Biaxial Indicatrix

We describe the optical properties of the biaxial minerals in terms of three mutually perpendicular directions : X, Y and Z

• Vibration direction of fastest possible ray is designated as X
• Vibration direction of slowest possible ray is designated as Z
• RI ( refractive indices) for the light vibrating in the three directions parallel to the X,Y and Z are α, β and γ respectively

Therefore we can say that “α” has the lowest RI, “β” has the intermediate RI and “γ” has the highest RI, “γ” being highest must be vibrating perpendicular to the optic axis.

Now follow the description below and try to found it in the above given figure –

• If acute angle b/w the optic axis (2V) is bisected by Z, it is Biaxial positive mineral/crystal
• If acute angle b/w the optic axis (2V) is bisected by the X, it is Biaxial negative mineral/crystal

In the Biaxial positive minerals, the intermediate RI “β” is closer in the value to “α” than “γ”

In the Biaxial negative minerals, the intermediate RI “β” is closer in the value to “γ” than “α”