Observations with the petrographic microscope - Mineralogy for Petrologists - page 40

Figure 2.10 Convergent light interference figure of a uniaxial mineral cut perpendicular to the
optic axis.

Figure 2.11 Non centered black cross.

γ γ

accessory
plate

accessory
plate

Red Blue Blue Red

Blue Red Red Blue

positive uniaxial U+

negative uniaxial U−

Figure 2.12 Optic sign of a uniaxial mineral.

2.6.2.1 Determining optic sign
Add an auxiliary plate (a quartz plate in Figure 2.12), the quadrants now
show contrasting colors:

￿ blue in the NE and SW and orange in the NW and SE for a positive

uniaxial mineral;

￿ the reverse: blue in the NW and SE, orange red in the NE and SW for a

negative uniaxial mineral.