Editing Strontianite (section)

== Optical properties ==
Strontianite is colourless, white, gray, light yellow, green or brown, colourless in transmitted light.  It may be longitudinally zoned. It is transparent to translucent, with a vitreous (glassy) [[Lustre (mineralogy)|lustre]], resinous on broken surfaces, and a white [[Streak (mineralogy)|streak]].

It is a [[Birefringence#Positive or negative|biaxial(−)]] mineral. The direction perpendicular to the plane containing the two [[Optic axis of a crystal|optic axes]] is called the optical direction Y. In strontianite Y is parallel to the b crystal axis. The optical direction Z lies in the plane containing the two optic axes and bisects the acute angle between them. In strontianite Z is parallel to the a crystal axis. The third direction X, perpendicular both to Y and to Z, is parallel to the c crystal axis.<ref name=Dana/><ref name=HOM/>  The [[Refractive index|refractive indices]] are close to n<sub>α</sub> = 1.52, n<sub>β</sub> = 1.66, n<sub>γ</sub> = 1.67, with different sources quoting slightly different values: 
*n<sub>α</sub> = 1.520, n<sub>β</sub> = 1.667, n<sub>γ</sub> =  1.669<ref name=Dana/>
*n<sub>α</sub> = 1.516 – 1.520, n<sub>β</sub> = 1.664 – 1.667, n<sub>γ</sub> = 1.666 – 1.668<ref name=Mindat/><ref name=Webmin/>
*n<sub>α</sub>  = 1.517, n<sub>β</sub> = 1.663, n<sub>γ</sub> = 1.667 (synthetic material)<ref name=AM61>Speer, J A and Hensley-Dunn, M L (1976) Strontianite composition and physical properties.  American Mineralogist 61:1001–1004</ref>
The maximum [[birefringence]] δ is 0.15<ref name=Mindat/> and the measured value of [[Conoscopic interference pattern|2V]] is 7°, calculated 12° to 8°.<ref name=Mindat/><ref name=Webmin/>

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 strontianite the effect is weak, with 2V larger for violet light than for red light r < v.<ref name=HOM/>