FMF - Friends of Minerals Forum, discussion and message board

[Gerhard Niklasch]

I've had an UV lamp for a while now, and more recently I had added a suitably-sized polarizing filter to my amateur-science arsenal. I had also noticed that some Apatites are slightly dichroic when viewed from their sides through the filter, and of course I had enjoyed the nice bright golden or yellow fluorescence from a number of them.

Last night I finally put two and two together, placed a succession of Apatites under the UV rays, and inspected their responses for (linear) polarization.

It was clear a priori -- since much yellow light is emitted through the basal pinacoids parallel to the c axis, where the electric field is necessarily at right angles to c -- that if any linear polarization is present, it could only be in the direction orthogonal to c. And indeed this is what I observed.

The effect was most pronounced with a dark-green specimen from the Sapo Mine, Minas Gerais (from Luis Menezes stock of course). Here, when the filter was oriented parallel to the c axis, it almost completely extinguished the yellow light emerging laterally through the prism faces.

The effect was weaker but quite definite with a Panasqueira Mine (Portugal) apatite, and quite weak though still detectable with a few specimens from Pakistan. (Those fluoresce exceedingly brightly, but are merely translucent. I suspect a lot of the emitted light is scattered off small internal fractures and inclusions and thus loses its original polarization plane before it leaves the crystal.)

So, if you're also in possession of the ingredients, this is an illuminating (in more than one sense!) exercise for long winter evenings!

Enjoy,
Gerhard

[Peter Megaw]

Two years ago, Brian Jackson at the National Museum of Scotland in Edinburgh showed me some extremely dichroic apatites they were working on...might be fun to run this past him and have him check those with your technique.

[Elise]

[Brian Jackson]

Hi. Elise notified these posts to me. My recollection was that the apatite from Slyudyanka showed no fluorescence. I checked this last night (SW-MW-LW UV) and indeed there was no fluorescence.

The effects shown by this apatite are exceptional pleochroism and polarisation by absorption. Being new to this I've tried to add some images but I only know how to do it one slide at a time.

Yes Elise I MUST write this up.

[Jordi Fabre]

Hi Brian, welcome here. A great pleasure read the scientific contents of your posts!

Jordi

[Peter Megaw]

Brian Laddie...welcome aboard, glad Elise was able to lure you in! For those who don't know Brian and his museum...the National Museum of Scotland, I strongly urge a vist to Edinburgh, and some other excellent kindred museums nearby in Glasgow, especially if you like classic base metal secondaries and classic malt elixirs! His exhibits here at Tucson have never failed to enthrall the cognoscenti.

Yes, not only do you need to write this up, but you need to put in an exhibit here in Tucson on this...perhaps linked/shared with the fluorescence question. Should be a shoe-in for educational award recognition (No, I am not on the judging committee for that!).

Thanks again for the time you spent with us last year!

[Gerhard Niklasch]

Many thanks, Brian, for chiming in and presenting your most illuminating slides!

I like those spectrum-to-hue translations, and using two crystals as a mutual polarizer and analyzer is a neat trick. (Of course, come to think of it, before the days of our modern filters, sliced tourmalines were used instead...)

It only took an evening of rummaging to unearth a Slyudyanka specimen from my own collection. This (about 25x40x35mm, with crystals to 30mm) comes with the original orange calcite matrix, so I can't directly replicate the two-crystals trick and am limited to front-lighting. And the apatites aren't as clear and gemmy as yours. The overall effect is therefore more subdued, but still quite obvious.

I get a very weak greenish response under SWUV, which may be due to impurities or inclusions; it isn't polarized as far as I can detect.

We certainly can't expect all fluorescence from optically nonisotropic crystals to be polarized. Just like there are many different kinds of color centers in visible light, some of which are and some of which aren't sensitive to the orientation of the surrounding lattice, there are many different kinds of luminescence mechanisms, and only some of them would involve transitions with a preferred orientation of the electric field. It seems this topic has not yet been widely studied...?

It's certainly fascinating what one can discover, non-destructively, with just a few simple tools!

Cheers, Gerhard

[Elise]

[nimfiara]

Wow! Elise, Fantastic example of polarization in the apatite.
I love this last picture.

Thanks!!!

[Gerhard Niklasch]

Fascinating, isn't it... I love minerals which "do" something (beyond just being interesting to look at) with the help of a simple tool or two, such as a UV light and/or a polarizing filter!

In the meantime, I've encountered two more examples where the fluorescent response to a SWUV light source is to some extent linearly polarized. Pictures to follow once I get round to preparing them.

One is... Benitoite, which changes color quite drastically depending on polarization already in ordinary visible light. Under SWUV, shining roughly along the c axis whilst viewing the crystal through the filter from the side, the blue response from the illuminated faces is brighter in the E||c orientation of the filter. But blue light traversing the crystal to emerge on the rear faces, those not directly hit by UV rays, is brighter in the other orientation. This may be the result of multiple scattering inside the Benitoite; of course any light rays travelling parallel to the c axis can't help having their electric fields at right angles to this axis, and some of this may survive a further scattering towards the viewer.

The latest example I've found is Scheelite, where the effect is less pronounced. I'm not yet sure of the orientation here, because the brightest Scheelites in my possession have very oddly distorted shapes and I'm not yet sure which axis is which in them!

However, the yellow response of Powellite seems to be unpolarized, as far as I can detect.

Enjoy,
Gerhard