What is pleochroism and pleochoric scheme ?

Banibrata majumder

Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light. Pleochroism is an extremely useful tool in mineralogy and gemology for mineral and gem identification, since the number of colors visible from different angles can identify the possible crystalline structure of a gemstone or mineral and therefore help to classify it. Minerals that are otherwise very similar often have very different pleochroic color schemes. In such cases, a thin section of the mineral is used and examined under polarized transmitted light with a petrographic microscope. Another device using this property to identify minerals is the dichroscope. A pleochroic mineral literally changes color as it is rotated on the microscope stage while viewed using plane-polarized light and is described by the pleochroic scheme. In order to see whether a mineral is pleochroic or not, you only have to turn the microscope stage and observe whether the crystals of a particular mineral experience changes in their colouring. Pleochroism may manifest itself by a change in actual colour (e.g. in some positions the mineral is seen as blue and in others as red) or by a change in the intensity of colour (from light blue to dark blue).