Pleochroism or dichroism is the change in color evident as the mineral is
rotated under plane-polarized light.
The primary cause of dichroism or pleochroism in minerals is due to
adsorption of particular wavelengths of light. This selective adsorption of
certain wavelengths of light causes the transmitted light to appear colored.
This color is a function of the thickness and the particular chemical and
crystallographic nature of the mineral.
If the adsorption of particular wavelengths of light differs according to the
optical path, the phenomena of pleochroism is evident. This is observable in
plane-polarized light when the polarizers select the light exiting from the
mineral. This color depends on which optical path is viewed.
Pleochroic colors are observed and recorded in the mineral data and are
generally diagnostic of the particular mineral.
Note: Colored minerals may not be pleochroic.
Note: Isotropic minerals are never pleochroic.
None (no variation in color)
Isotropic minerals are always dark under crossed polarizers.
Anisotropic minerals are not. If no color variation is observed on rotation
under plane-polarized light then the mineral is non-pleochroic.
Dichroic (two colors observed)
Dichroic minerals are generally always hexagonal, trigonal,
Pleochroic (three color observed)
Pleochroic minerals are generally always orthorhombic,
monoclinic, or triclinic.
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