Mineralogy Database

Reflection Anisotrophism

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Reflected Light Anisotrophism

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

Anisotropy = Variation in color or intensity of a mineral viewed under crossed polarized light.

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. Anisotrophism is an optical effect where the mineral appears to change colors as it is rotated while illuminated by crossed polarized light. The polarizers are not crossed as in the case to observe reflection pleochroism. 

Isotropic minerals (eg, galena, pyrite) and the basal sections of hexagonal, tetragonal, and trigonal minerals do not show any anisotrophism when rotated in cross polarized light. Minerals which are pleochroic or bireflectant are generally also anisotropic. Care must be taken when observing anisotrophy to follow these rules:

  •  Sample is freshly polished and does not have any tarnish.
  •  Illumination level is set generally high to view the muted colors under crossed polarizers.
  •  Anisotropic colors, when obvious, are characteristic of the mineral.
  • Some, but not all, isotropic minerals are anomalously anisotropic (eg. pyrite).

Other References to Anisotrophism

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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