FMF - Friends of Minerals Forum, discussion and message board

[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 ;((

[Mark Ost]

Ok, so maybe it's not our fingers but you never know about low level radiation! Jordi may have to support my five eyed children.

[Mark Ost]

Now that you mention it Pierre, it occurred to me I had a beryl from the same location with opal and it seems to do the same thing yours does. Much more subtle than the Mexican opal but it too reacts in daylight.

[Mark Ost]

Pete
I just checked my Namibian beryl (aquamarine) / schorl/ opal. Under incandescent light it appears a frosted white with no hint of green color. In the late evening light it takes on a green hue. Glass does not seem to filter the effect. I will try in direct sunlight tomorrow. Fluorescent light seems to wash it out. Standing away from a south facing window the green is evident but washes out closer to the window due to brightness. Very responsive to SW, MW even in daylight. Less so LW.

[alfredo]

I tried to measure the radioactivity of daylight fluorescent opal with a geiger counter and got nothing above background. Ditto for highly fluorescent artificial "uranium glass". The amount of uranium present is simply too low. As for the dangers of low-level radiation, remember that we are surrounded by it all the time; a specimen of daylight fluorescent uranyl-bearing hyalite seems to add no appreciable quantity to that level. Nothing to worry about.

[Cesar M. Salvan]

As I said in my report, the uranium content can be detected only by sensitive instruments. Hand-held Geiger monitors are not adequate to detect it. GM monitors equipped with thin window detectors, used correctly, can detect the small amount of activity of these samples, as are too close to background.

[Jesse Fisher]

At the TGMS show I measured the activity level of a specimen using a hand-held scintallometer that was available in the GSA booth in the convention center lobby. The measured activity level was 1-3 microSieverts per hour and 0. - 0.3 milliRoentgens per hour. According to references, the former measurement is about the some as the exposure one will receive while flying in a typical airliner at cruising altitude (around 40,000 feet). I'm not sure how many hours I have logged flying at 40,000 feet in the many trips between California and London over the past 20 years, but I am sure it is far more than I would ever spend holding one of these specimens, no matter how pretty they are!

[Cesar M. Salvan]

I received a lot of questions by email and even by phone, about this opal and its uranium content. Two of these questions are 'faq', so I decided to share it here for general information:

- In the spectrum of the radioactivity of the opal, why appears uranium-235 and other elements? if U-238 is the major isotope in natural uranium, where is it?. The U-235 is dangerous?

There is certain concern about uranium. Some people does not expect the presence of U-235, well known by their nuclear uses. Well, the gamma spectrum showed in my report is the typical radioactiviy spectrum of uranium in naturally occurring radioactive materials (NORM), as rocks or minerals. If we measure the spectrum of U in any mineral or rock, is always the same.
The main component in uranium, the U-238 (99.28% of natural uranium) is an element with a long half-life and, hence, a poor radioactive emitter. It is transmutated by alpha emission in Thorium-234. This transformation is accompanied by emission of gamma rays, but with a very low energy. Its energy is predated in the spectrum by the daughter nuclides and the X-ray emission. This is why, despite its abundance, the U-238 is invisible in this spectrum. The U-235 has a lower half-life and is a more intense emitter. The majority of radiation emitted natural uranium does not come from uranium, but from its daughter nuclides in the uranium series. These nuclides appears at constant concentration in NORM, because the uranium older than 700000 years is in "secular equilibrium". This is why the spectrum is the same in all minerals and rocks and this is why the uranium in minerals is more radioactive than purified uranium.

Do not be afraid by terms as uranium, X-ray, etc. This is a completely natural process and, in the quantities contained in opals, is completely safe.

- Is possible that the opals are a fake, produced by treatment of opals with uranium salts?

This is absolutely impossible. And, in the hypothetical case of a fake, the gamma spectrum would reveal the fraud, as the spectrum of uranium in laboratory chemicals or other processed materials is completely different.

[Peter Megaw]

I have been playing around with another piece of this material and have taken a series of new pictures showing the different fluorescent responses to various lighting conditions. (Incandescent W light; daylight; a combination of Incandescent and SWUV and full SWUV).

I discovered that there appears to be a distinct difference in the daylight fluorescence between early morning (8:00) versus early afternoon (14:00); the later is stronger and yellower than the early shot. Both were taken in the same place under full shade with otherwise unrestricted view to the sun. No cloud cover whatsoever. The only real difference in the photographic conditions seems to have been the sun's position...and of course amount of atmospheric filtering.

All the photographs are of the same piece

[Vinoterapia]

Amazing, this really is a kind of "Chameleon" mineral.

[waterdog]

Our specimen arrived today, thanks Jordi!

I checked the specimen with a Ludlum Type 3 radiation detector with a Model 44-9 pancake probe. The probe picks up alpha/beta/gamma. The response was 600 counts per minute at 0.5 cm from the detector face. The sample covered about 70% of the detector face. This count translates to 0.2 mrem/hour (2 μSv/hour), a worthless number for actual dose calculations but it gives a crude estimate of the radioactivity of the sample. The sample clearly radiates above background (less than 50 counts per minute in a dusty room). The specimen kicks out 18 mSv/yr while the average annual radiation dose (in US) is 6.2 mSv/year. So don't keep one in your pocket. :)

P.S. The meter hasn't been calibrated in 20 years.