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.

Michel-Lévy interference colour chart issued by Zeiss Microscopy

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 (this situation can be revealed with the conoscopic interference pattern).