What is pleochroism in mineralogy ?

Pleochroism : definition


Pleochroism refers to a variation in the color of certain minerals according to their orientations under unanalyzed polarized light.

To highlight and appreciate this phenomenon, you need a source of polarized light, which can be obtained simply with several methods :

- The simplest method is to use an LCD screen and make it display white (by opening a blank wordpad document, paint, messaging software, etc...). The crystal is placed in front of the screen and rotated to contemplate pleochroism. However, with the advent of LEDs, modern screens unfortunately no longer emit polarized light.

- The second method requires the use of a polarizing filter. Polarizing filters are used in photography but also constitute the lenses of 3D cinema glasses. This filter is placed in front of a light source and the crystal is rotated in front of the filter to observe pleochroism.

Pleochroism is observed only on colored anisotropic minerals.
Pleochroic twinned diaspore from Aydun, Mugula, Turkey
Pleochroic vorobyevite from Deo Darrah, Afghanistan
Pleochroic baryte from Puy Chateix, Puy-de-Dôme, France
Pleochroic vorobyevite from Deo Darrah, Afghanistan
Pleochroic aAllanite-dissakisite from Trimouns, Ariège, France
Pleochroic aquamarine from Vietnam

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