During a natural coesite crystal study, I observed a remarkable strong Raman band in the low-frequency range at 339.4 cm-1 between the 326 and 355 cm-1 band positions. The spontaneous Resonance-Raman-Effect produces this band (I think so). Using the 405 nm laser, the intensity of the 339 cm-1 lines is medium or low. The intensity and the shift of the 339 cm-1line can be used to study the specific electronic transition. Have somebody an idea what the origin of this remarkable shift and increase of intensity is?
Dear Rainer Thomas , did you considered the possibility of some kind of contamination or inclusion of some other mineral?
I thought about this possiblity because coesite Raman spectrum is quite well defined in the range described at quite different conditions (temperature, high pressures etc) and just the bands at 326 and 355 cm-1 appear there.
Seeking for something close to 339 cm-1 only wollanstonite (CaSiO3), and bronzite (Mg,Fe2+)2Si2O6) show a band at such Raman shift due to M-O stretch/bend. May be you could find some bands that fit with one of these minerals but if that isn´t the case, it could be a good clue to look for the right M-O Raman band of some other minerals.
Here you can see the references I used for these two minerals:
Wollanstonite:
https://rruff.info/tags=758/R120016
Bronzite:
Article THE RAMAN STUDY OF SINGLE-CHAIN SILICATES
Hope it helps.
Yes, I dit. There is no other mineral with such a strong band present. The dependence on laser wavelength is striking. However, I have to check other wavelengths (not possible at moment).
A further remark: I have checked nice synthetic coesite crystals and have not found such a strong band using the 532 nm wavelength.
Furthermore, the water content is high - a first approximation gave values of about 1% (study not finished).
I think the 339.4 cm-1 band in the coesite spectrum is the result of a spontaneous Resonance-Raman effect.
My interpretation is not correct. I can show that this Raman band is the medium-strong 326 cm-1band of poor coesite, shifted slightly to higher values, maybe by significantly higher Al content. The blueshift of this band, as a function of the laser energy, is similar to the other coesite bands.
Dear Rainer Thomas , thanks for the updates. Re reading your query now I thought about the posssible effects of crystal orientation with respect to the laser beam on the Raman spectrum. So if your crystal is a single one and big enough to expose it at different orientations, may be you could see if the band is shifting or not.
Dear Manuel Gomez,
Thank you for your comment. I have tried different orientations.
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