A biaxial optic normal figure is similar to the uniaxial flash figure, and is produced when the optic normal (ON) is vertical. In this orientation the optic plane, containing the Bxa, Bxo and optic axes, is horizontal.
The grain which produces this interference figure will display the maximum interference colour for this mineral in the thin section.
The vibration directions for the two rays exiting in figure are similar to those for a uniaxial flash figure.
When the X & Z indicatrix axes parallel the polarization directions, the flash figure is a broad fuzzy cross with only the outer edges of each quadrant of the field of view allowing some light to pass.
As with the uniaxial flash figure a very small rotation, < 5° of the stage causes the isogyre cross to split and leave the field of view in the quadrants into which the acute bisectrix is being rotated.
If 2V = 90° the cross - shaped isogyre does not split as the stage is rotated, it simply dissolves away, with a 5° rotation.
Optic Normal - Biaxial Flash Figure Overhead