IVCT, intervalence charge transfer, dichroism in tourmaline.

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

 

About Bruce Fry

I was born in Summit, NJ in 1947 and graduated from Summit High School in 1966. I graduated from the Colorado School of Mines in 1970 and after spending another year in graduate school, I left to see the world of Brazil. After spending some more time discovering myself, I ended up working for the Commonwealth of Pennsylvania for 32 years as an Air Quality Engineer in the Department of Environmental Protection. I retired in 2007 and took up faceting gemstones again after a long hiatus that reached back to my twenties. I had started cutting cabochons when I was 13 and bought my first faceting machine when I was 15, but ran out of money and time until I retired. My great love in gemology is tourmaline and the collection presented here represents my effort to get as much beauty and variety in the colors of tourmaline as I can. I was particularly lucky in being able to get unheated cuprian tourmaline before copper was discovered in gem grade tourmaline from Mozambique.
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