[Rockhounds] Terlingua-type calcite

[DonH]

Doug Leeper wrote:

> If you have a piece of this calcite that does this that you can part 
> with, I can obtain a very accurate response in the visible spectrum, 
> possibly that may show the source?
> 
> I'm thinking like emission lines you get from various elements...
> 
> I have sources with 365nm, 290nm, 390nm, and 460nm dominant wavelengths 
> I could stimulate it with.


Hi Doug,

I think the issue here is that we don't know the exact electron 
transitions that cause the colors.  I encourage your experimentation, 
but you might like to know what has gone before.

Some experiments were performed on this calcite by a young student who 
was working with George Rossman at CalTech.  The results were published 
in UV Waves, a publication of the Fluorescent Mineral Society.  I don't 
have the issue at hand and I don't remember exactly what was, and was 
not, determined by those experiments.

Along with Earl Verbeek at the Sterling Hill Mining Museum and the 
Thomas S. Warren Museum of Fluorescence, I used a Varian Cary Eclipse 
UV-VIS spectrophotometer to run continuous scans on Terlingua-type 
calcites.  Their emissions are complex, with at least three and possibly 
four emission peaks in the LW UV as well as in the visible, along with 
the issues of primary and secondary fluorescence and phosphorescence.  I 
was speaking with an experienced spectroscopist last week about another 
matter, and the subject of this calcite came up.  The consensus was that 
more detailed work needs to be done, possibly using laser ablation 
inductively coupled mass spectrometry to ascertain the trace elements, 
and I think it would be useful to conduct single-crystal x-ray 
diffraction to try to determine the distributions and ordering of those 
trace elements.  If anyone has done these things, neither of us was 
aware of it.

Certain substitutions and their effects are well known; others are not. 
  For example, divalent europium can substitute nicely for calcium in 
many structures (depending on how many oxygen surround the calcite in 
the coordination polyhedron), due to its size and charge.  Europium is 
known to produce a blue fluorescence, though I imagine this is not an 
iron-clad rule and may depend upon what mineral it is substituted into 
and also what co-activators exist as well, and also structural defects 
introduced into the mineral.  On the other hand, some activators are not 
well documented at all, though there may be educated speculation for 
some of them.

The end result is that you may get a well-defined emission spectrum for 
Terlingua-type calcite, but the real challenge is defining the cause of 
each primary and secondary fluorescence and the phosphorescence seen at 
each excitation wavelength, since it all changes as the excitation 
wavelength increases.

Unfortunately, all my specimens are packed away in preparation for 
moving to wherever my new job (when I get one) will be, or else I would 
send you a piece.  If no one on the list has some to spare, you can 
always buy a small piece from an Internet dealer.  I encourage you to 
see what you can discover.


best,
Don