PLM Fundamentals:               

Color and Pleochroism            

What is Pleochroism?

Tiny pieces and slices of many minerals have a distinct color when examined under a microscope, although their color may not match the color of larger pieces of the same sample. However, the color of thin slices of a mineral can be far more diagnostic than that of the hand sample and so should always be documented.
PLEOCHROISM is a change in color that occurs when an anisotropic colored mineral is rotated. 

Pleochroic minerals can exhibit either two or three pleochroic colors. The extremes of color occur when crystals are oriented up-down and side-to-side in the field of view of the polarized light microscope.

The different colors and the orientation of the mineral at the time that they occur should be noted.

Why Does Pleochroism Occur?

The color of a material results from the absorption of certain wavelengths of light. For example a blue mineral is one that absorbs red and yellow wavelengths of light, leaving only blue wavelengths to pass through the sample on the microscope stage and eventually reach our eyes.

In pleochroic minerals, the absorption of light is dependent upon the direction in which it travels through the crystal. For example, in a mineral that is pleochroic from yellow to green, light traveling through the mineral in one direction has the red and blue light absorbed (leaving only yellow) whereas light traveling through the other direction has only the red light absorbed (leaving blue and yellow light that mix to form green light).

Example of Pleochroism

How to Determine Pleochroism on a Polarized Light Microscope

Set up the microscope for plane polarized light (Analyzer Out) with any immersion medium
Find a grain to study that is oriented approximately up-down or side-to-side in the field of view
Rotate the stage through 90 degrees
Observe and document any color changes in the grain being examined

How would you describe the pleochroism in the example in the rollover above?


To see how your description compares to the instructor's...

© 2001 Wayne G. Powell