BIAXIAL MINERALS

Include orthorhombic, monoclinic and triclinic systems, all exhibit less symmetry than uniaxial and isotropic minerals.

Minerals in these crystal systems exhibit variable crystal structure, resulting in variable chemical bonding.

The crystallographic properties of orthorhombic, monoclinic and triclinic minerals are specified by means of the unit cell measured along the three crystallographic axes.

It is also necessary to specify 3 different indices of refraction for biaxial minerals:

nalpha, nbeta, ngamma are used in text.

where nalpha < nbeta < ngamma

A variety of other conventions have been used or suggested, make sure that you are aware of the convention used in the text you are using, if it is not Nesse.

The maximum birefringence of a biaxial mineral is defined by

(ngamma - nalpha)

Clarification

1) It takes 3 indices of refraction to describe optical properties of biaxial minerals, however, light that enters biaxial minerals is broken into two rays - FAST and SLOW.

2) Ordinary - extraordinary terminology is not used. Both rays behave as the extraordinary ray did in uniaxial minerals. The rays are both extraordinary and are referred to as SLOW RAY and FAST RAY.