Reflected Light Bireflectance

See Also: Reflectivity, Color, Pleochroism, and Anisotrophism.

Reflected light microscopy is used to examine opaque minerals (and other materials, e.g.. ceramics) to determine the paragenetic relationships between different mineral phases and their identification. Often, the same specimen which is viewed using the light microscope can be analyzed using advanced x-ray and ion microprobe techniques.

The sample (polished thin section, epoxy grain mount, or polished section) is placed in the appropriate reflected light microscope. Bireflectance is an optical effect similar to pleochroism where the mineral appears to change in intensity as it is rotated while illuminated by plane polarized light. The polarizers are not crossed to observe bireflectance.

Isotropic minerals (eg, galena, pyrite) do not show any bireflectance (or pleochroism) when rotated in plane polarized light. Minerals which are pleochroic are also bireflectant. Care must be taken when observing bireflectance to follow these rules:

Sample is freshly polished and does not have any tarnish.
Illumination level is not too excessive (intensity changes the perceived relative intensity effect).
Minerals which are pleochroic (non-isotropic minerals) are also bireflectant.
Bireflectance is better observed under oil.

Other References to Bireflectance

An Atlas of Opaque and Ore Minerals and their Associations from the SME

Reflected-light Microscopy from the University of Utah.

Data from the "Visible Light Spectrum" program from efg's Computer Lab was used to obtain the spectral colors used in the calculation of the macroscopic color based on reflectance measurements.

Dan Bruton's COLOR SCIENCE web page.

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