Interference colour chart
In optical mineralogy an interference colour chart, first developed by Auguste Michel-Lévy, is a tool to identify minerals in thin section using a petrographic microscope. With a known thickness of the thin section, minerals have specific and predictable colours in cross-polarized light, and this chart can help identify minerals. The colours are produced by the difference in speed in the fast and slow rays, also known as birefringence.
When using the chart, it is important to remember these tips:
- Isotropic and opaque (metallic) minerals cannot be identified this way.
- The stage of the microscope should be rotated until maximum colour is found, and therefore, the maximum birefringence.
- Each mineral, depending on the orientation, may not exhibit the maximum birefringence. It is important to sample a number of similar minerals in order to get the best value of birefringence.
- Uniaxial minerals can look isotropic (always extinct) if the mineral is cut perpendicular to the optic axis. (But this situation can be revealed with the Conoscopic interference pattern).
- Nesse, W. D., 1991, Introduction to Optical Mineralogy, 2nd edition.
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