Optical Properties
Alstonite is a biaxial (-) mineral with refractive indices nα =1.526, nβ = 1.671, nγ = 1.672. The maximum birefringence (the difference in refractive index between light travelling through the crystal with different polarizations) is δ = 0.146.
The optic angle 2V is the angle between the two optic axes in a biaxial crystal. The measured values of 2V for this mineral is 6°. It is also possible to calculate a theoretical value of 2V from the measured values of the refractive indices. The calculated value for alstonite is 8°. If the colour of the incident light is changed, then the refractive indices are modified, and the value of 2V changes. This is known as dispersion of the optic axes. For alstonite the effect is weak, with 2V larger for red light than for violet light (r > v).
The optical directions X, Y and Z are the directions of travel of light with refractive indices nα, nβ and nγ respectively. In general they are not the same as the directions a, b and c of the crystallographic axes. For alstonite X, Y and Z are parallel to the c, a and b crystal axes respectively. Alstonite fluoresces weak yellow under shortwave and longwave ultraviolet light.
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