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
Anisotropic colors, when obvious, are characteristic of the mineral.
Some, but not all, isotropic minerals are anomalously anisotropic (eg.
Other References to Anisotrophism
An Atlas of Opaque and Ore Minerals and their Associations from the