Saturday, April 21, 2012

Optical properties of Minerals under Plane Polarized Light (PPL)

The PPL arrangement of the Petrological microscope is with the Analyzer in the "out" position. The optical properties that can be seen under this arrangement are as follows,

1.  Optic Colour
This is the colour of the mineral as seen through the microscope. Often the optic colour is different to the physical colour of the mineral.
eg.  Feldspar - Colourless,  Quartz - Colourless,  Biotite - Brown,  Hornblende - Green/Yellow,  Alamandine Garnet - Pale pink

2.  Pleochroism
The change in optic colour or colour intensity when observed under the PPL arrangement while rotating the microscope stage is known as Pleochroism. This occurs when the absorption of light varies with the direction of   observation.
eg.  Feldspar, Quartz - No pleochroism  Biotite - Strongly pleocroid (yellow to dark brown)  Hypersthene - Strongly pleochroid (pink to green)  Hornblende - Moderately pleochroid (yellow to green)  Garnet - Weakly pleochroid (pale pink to pink)

3.  Relief
The effect of the mineral grain standing out with respect to the surrounding minerals or medium when viewed under the PPL arrangement is known as Relief. The relief depends on the difference between the Refractive Index of the mineral and that of the surrounding medium. If the refractive index is high with respect to the surrounding medium, so is the relief. Relief is categorized as High, Moderate and Low.

4.  Twinkling
The change in relief observed when the microscope stage is rotated is known as Twinkling. The reason for this is the variation of the refractive index of the mineral with the direction observed. Only certain minerals exhibit twinkling.
eg.  Calcite

5.  Cleavage
 The planes along which a mineral shows a tendency to split (planes of relative weakness) are known as cleavage planes. These planes can be observed as straight lines under the microscope. Cleavage angles help to identify the minerals.
eg.  Hornblende - 120degree cleavage sets.  Biotite - parallel cleavages  Pyroxene - 90 degree cleavage sets

6. Shape
The shape of the crystal also helps in the identification process.
eg.  Accicular(needle shaped) silimanite  Rounded grains of Garnet.