[John Layne]

Is it possible to create a MXM material (not IOR) with Birefringence (double refraction)?

http://en.wikipedia.org/wiki/Birefringence

[JDHill]

Sure, just combine two dielectric BSDFs with different Nd values. The simplest steps would be: a) Wizards > Water, b) right-click BSDF > Duplicate, c) change Nd in the duplicate BSDF to something other than 1.33.

[John Layne]

Thanks JD