Now come to the major means of distinguishing whether an anisotropic mineral is uniaxial or biaxial and for determining the optic sign for an anisotropic minerals - THE INTEFERENCE FIGURE.

To obtain and observe an interference figure using the microscope.

  1. With high power, focus on a mineral grain free of cracks and inclusions
  2. Flip in the auxiliary condensor and refocus open aperture diaphragm up to its maximum.
  3. Cross the polars
  4. Insert the Bertrand lens or remove the ocular and look down the microscope tube.

Will not see the grain, but the interference figure, which appears on the top surface of the objective lense.

The interference figure consists of a pattern of interference colours and a black band which may form a cross. Nature and pattern for the figure is dependent on the orientation of the grain.

For Uniaxial Minerals three types of interference figures will be considered.

  1. Optic Axis Figure - OA vertical
  2. Off Centred Optic Axis Figure - OA inclined.
  3. Flash Figure - OA horizontal

Each figure type is a direct reflection of the different cuts through the indicatrix.