillate in more than one orientation) have of presenting a privileged distribution of the orientation of the vibrations that compose them. Electromagnetic waves, such as light, thus have polarization properties.

In practice, polarized light is obtained by means of a polarizing filter. These filters are used in photography to eliminate or intensify certain reflections, as well as the blue of the sky. They are also found on all liquid crystal screens (LCD, TFT, etc...) but on the other hand not on LED screens, 3D cinema glasses are also polarizing filters.

The examination of crystals under polarized light makes it possible to highlight the pleochroism of anisotropic minerals. It is highlighted by a color change depending on the orientation of the crystal (we speak of dichroism when a crystal has 2 distinct pleochroism colors and trichroism if it has 3 like tanzanite). Below is the illustration of the pleochroism of a baryte from Olloix, Puy-de-Dôme, France.

Polarized-analyzed light : definition

In transmission microscopy on thin rock sections, 2 polarizing filters positioned at 90° relative to each other are used, says in a crossed position. This is called polarized-analyzed light. This makes it possible to determine the birefringence of minerals, a property which results in chromatic phenomena calibrated by the Newton scale, and provides an often determining criterion in the identification of minerals.