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K-V mica

Posted by Corrado Balestra  
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Corrado Balestra December 20, 2011 02:05PM
Hi everybody,
I have a question.
Is it possible in principle to classify as roscoelite an aluminum free K – V mica ?
I have just checked by EDX a mica with the above mentioned composition and I noticed that in the official formula for roscoelite KV3+2<(AlSi3)O10>(OH)2 aluminum in the T4 site seems to be essential.
To be honest I hope to have a new mica member on hand...
Thanks in advance for your thoughts.

Corrado
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Peter Haas May 28, 2012 10:51PM
Vanadium dominance in the anion would not only make it a new species, but also a different compound type - a vanadosilicate (i.e. a V-Si heteropolyacid anion) as opposed to the alumosilicate (Al-Si heteropolyacid anion) in roscoelite. Also note that V3+ (unlike Al3+) is not easily incorporated in heteropolyacids (also, V(III) does not form homopolyacids on its own). If aluminium is really entirely missing, it is more likely that you have either an un-substituted silicate (and vacancies on the M+ site) or another element "substituting" (this is formally meant since you can't exchange a metal atom in the anion as you can with metal ions on a cation site) for aluminium. An IR spectrum will quickly tell you the situation in the anion - whether it is a pure silicate, whether it also contains Al-O groups and whether different M-O groups are present.
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Marco E. Ciriotti May 29, 2012 12:43PM
Thank you Peter.
The phase in under study.
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