[Rockhounds] Scapolite again

[Axel Emmermann]

Steve, 

I 'm afraid that it's more complicated than that. 
If my interpretation of the physics concerning this fluorescence is correct,
then it would certainly be more than just a band gap shift.
What you say may be applied, however, if we reverse it.
Looking at the difference of the fluorescence emission spectra of both
scapolite and feldspars may be easier than proposing a fluorescence spectrum
based on a theoretical composition. 
Especially so since Fe4+ is more than just an "ionic" activator. If I recall
right (mind the "ifs") the emission stems from Fe3+ PLUS a charge deficiency
in one of the oxygen atoms surrounding it.
I would assume that bond lengths play a very large roll in this.  When large
anions are lodged in the anion cage (CO3, SO4) that would affect both Si-O
and Al-O (and by proxy Fe-O) bond lengths AND angles between those bonds.,
wouldn't it?
I'm not sure but I would think that crystal field effects would have a
greater impact when acting on a fluorescent site ( [Fe3+ + O-]4+ ) than on a
pure ionic fluorescence (REE) or "conduction band" fluorescence (Mn2+).
A few picometers could go a long way...  I'm a bit unsure about that (LOL)

In this case I would prefer to wag the dog... try to correlate the spectra
of scapolite and feldspars rather than predict them from a proposed model.
Certainly in light of the complexity of scapolites! 

Axel



> -----Oorspronkelijk bericht-----
> Van: rockhounds-bounces at lists.drizzle.com
[mailto:rockhounds-bounces at lists.drizzle.com]
> Namens Steve Sellepack
> Verzonden: zaterdag 4 oktober 2008 18:56
> Aan: rockhounds at lists.drizzle.com
> Onderwerp: RE: [Rockhounds] Scapolite again
> 
> Based on the below discussion the problem may have a computational
solution.
> As fluorescence is a band structure property the influence of the
concentration of the
> specific element in a specific site will be displayed in the band
structure.  Computational
> modeling of the band structure correlating to the site replacement may not
display an energy
> gap equal to a red fluorescence but modeling of the series should display
enough of a band
> gap shift to be able to correlate the properties from natural specimens to
concentrations of
> iron or aluminum in the T2 sites.
> Steve
>