Conoscopic interference pattern
 | This article may be too technical for most readers to understand. (December 2009) |
- 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 with a petrographic microscope. To view this figure, you must focus on a mineral on the highest-power objective, in cross-polarized light, condensing lens, and Bertand lens. This figure essentially shows all possible birefringence colors at once, including the extinctions (in dark bands called isogyres).
A uniaxial mineral will show a straight cross shape with its isogyres, which will rotate in a solid form around the center of the figure.
A biaxial mineral can show a crossed 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.
References
W.D. Nesse (1991). Introduction of Optical Mineralogy (2nd ed.).