Post subject: A cause of color complexity in tourmaline.
Posted: Sat Oct 23, 2010 2:28 pm
Gemology Online Veteran
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
Please read the following article:
Non-integral hybrid ions in tourmaline: buffering and geo-thermometry
It is in the European Journal of Mineralogy 2009, volume 21, pages 241-250
A short review by yours truly.
I have read many papers which state that tourmaline has this color from this element or from that treatment without an adequate explanation of why the color of tourmaline is so complex. I have been told that Mossauer spectrometry has been used extensively with other minerals to determine structure and explain color etc., but tourmaline has the most complex Mossauer spectrum in silicate minerals and this has severally limited Mossauer work with tourmaline.
Well this paper purports to have broken the complexity barrier and found a wonderful mechanism for the complexity of color in tourmaline. The mechanism is caused by the release of strain imparted by sharing a common crystallographic edge between the y and z sites. This is done either by distributing impurities in the sites or changing the relative bonding lengths of the parts of the edge, associated with the y and z sites. These adjustments and the multi valence nature of the chromophores produces non-integral hybrid ions. These ions partially share electrons and interact with light as if they have a fractional charge. The color possibilities are fantastic as we all should know.
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