Conoscopic interference pattern

This page is about the geology/optical mineralogy term. For general information about interference, see Interference (wave propagation) or Interference patterns.

A conoscopic interference pattern or interference figure is the best way to determine if a mineral is uniaxial or biaxial and also for determining optic sign in optical mineralogy. The observed interference figure essentially shows all possible birefringence colors at once, including the extinctions (in dark bands called isogyres).

In optical mineralogy a petrographic microscope and cross-polarized light are often used to view the interference pattern. This is done by placing a Bertrand lens (Emile Bertrand, 1878) between a high-power microscope objective and the eyepiece. The microscope's condenser is brought up close underneath the specimen to produce a wide divergence of polarized rays through a small point. There are many other techniques used to observe the interference pattern.

A uniaxial mineral will show a typical 'Maltese' cross shape and its isogyres, which will revolve/orbit around a projection of the optical axis as the stage is rotated.

A biaxial mineral will typically show a saddle-shaped figure (with one isogyre thicker than the other, typically) that will often morph into to curved isogyres with rotation of the stage. The difference in these curved isogyres is known as the "2V" angle. In minerals that have far-off-center optic axes, only one part of the above sequence may be seen.

A Michel-Levy Chart is often used in conjunction with the interference pattern to determine useful information that aids in the identification of minerals.

Simplified discussion

A Conoscopic interference pattern or interference figure is the best way to determine if a mineral has one direction of single refraction or two directions of single refraction. (Herein after referred to as Birefringence) It is also used to tell which ray of light is faster in birefraction. The observed interference figure essentially shows all possible birefringence colors at once, including the extinctions. (in dark bands called isogyres)

Light as we see it is seemingly white, but when it is viewed through a mineral we can see that it is separated into a body of colors and each individual color represents a particular wavelength. As a wavelength each color has a different speed. Also minerals can inhibit light, limiting lights movement; So when light passes through a mineral it is separated into many different wavelengths, sometimes getting trapped in certain places resulting in a black color. For example: When a mineral is observed through a petrographic microscope with a cross-polarized light you will see multiple colors and some darker lines within the mineral. (The trapped light referred to as isogyres) In summation, Conoscopic interference patterns are essentially what we use to see how light reacts to certain minerals, and how we can see how much or how little light is inhibited by said mineral.

References

• W.D. Nesse (1991). Introduction of Optical Mineralogy (2nd ed.). 

• Albert Johannsen (1914). Manual of Petrographic Methods.