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Techniques for CollectorsYellow stains appear after specimen drys up

2nd Mar 2024 14:06 UTCAqua marine

Hello

We cleaned this specimen by placing it in high concentrated HCl for one hour, it became totally rust free. Then placed it in sodium dithionite solution for 24 hours andeft to dry. After it dried up this dark yellow stain appeared.

Then I placed ot in sodium bicarbonate solution for 24 hours but it had no effect.

What could be the problem and how to get rid of these stains.

2nd Mar 2024 15:42 UTCAlfredo Petrov Manager

Go from HCl to phosphoric acid or citric acid, not sodium dithionite.

3rd Mar 2024 06:35 UTCAqua marine

Or i should only use phosphoric acid instead of HCL and then sodium bicarbonate?

3rd Mar 2024 07:50 UTCKeith Compton 🌟 Manager

The two sodiums are different beasts:

Sodium dithionite Na₂S₂O₄

Sodium bicarbonate NaHCO₃(baking soda)

3rd Mar 2024 08:25 UTCAqua marine

Does phosphoric acid hurt muscovite?

3rd Mar 2024 09:49 UTCAlfredo Petrov Manager

Aqua marine ✉️

Does phosphoric acid hurt muscovite?

Haven't tried it myself, but probably not. Muscovite isn't much affected by acids, other than boiling sulphuric acid, which no one in their right mind would use.

Keep in mind though that the abundant incipient cleavages in micas means that any chemical is going to penetrate deep into the crystals and be very difficult to get out again. Superficial rinses aren't going to help. You need multiple long soak times - several times longer than the original acid treatment - to clean out the inside of the muscovite. Not a job for the impatient. Ultrasonic cleaning can speed things up, but can also speed up the development of cleavages.

4th Mar 2024 00:57 UTCAqua marine

In my experience using HCl always effects muscovite. The bigger muscovite loses its luster while the very small muscovite disappear.

4th Mar 2024 07:53 UTCRuss Rizzo Expert

the very small muscovite disappear.

HCL shouldn't dissolve muscovite.

4th Mar 2024 08:32 UTCAlfredo Petrov Manager

Aqua marine ✉️

The bigger muscovite loses its luster while the very small muscovite disappear.

Then perhaps it is not muscovite, but rather lepidolite or a different mica. Color is not the best guide for ID.

4th Mar 2024 08:55 UTCRuss Rizzo Expert

Maybe I should've been more succinct; muscovite or any mica shouldn't dissolve in HCL.

3rd Mar 2024 12:48 UTCAmir C. Akhavan Expert

Sorry for being blunt, but I really don't understand what the point of using dithionite or bicarbonate or anything else after HCl is. when the stains have gone.

This simply creates a mess.

When the iron oxide/hydroxide stains have been removed by HCl, all you need to do is to rinse in slightly acidic water (a few droplets of HCl).

Patiently, for a week or so. And then rinse in pure water.

When you place a specimen in an acid, the acid will quickly penetrate the entire specimen because the specimen is dry, but it takes much much longer to replace the acid and the dissolved iron in it with pure water because this can only happen by diffusion, and that takes time.

It is a bad idea to add stuff that raises the pH, like bicarbonate, to a solution of Fe3+. Fe3+ is only soluble at low pH, at neutral pH it will form insoluble Fe-oxychlorides or precipitate as Fe hydroxides and then you are exactly where you have started and the specimen turns brown again.

Dithionite will reduce Fe3+ to Fe2+ which is more soluble at neutral pH, but if the iron is still in the crevices, it does not help in the long run, the dithionite and iron will get oxidized and the specimen turns yellow-brown again. You want the iron out, not just "invisible", so whatever chemical you use, you need to wait until all has diffused out of the specimen's crevices. One cannot speed up this process by adding more chemicals.

4th Mar 2024 01:00 UTCAqua marine

Thank you for the detailed reply.

Sodium bicarbonate is used for neutralising the mineral when treated with acid.

However if a bath in water with few acid drops for a week does the job then its great aswell.

I will surely try it.

Sometimes yellow stains appear when the minerals dry after treatment of only sodium dithionite. And we have to repeat the process a number of times.

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