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 a pattern of birefringent colours crossed by dark bands (or "isogyres"), which can be produced using a geological petrographic microscope for the purposes of mineral identification and investigation of mineral optical and chemical properties. The figures are produced by optical interference when diverging light rays travel through a optically non-isotropic substance - that is, one in which the substance's refractive index varies in different directions within it. The figure can be thought of as a "map" of how the birefringence of a mineral would vary with viewing angle away from perpendicular to the slide, where the central colour is the birefringence seen looking straight down, and the colours further from the centre equivalent to viewing the mineral at ever increasing angles from perpendicular. The dark bands correspond to positions where optical extinction (apparent isotropy) would be seen. In other words, the interference figure presents all possible birefringence colours for the mineral at once.

Viewing the interference figure is a foolproof way to determine if a mineral is optically uniaxial or biaxial. If the figure is aligned correctly, use of a sensitive tint plate in conjunction with the microscope allows the user to determine mineral optic sign and optic angle.

In optical mineralogy, a petrographic microscope and cross-polarized light are often used to view the interference pattern. The microscope's condenser is brought up close underneath the specimen to produce a wide divergence of polarized rays through a small point. This is done by placing a Bertrand lens (Emile Bertrand, 1878) between a high-power microscope objective and the eyepiece. 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 two curved isogyres (called brushes) with rotation of the stage. The difference in these curved isogyres is known as the optic angle, or"2V". In minerals that have far-off-center optic axes, only one part of the above sequence may be seen. On either side of the saddle the interferences rings surround two eye like shapes called melanotopes. The closest bands are circles, but further out they become pear shaped with the narrow part pointing to the saddle. The larger bands surrounding the saddle and both melanotopes are figure 8 shaped. By combining interference pattern microscopy with use of a sensitive tint plate, the optic sign and optic angle can be determined together. This information can help both with mineral identification, and with interpreting the chemical composition of some minerals (for example, feldspars).

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

References

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

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