Michel-Lévy interference colour chart (21257606712)

In optical mineralogy an interference colour chart, first developed by

Auguste Michel-Lévy Auguste Michel-Lévy (7 August 184427 September 1911) was a French geologist. He was born in Paris. Biography He became inspector-general of mines, and director of the Geological Survey of France. He was distinguished for his researches on extr ...

, 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 Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the ...

, 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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...

. When using the chart, it is important to remember these tips: *

Isotropic Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...

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).

References

*Nesse, W. D., 1991, Introduction to Optical Mineralogy, 2nd edition.

External links

Example chart from Webmineral
Optical mineralogy Petrology {{petrology-stub