|A yellow color center in Paraiba tourmaline?
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|Author:||bruce_tourm [ Thu Jan 31, 2019 12:24 am ]|
|Post subject:||A yellow color center in Paraiba tourmaline?|
In this post I will be referring to the following research paper that is available on the inter net.
Correlation of irradiation-induced yellow color with the O- hole center in tourmaline.
By K. Krambroock, M.V.B. Pinheiro, K.J. Guedes, S.M. Medeiros, S. Schweizer, and J.M. Spaeth
I would like to start this post with the assumption that this paper's findings are valid. Adding a yellow vector to the color of tourmaline threw irradiation has certainly been demonstrated earlier. (Dr. Nashua of Bell Labs fame, for one.) The repercussions of having an explanation of radiation induced color without a transition-element chromophore is important in more varieties of tourmaline than Paraiba, but i will limit my post to Paraiba.
From earliest work done on Paraiba tourmaline the question of why some Paraiba tourmaline is green has not been properly addressed. Titanium in combination with manganese was postulated to produce the yellow color vector in Paraiba, when only extremely low concentration of titanium had ever been found in testing Paraiba tourmaline. Titanium was soon dropped in popular explanations of why there are green Paraiba and it was not mentioned as a chromophore in the Harmonization final definition of Paraiba tourmaline.
So what replaced the magical titanium in making some Paraiba green? Why "high" concentrations of copper. Some inventive people even dragged out malachite and azurite to show that copper can produce both colors. Now this idea that somehow different concentrations of copper, alone, can produce cyan and green color is amazing. Research has shown that copper only exists in tourmaline in one oxidation state, unless it is exosorbed from the tourmaline as a native element or other mineral. It also finds a home in only one crystal location in Paraiba. To compound the popular explanation of the magical powers of copper as a chromophore in Paraiba is the fact that some greens can be heat treated to cyan and others can not. (At least Mozambique have greens that will not respond to heating and I have send a graphic display of heating out the green in Paraiba, in an EnglishLapis magazine article)
Now iron needs to be addressed as a possible chromophore in Paraiba. It certainly produces a green color that can not be be eliminated by low level heating. And I think that iron is a factor in some Paraiba, but most Paraiba I have seen tested or reported on have very low iron levels. So what is left to make the beautiful green Paraiba that will never be heated to those valuable cyan treasures. How about small areas of high manganese that crystallize in the tsalisite structure? It would be inline with Elbaite and Liddacoatite, that is common in some tourmaline.
Finally I am ready to look at the color centers researched in the cited paper above. The yellow produced by the O-hole center, induced by radiation, in tourmaline is developed around two aluminium and one lithium atom. The hole is not stable above about 450 degrees F. Therefore it would be affected by the common practice of heating Paraiba to and reduce manganese and eliminate the red factor in Paraiba, to make the cyan color. The time that it takes, at elevated temperatures, to reduce manganese and eliminated the irradiation induced color centers varies with temperature. This is enough material from the paper and other reading to go on.
I wondered why heating Paraiba to change its color was a delicate and convoluted process if it only was used to reduce manganese. Then I found out that bleaching Paraiba by over heating it was a common problem. Now if part of the color was from iron (Indicolite) then eliminating its color was easy to see. But as I have related most Paraiba has extremely low levels of iron.. So what is left to not over heat since the copper chromophore has been shown not to be heat sensitive. Now a step back to say that Paraiba's visual presences is really enhances by its illumination under yellowish light. It is remarkable to me. Now if we take that fact and not completely eliminate all the radiation induce color centers (yellow)while eliminating all the red by reducing the higher oxidation state of manganese, you could get the best and brightest gem possible. Go too high in temperature or stay too long and its looking lot more like Aqua time. You wash up, but maybe you could try again by irradiating the stone before reheating it a bit differently.
Finally the determination that the generation of color centers depends on lithium adds an interesting twist to the wonderful of color in tourmaline. Lithium is not a transition element like copper and iron etc. and does not have the proper electron configuration to color substances (Elbaite Rossmanite etc.) directly. But it now appears that the concentration of lithium, which varies significantly in lithium bearing tourmaline, can effect color. And the color yellow can be a subtle enhancer of a dominant color without greatly effecting the basic color of the gemstone. I love to look at my collection under incandescent light. I find that greens and blues really respond positively while some reddish and purples do not. For another great view, which I am sure is effected by the yellow content of the light, is viewing the collection under indirect light reflected off new fallen snow. I could go on, but I think you get the idea.
Ending this post with visions of colorful tourmaline running around in my head should be enough, but I just had an idea. It has a lot of ifs, that I can not answer, but if the O- hole is nearly universal, like the oxidation of manganese +2 to manganese +3 by natural radiation (It has been proposed that all pink/red tourmaline initially crystallized as a colorless crystal) then the lack of the color center would indicate that the tourmaline was heated. This seems important to some people and the gemological labs can not determine whether a tourmaline has been heated or not. They will say that there is NO indication heating which in the case of Paraiba is based on having a purple cast in the gemstone. As I have said in other posts, this is not valid because it takes time to completely eliminate the red tone threw heating. At least with Mozambique material, I am sure that some material was only partially changed to a lighter lavender, not cyan threw heating.
My wish would be to test all my speculations, but that will never be for me. With material so expensive I doubt anyone is going to risk much Paraiba by running tests. But the day we can make Elbaite easily and dope it to our heart's content will a brilliant day for understanding color in tourmaline.
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