FMF - Friends of Minerals Forum, discussion and message board

[Pete Modreski]

Josele, that's a nice assortment of calcites and a nice photo of them.

So, as you probably know, the blue-fluorescing calcite is very much the exception among calcites, even among fluorescent calcites. We often call it "Terlingua-type" calcite, and it has the unusual suite of properties of blue fluorescence SW, pink LW, and very bright and long-lived phosphorescence after the SW exposure. And there are only a few known localities where calcite with these properties occurs; the main ones being Terlingua TX, one locality in Mexico, and Hope, Indiana (there may be a few others, less well know). And my understanding is that it's divalent europium, Eu2+, that emits this blue fluorescence, though there may indeed by other rare-earth elements present that help absorb the UV light that is re-emitted by the europium (and thus, they would be co-activators). But I don't think there exists any hard data as to whether or not there are such co-activators, and just what they are or what their role may be. (There's still a lot of research left to be done to really understand this sort of thing!)

And of course, the only way to synthesize such calcite, would be to precipitate calcite from a solution that contains europium salts (and varying the oxidation state of the europium, Eu2+ or Eu3+, is another variable that one could experiment with), and see if the presence of other REE, or the temperature at which the calcite was deposited, etc., had any effects on the fluorescence. This type of synthesis experiment, unless done under high pressure hydrothermal conditions, would only produce a very fine-grained powdered form of calcite, not macroscopic crystals.

I'm about run out of ideas for a response, after saying this!

[Josele]

While it is true that some information talk about Eu as activator of blue fluorescence in calcite, I can not found any serious paper or reference confirming this assertion. Will appreciate any help to find it.

At the moment, after re-reading all relative to calcite in Luminescence Spectroscopy of Minerals and Materials of M. Gaft et al., must insist in uncertainty of which activator causes blue fluorescence in calcite.
Is well stablished that Eu2+ can activate blue fluorescence in fluorite, apatite, Baryte, danburite, feldspars, anhydrite, zoisite and charoite (page 331, Table 12.1, REE luminescence centers detected in minerals, where calcite is not listed in Eu2+ activated minerals). This does not mean that can't occur, just that the authors they have not found it.
In chapter Unidentified Luminiscence Centers (paragraph 5.11.4 Calcite, page 250), authors say: "Under short-weved UV lamp excitation (254 nm) visually observed luminiscence of calcite is violet-blue with very long phosphorescence time of several seconds. ..." whose characteristics closely resemble to Terlingua-type calcite. After a discussion of tests results they don't arrive to a consistent identification of the activator and concluded "Additional research needed in order to clarify this problem".

In other hand, regardless of activators, I think, as Pete, that must be not easy to crystalize fluorescent calcite with home-made systems. That's why I encourage Parvin to explain his method.

[Pete Modreski]

Josele, I want to retract/amend my statement that europium (Eu2+) is responsible for the blue fluorescence of some calcite. I agree with what you've quoted; there really is no hard evidence for this, it is just a statement that was based on supposition. I shouldn't have been making that claim without checking out the sources (if there even were any!). Thank you for pointing this out.

[Josele]

Thank you, Pete, for relating Parvin's question with Terlingua-type calcite. My knowledge is very limited but I became curious and I was lucky for having the right consulting book.

Let me take this thread to another question about fluorescence. I have understood that each activator emits in a determined range which corresponds to a color, then, what causes white fluorescence? White is the addition of all visible colors together. When we see a white fluorescence, that means there is three or more activators emitting in a range that added to others create white light?
Very strong emission in any color also is seen white, but usually can see the color in the edges of the piece. However sometimes fluorescence is pure white, how does it work?
Comments will be appreciated, thanks for your interest.

[CranCowan]

The most commonly blue fluorescing calcite (under SW) is probably the so called Terlingua-type.

It's activator is thought to be organic. (Robbins, Graft ...)