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.
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.