Optical properties of Minerals under Cross Polarized Light (CPL)

The CPL arrangement of the microscope is withe analyzer in the "In" position. The optical properties that can be observed under this arrangement are as follows,

1.  Isotropic and Anisotropic Minerals
All minerals are either Isotropic or Anisotropic. Isotropic minerals are the minerals that do not show any colour under the CPL arrangement whatsoever. These minerals appear dark. Isotropic minerals are the minerals that are very symmetric and usually belong to the cubic crystal system. Examples include Garnet and Spinel. (Garnet having pink as its optic colour turns dark when the analyzer is in. This property is used to identify garnet).
Anisotropic minerals are the minerals that show colours under the CPL arrangement. These colours are known as interference Colours. Anisotropic minerals belong to crystal systems having lower crystal symmetry. (Systems other than Cubic)

2.  Extinction Position and Angle.
Extinction position is a position reached when the microscope stage is rotated under the CPL arrangement and no colours are seen (dark positions). All Anisotropic minerals have extinction positions. All minerals show 4 extinction positions in a single rotation (90 degrees apart). At he extinction position the vibration directions of the mineral are parallel to the planes of polarization of the polarizer and the analyzer. Therefore no light reaches the eyepiece thereby giving the dark extinction position.

The extinction angle is the angle between one of the cross-hairs and the lines of cleavage of the mineral at the extinction position.

3.  Interference Colours
The colours that anisotropic minerals show under the CPL arrangement are known as Interference Colours. The maximum intensity of these colours are seen in between two extinction positions. (at 45 degree positions).

4.  Birefringence
Birefringence is the difference between the maximum and minimum Refractive index value shown by a mineral. The interference colours shown by a mineral depend on its birefringence and therefore birefringence is very useful in mineral identification.

5.  Optic Sign
Double refraction of all anisotropic minerals results in them having two refractive indices RIO and RIE. According to these values, The Optic sign is determined as follows.
RIO  >  RIE.  -  Negative (-)
RIO  <  RIE.  -  Positive (+)

6.  Interference Figures
Interference figures are important optical properties that are used in the identification of minerals, as well as to determine other properties of minerals such as,
-  Optic Sign
-  Whether a mineral is Uniaxial or Biaxial
-  2V angle of a mineral (2V angle is the angle between the two optic axes of a mineral)
Interference figures are observed under the Conoscopic Arrangement of the microscope. The thin sections have to be specifically prepared in order to observe Interference figures.