I was searching threw my papers for tax time, when I ran across a copy of a personal email from George R. Rossman of Rossmanite and Caltech. I want to quote a paragraph on dichroism in tourmaline from that April 30, 2007 email.
“IVCT would be much more intense when the vibrational direction (the E-vector) of the polarizing light is perpendicular to the c-axis. This is obviously is the case with Fe2+-Fe3+ IVCT in the dark green to black tourmalines. The reason is that the active ions are in the y-sites which are oriented such that adjacent ions in these sites are in the perpendicular to the c direction.”
This appears to me to answer why some blue tourmaline I have cut is not dichroic to any discernable degree with my eyes and other examples are strongly dichroic. This also holds for other colors of tourmaline.
As I have said in other places, pure tourmaline is not dichroic/pleochroic, it is colorless. Therefor the dichroic nature of tourmaline is not inherent, but depends on the interaction of chomophores, in this case intervalence charge transfer reactions (IVCT) that are oriented by the tourmaline to produce a darker more intensely colored c axis. (Probably the most important IVCT is the interaction between titanium and iron to produce the wonderful blue of sapphire).
Bruce
