Failure using Super Iron Out on Azurite

Hi,

I tried cleaning some residual limonite from a specimen of azurite & malachite from the Lost Lake claim and the azurite turned black after only a few minutes in a concentrated solution.

Any ideas?

Bob

Is sodium dithionite a strong enough reducing agent to reduce Cu(II) to Cu(I)? I've often wondered whether it was safe to use it on secondary Cu minerals.

Probably, as the copper-bearing Fehling solution is used to detect the much less reducing monosaccharides. Presence of monosaccharides is indicated by precepitation of copper (I) oxide.



Edited 1 time(s). Last edit at 05/20/2011 08:25AM by Amir C. Akhavan.

Purely based on the standard reduction potentials at 1atm. pressure and at 25oC, dithionite has a more negative reduction potential (-0.66V vs NHE) than copper(II) to copper(I) (+0.52V) so in aqueous solution sodium dithionite would reduce copper(II) to copper(I).

Alan

I posted more detail in 'What minerals are not compatible with Super Iron Out?' about this very subject, but basically went through this same learning curve- as I live in Arizona, so tend to have a lot of it laying around. Generally, on AZ (and surrounding region) specimens containing azurite- the only cleaning I do is a rinse in water then leave them as is.

When I tried using alkaline dithionite solution on azurite I found that it turned green (presumably into malachite?). Either way, I'm quite sure that it is unsuitable for azurite (unfortunately).

Barry

Try consulting "Mineralin reinigen und aufbewahren". Its a great work on mineral cleaning, if you can read scientific German. The English translation, I imagine, would be quite popular.

I have had the same problem with Super Iron Out (SIO) when cleaning oxidized copper specimens. I wanted to remove iron stains on malachite and azurite that I collected from the Whipsaw Mine north of Wickenburg, AZ and from the Turquoise Mountain area of SE AZ. The SIO removed the iron stains but everything wound up covered with a tenacious black stain. The first time it happened with the Whipsaw Mine specimens, I thought that it may have been some kind of manganese stain. I removed the stain with dilute acetic acid (vinegar) but the specimens were somewhat damaged by the acid.

I just did it again with the Turquoise Mountain specimens. Another tenacious black stain that ruined the specimens. This time it finally dawned on me that I was reducing the soluble copper in the specimens to Cu+ and probably forming a black copper compound, maybe copper sulfide, that is ruining the nice bright colors of the specimens (I didn't think to check mindat.org or I would have known what the stain was). Rather than using a long soak in dilute acetic acid like last time that might further damage the specimens, I remembered that peracetic acid is a strong oxidizer. So I dumped in some drugstore strength hydrogen peroxide into vinegar and used it to clean the black stained specimens. Literally within minutes, the black stain was dissolved off of the specimens and the colors were back to normal - green malachite and blue azurite.

After learning this the hard way, I will still use SIO to remove iron stains from oxidized copper minerals and then give them a quick dip in a vinegar/hyrogen peroxide solution to get rid of the black stain.

Steve

Hi there!

SIO if I'm not mistaken has Citric acid.S.Dithionite is the active compound.

S.Dithionite not always removes Limonite in total.In any case,I prefer removing Limonite mechanically-ultrasonic.

Water and dish soap-NOT hand soap,it is dangerous since it contains Citric Acid-works,but very slowly.

Alkaline solutions will damage most species.S.Dithionite needs no regulation of pH.

For some reason,it changes the color of one of the four indicators on indicator paper,but it has to be due to reduction and not because the pH is lower.

Quick acid bath is something I always avoid.It is artificial etching of the specimen.

Kostas.

hi.i touch a quartz coated by rhodochrosite with my finger after than i touch lemon juice without know and everywhere i touch the rhodochrosite the specimen turn black.i think than all carbonate are really not compatible with acid .you must check for the possible reaction of the specimen with all the agent before to use .im very angry about this accident cause this quartz point are one of my favorite specimen from peru in my collection and i can do nothing to give him back is pink color.but i dont know if you apply just a little bit of iron out with a Q-tips if that's can work .test it in hidden place before.bye