FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

I picked up some interesting new material here at Tucson the other day

it is Hyalite opal from somewhere (Jalisco?) in central Mexico that shows what appears to be daylight fluorescence

Indoors under fluorescent or incandescent light it has almost no color at all (picture 1) but in daylight it has an attractive "'uranyl" yellow-green color (picture 2) similar to, but not as intense as the strong color it shows under Short-Wave UV (picture 3).

I have heard of similar material from Erongo? but not seen it.

The best pieces of this material is completely transparent with smooth botryoidal surfaces and small free-standing druplets.

Hopefully we will be able to get more complete location information someday

[Mark Ost]

That is remarkable considering that the vast majority of fluorescence is so low level as to be totally undetectable in daylight. A very interesting piece Pete.

[waterdog]

Peter,

Would you care to share the name/location of the dealer? I'd like to see that in person.

[Susan Robinson]

My husband says that without analysis, there may be enough ultraviolet radiation in sunlight to cause the mineral to fluoresce. A similar effect can be seen in some of the green fluorites from England (particularly the green ones from the Heights mine). In the case of hyalite opal, the activator is thought to be the uranyl ion.

[cascaillou]

A similar phenomenom is seen in "green transmitter" (aka "chartreuse") diamonds, which show intense green luminescence under visible excitation (thus green emitter would be a better terminology for such diamonds), so that they look yellowish green or greenish yellow under fluorescent light (that includes daylight), while they will look yellowish brown under incandescent light.

Very interesting opal indeed. That is exactly the kind of specimen I was refering to as 'scientifically eloquent' in the discussion about "what defines a mineral quality", as the phenomenom definately adds great interest to the piece. But maybe I'm just one more rock nerd hahaha

[Pierre Joubert]

Here is a specimen from Erongo, Namibia. The photos where also taken in sunlight. I thought that the green colour was due to some mineral. Please help me right.

[Peter Megaw]

Pierre, several people mentioned the Erongo material to me during Tucson...thanks for posting pictures. Is the fluorescence strongest in direct sunlight or indirect? The Mexican material seems to wash out in direct sunlight, with the strongest response in shade or "filtered" sunlight. Glass clearly dampens the effect.

Having obtained more material since my initial post, I can make more observations.

1. There is a range of daylight response from almost none to a pale yellow to the intense yellow-green shown here.

2. There appears to be no correlation between thickness of opal and fluorescence

3. SWUV response is much greener in the daylight fluorescing material. The less responsive material gives off a yellower fluorescence under SWUV.

4. LWUV response is also strong, but I don't have a good LW light so I can't make systematic observations.

5. A few pieces have opal grown over sprays of yellow acicular crystals that look like sklowdowskite. In one instance the opal overgrows individual fibers giving an effect that looks like a sea anemone.

6. Despite the obvious uranyl activator, the material gives off little radiation (<0.3 mrg/min indicated by a hand-held detector)

[Gerhard Niklasch]

Looking forward to seeing a specimen in person (fortunately there are good sources :) )…

Let me encourage anyone in possession of one, and of a gem spectroscope and a not too cloudy sky, to aim the second at the first under the third! The UO2 group produces a very characteristic response, with three narrow emission bands side by side in the green, flanked by two weaker ones, one on the yellow side and one on the blue-green side, like the fingers of a hand. The spectrum looks very different from other bright-green fluorescent minerals such as Franklin Willemite or MSH Genthelvite, whose response comes in a single wide green band.

[Pierre Joubert]

[alfredo]

"Daylight fluorescence" is a more widespread phenomenon than mineral collectors realize, but is usually overlooked. Several minerals fluoresce when exposed to the shorter wavelengths of visible light (blue and violet). The best way to observe it is to look at minerals illuminated exclusively with blue light, no other wavelengths of illumination being present. Then if your mineral glows in green, yellow or red, you are observing fluorescence and not just reflection of daylight.

A century ago someone tried this by blacking out the windows in a room, leaving only one small open spot for a beam of sunlight to enter, which was then filtered through a glass jar of copper sulphate solution. Nowadays it is easier (but more dangerous) to just use a blue laser at night. You will find several yellow secondary uranium minerals fluoresce under blue light, some diamonds, rubies, red spinel, calcites, organic minerals.... and probably more that no one has thought to try yet.

[Jordi Fabre]

César Menor Salván is a former researcher from the main Spanish scientific research center (CSIC) and we are enough lucky in Spain to have him as an analyzer through his company GeoSpectra. You can check APXS: from Mars to the mineral collector's workbench - (17) to know more about the analytic methods he is using.

Of course I submitted to him one of these fascinating new Mexican Opals and here you have a result of his studies. Interesting, right?

[Don Lum]

Jordi,

This is very interesting information. Thank you for the analysis.

Don

[Gerhard Niklasch]

Indeed!

To put the quantitative results in perspective: What was the size of the sample, not counting the matrix? And what are the y-axis units on the gamma spectrogram - "Counts", but accumulated over what length of time?

Cheers, Gerhard

[Mark Ost]

Thank you Jordi for the analysis. This will be a nice compliment to the owners of those samples.

[Cesar M. Salvan]

Thank you, Jordi, for the reference and friendship.

Gerhard, take into account that this is a summary. My policy is that all data and technical details belong to the owner of the samples :).
In this case, the sample weight is 2,53 g and the gamma spectrum accumulates during 4 hours, measured without matrix, in a background isolated "lead castle".
In spectrometry, the geometry of the system is a critical factor anyway. But, anyway, as you can see, the radioactivity is very low.

[Gerhard Niklasch]

Thank you much for the clarification - yes, now the point is clear! It would have been something else if a 50µg sample had rivaled 60µg of Autunite. ;)

I'll soon have a chance to look at the luminescent response through my spectroscope, and will report on what I see. Unfortunately I have no way of capturing this photographically.

Cheers, Gerhard

[Gerhard Niklasch]

Pleased to report that a specimen was delivered to my door today, and was eagerly unwrapped! Even behind closed windows under completely overcast skies, it is all yellowish-green and greenish-yellow (in separate intercalating areas) contrasting nicely with the sandstone(?) matrix. A white LED lamp strengthens the effect. It turns bright green under LWUV, and brighter yet under SWUV (I needn't even bother to darken the room!), showing (as expected) the marked uranyl bands when viewed through the spectroscope. While either UV wavelength is capable of exciting all the bands up to the blue-green one, it seems the blue component of the cloud and glass filtered daylight mostly excites the two or three bands towards the lower-energy, yellow side of the spectrum, resulting in a yellower hue. (The variation in hue across the specimen area under daylight or LED light might be due to varying concentrations of some co-activator.)

Excitedly (me too! :) ),
Gerhard

[Mark Ost]

Look forward to getting mine. Now if all our fingers fall off due to the Uranium content..........................

[Gerhard Niklasch]

Well, for a typical hand specimen of this material, we're talking about an order of magnitude of one U238 decay per second on average, plus the same rate from each of the unstable daughter isotopes along the decay chain (that's what "secular equilibrium" amounts to). This really is low activity. If I were to worry about my fingers, I'm more worried of cutting myself on a cooking utensil or pinching them in a door by sheer personal stupidity. :)

I've played around a bit more with my LED flashlight and found that the yellow and green patches move when I move the light source: so they don't arise from different material properties at all. Rather, the opal blobs act as little lenses, focusing the incoming light onto a few bright spots on the rear side where they contact the matrix - and these spots move as the angle of illumination changes. And then the blobs again act as loupes, showing magnified versions of the bright spots to the observer's eyes. They now appear yellow because the violet component of the incoming light has already been absorbed and turned into green fluorescence. Those bits of opal through which I'm not looking at such a bright spot appear green rather than yellow.

Along the way, I also learned that this particular LED flashlight emits practically no blue light… it contrives somehow to produce a continuous spectrum from red through yellow to green, then there's a gap, and then there's a violet band centered around 450-460nm; the whole arranged to fool a human eye into seeing white when there's no spectroscope in the light path.

Cheers, Gerhard

[alfredo]

Azurite must look awful in that light ;((