WHICH GRAINS ARE SUITABLE TO PRODUCE USABLE INTERFERENCE FIGURES?
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 figur
