For biaxial minerals optic sign is dependant on whether the X or Z indicatrix axis is the acute bisectrix.
In the special case where 2V = 90°, mineral is optically neutral.
Another convention used is to identify the angle between the optic axes bisected by the X axis as the 2VX angle; and the Z axis as 2VZ angle.
These two angles can vary from 0 to 180°, such that the following relationship holds:
Using this convention the optic sign is determined by the following:
Light travelling through biaxial minerals is split into two rays - FAST and SLOW rays which vibrate at 90° to each other.
The vibration directions of the FAST and SLOW rays are defined, or determined, by the axes of the ellipse or section through the indicatrix, which is oriented at 90° to the wave normal.
The Refractive Index corresponding to the FAST ray will be between nalpha and nbeta, and is referred to as nalpha'.
The Refractive Index corresponding to the SLOW ray will be between nbeta and & ngamma, and is referred to as ngamma'.
With this convention the following relationship will be true for all biaxial minerals: