FMF - Friends of Minerals Forum, discussion and message board

[bob kerr]

Most of us on this forum have heard of or have seen color enhanced anglesites from Morocco. In an old MR I found a statement something to the effect that the color enhancement (caused by a bleach??) can be reversed with a quick dip in a bromine solution.

Well, at the Flagg Show in Phoenix last year, I came across a quite nice anglesite crystal clearly marked as color enhanced and curiosity got the best of me so I bought it at a fairly reasonable price with the full intent of trying the bromine solution.

before and after photos follow. the piece was dipped twice for 30 seconds each dip.

to summarize: the color did indeed change from a lightly burnt orange to yellow so whatever was causing the color change was clearly superficial. However the luster seems to have been negatively impacted. Note in the two photos I tried to show the reflection on the one crystal face.

so, results are mixed and I have some issues for discussion:
1 - assuming this yellow is the original color, why would anyone want to turn it to a burnt orange in the first place?
2 - does anyone know what the chemistry is here? how does lead sulfate change color and how does this reverse?
3 - what can/should be done next to possibly get back the luster? a hopeless case?

thoughts are welcome.
thanks,
bob

[alfredo]

Logically, any colour change to a crystal face (not counting mere coatings of another substance) involves a chemical reaction - atoms leaving and being added to the surface. Reversing that might often be possible but I wouldn't expect it to *ever* be possible to restore the original luster, as the surface has been changed.

For the same reason, it makes little sense to "clean" the surfaces of oxidized or otherwise chemically altered crystals - like taking limonite off siderite, or cerussite off galena, because the surface luster is destroyed and can never be restored to its original condition.

You might be able to "hide" the dullness with silicone spray or some such artifice. :((

[Jordi Fabre]

Same thing happens with the Realgar-Pararealgar (see: Realgar changes into Pararealgar or Orpiment?) if you heat the Pararealgar at 220 ºC you will get back Realgar, but matt and ugly Realgar.

[Dale Hallmark]

i like the restored color better. How do we know that the luster on the first piece isn't artificial too?

Dale

[kushmeja]

[alfredo]

In the case of the orange anglesites, it was a chemical change on the surface, not just mere coating ("paint"), so it is more likely to etch the surface and leave micro pitting than to "fill in" any irregularities.

[bob kerr]

[bob kerr]

[alfredo]

Just a guess, but perhaps bleach changes anglesite bright orange-red because the oxidizing action and chlorine create a surface layer of Pb-oxychloride mineral like lorettoite, which are orange and not too difficult to form?

No idea why Br would destroy lorettoite (if in fact it does). That much chemistry is over my pay grade.

[Roger Warin]

Hi Bob,
Here's an interesting topic. I remember in the 90s and before, Moroccans selling natural Baryte and other tinted brown. The rumor in France said at the time that crystals were immersed in chlorine bleach, aqueous solution of NaClO, for domestic use.
When you talk about a solution of bromine, what? Bromine (chemical element, non polar) is very slightly soluble in water (polar solvent). It is also prohibited for sale in Belgium. Would it be a solution NaBrO? To my knowledge this is not a consumer product.
There was therefore oxidation of crystal. I suspect Moroccans have let crystals immersed in the reagent bath for a long time. The attack would be deeper than we imagine, especially in crevices.
The first idea that comes to my mind is that cations that turn brown are iron, Fe 2 + slightly greenish blue, becoming yellow brown.
Should therefore use a reducing reagent to find the initial coloring, I think of salts of sulfurous acid (not sulfuric).Should therefore use a reducing reagent to find the initial coloring, I think of salts of sulfurous acid (not sulfuric). As Na2S2O3 or better, sodium dithionite, Na2S2O4.
Do we know the chemical composition of these Barytes?
This is a very interesting topic and it's the first time I think about. I emit this hypothesis to initiate the debate.
Roger.

[bob kerr]

[Jordi Fabre]

To clarify:

- The origin of these red Anglesites was just accidental. As they appeared frequently coated with Galena, which detracts from its beauty, someone in Morocco tried to clean the Galena and submerged one Anglesite in "Eau de Javelle" (bleach), a very-easy-to-find chemical agent in Morocco, and the Anglesite became red. They immediately repeated this trick with the MEDIUM-BAD Anglesites, but very soon the forgeries were discovered and they stopped doing it. In fact, not many Anglesites were faked, so they are overvalued forgeries considering (for example) the number of "silvers" from Imiter that went (and still are) in many collections even though they were proved to be of anthropogenic origin. See: Wire "silvers" from Imiter (Morocco)

- I'm firmly convinced that any attempt to clean, recover back the color, wash... anything related with Anglesite will be a disaster because it is a mineral species extremely sensitive to all kinds of chemical agents, even water (city water non rainwater)

[lluis]

Hi, all

I think that Alfredo hit the nail with lorettoite.
If you place a big amount of javel water (bleach), you could alter the surface of anglesite from sulfate to an oxide (javel water is basic), and due to be in essence practically "wet" chlorine, an oxychloride is easy to imagine. A thin film over anglesite would give the reddish/yellow appearance.
Bromine is slightly soluble in water. Bromine water is used (or was...) in cation analysis when I studied my career of chemistry. Easily done ...if you are a chemist and have access to it (as Roger said...). Not that easy if you do not have a lab, and, besides, not at all recomended: injuries if bromine hits skin could be (and are...) severe.

Why it cleans (bleach at least some colour....)?
I think that simply it makes lead bromide, that is off white. Being bromide more insoluble than oxychloride, well, it happens...

In all cases, as Alfredo said, you have a pitted surface, so it would not be mirror like, so...
And as Alfredo said, only a coating with an oil (silicone could be....) or anything could repair aspect. I would never do this, but that for others to decide.

Sodium dithionite I fear would not work: it is a reducer agent, but Pb would not be reduced to metal, and Pb2+ is the lesser state, so...
Less with sodium sulphite, because it is a very poor reducer....

To clean the yellowish color....
I can think in a bath in warm (or hot) diluted nitric acid (that disolves lorettoite...), but could make not exactly nice injuries to skin. so, only for chemists and in a lab, or simply placing in muriatic acid to convert oxychloride to chloride. But that I fear would make a dull or whitish surface.....

I simply would let...

For case of Barytes, I do not think in Fe3+. Yes, Fe3+ in presence of chloride ion makes very deep yellow/brown/red colours. but Fe3+ is a very pale lilac colour (take a look at ammonium iron (3+) sulphate: it is very faint lilac.
If case was that, then they would let all yellowish iron chloride on the surfacer, but it would lead to a stained appearance... In case they do not clean the javel water (which would take color off).
If it was a case of Baryte that had some substitution of Ba by Pb, not impossible, then, you would have same as with anglesite....

My two cents

With best wishes

Lluís

[cascaillou]

here is a decription of the treatment by Kurt Nassau:

pale yellow to near colorless anglesite can be turned red by briefly dipping them into bleach solution, this can be reversed by immersion in a saturated bromine solution.