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 Post subject: Non cuprian blue/purple tourmaline. Why is it rare?
PostPosted: Wed Feb 27, 2019 12:58 pm 
Gemology Online Veteran

Joined: Thu Sep 25, 2008 10:33 am
Posts: 833
Location: Mars PA
One of the great effects of discovering gem quality cuprian tourmaline was in the world of blue purple color. I consider the part of the color wheel populated by blue purples to be the rarest area of color wheel in tourmaline. But with copper the blue purples kept coming. Even exotic stones like "Laurellite", a name I have give to a cuprian reverse Alexandrite color changer (blue to purple)from Mozambique appeared.

Since it is well established that copper and iron can produce the same cyan color in tourmaline, you should only need the higher oxidation state of manganese to make purple from both. Where are the missing iron purples? There are certainly many tourmaline with significant amounts of both manganese and iron.

It has been proposed that all manganese in tourmaline is in the lower oxidation state when it crystallizes. Then natural radiation oxidizes the manganese and the tourmaline becomes red/pink. (There are very few colorless tourmaline left in nature.) Now iron can be oxidized by natural radiation too and rather than manganese's colorless and pink/red color, it can be blue or green. Now the variations in the production of color in tourmaline, that is caused by natural radiation, complicates the picture, it does not explain the lack of comparable iron blue purples to copper ones.

So what is happening that keeps the freshly crystallized manganese from being oxidized? I believe that it is a combination of the the number of electrons generated by the oxidation of iron and perhaps some manganese and the limited number of electrons the matrix of the tourmaline can handle and maintain electrical neutrality. Electrical neutrality is essential for all stable compounds in nature. This is the same effect, I address in an earlier post on the colorless tourmaline that has both a significant amount of manganese and most of 1 per cent lead. I bet it is also responsible for the the colorless band that is always seen between the green rind and pink center of watermelon tourmaline.


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