Observations with the petrographic microscope - Mineralogy for Petrologists - page 24

optic
axis

optic
axis

γ

2V

circular
section

α

circular
section

β

positive biaxial mineral

Figure 2.2 Indicatrix: biaxial mineral.

If the indicatrix is a triaxial ellipsoid (orthorhombic, monoclinic and
triclinic systems), there are two circular sections . These sections are optically
isotropic. Directions perpendicular to the two circular sections are the optic
axes : the mineral is biaxial.
The acute angle between the optic axes is the optic angle or 2V angle.
If is the bisector of the acute angle of the optic axes, the mineral is

positive biaxial ;

γ

α is then the bisector of the obtuse angle of the optic axes.

is the bisector of the acute angle of the optic axes, the mineral is
negative biaxial.
Birefringence of any section (Figure 2.1) is:

If α

n γ − n α = ( γ − α ) sin θ sin θ′

where θ and θ′ are the angles between the normal to this section and the two

optical axes.