As with uniaxial minerals, grains which will produce a centred to offcentred biaxial optic axis figure are easiest to identify because of their low interference colours due to the optic axis being vertical.
If the birefringence for the mineral is low, grains with their optic axis vertical will remain extinct or nearly extinct as the stage is rotated.
Optic axis figures are used for most routine work because these orientations produce interference figures where the both the optic sign and the 2V angle can be determined.
Optic normal figures (flash figures) have the Y indicatrix axis vertical and X and Z axes horizontal.
In this orientation, birefringence is at a maximum (ngamma - nalpha), and this orientation will also display the highest interference colour for that mineral in the thin section being examined. This interfernce figure is not useful for determiningthe 2V or the optic sign of the mineral.
The location and identification of grains which will produce an acute bisectrix figure is a matter of trial and error. Many grains and their interference figures must be examined before one with a nearly vertical Bxa is obtained.
The interference colour for this section will be in the lower range of that displayed for the mineral in the sample. Reason: birefringence for this orientation must be less than:
It is very time consuming to search for a suitable grain which will produce a centred Bxa figure, but it is essential because these figures give a better estimate of 2V than optic axis figures.
Locating grains which produce an obtuse bisectrix figures is again a trial and error procedure. Birefringence is higher than that exhibited for a grain producing a Bxa figure but less than a grain producing an optic normal figure. The result is that the interference colour will be in the upper portion for that exhibited by the mineral in the thin section.