Removing yellow Staining due to Hcl soaking

Hi all,

I'm hoping someone can hlep me out or give me some suggestions on how to remove HCL

I've been soaking a few quartz pieces I found in ellenville, NY in a strong solution of HCl to remove a thick "sphaleritic" and rust coating that has formed over the zstals. After a few days of soaking in the Hcl, I took my rocks out of the acid, rinsed them in water, and them stuck them in the ultrasonic for an hour or so. The coating has disolved, but when the rocks dryed, a few areas on the plate of crystals seemed to be stained yellow from the HCl. I think that is is more of a surficial stain, but not too certain. I say this because on a few of the larger xstals have smaller "micro" crystal spurs growing out of the sides, and this is where the yellow staining seems to be concentrated in the nooks and cranies of the micro crystaline clusters, if you will. I'm trying to let the pieces soak in a water-baking soda solution for a while and see if that works, trying to neutralize any residual acid left on the rock. I'll also try to ultrasonic them again and see when happen. Does anyone else have any suggestions on how to remove this staining?

ANy help much appreciated!

Erica

Usually, in such cases putting them back in some fresh clean acid will remove the stain. But first look at the yellow stained areas, preferably under a microscope and satisfy yourself that the 'iron stains' are really in the cracks and on the surface of the crystals and not intergrown on the surface or just under the surface. Use a needle and try and scratch it off. Don't worry about hurting the quartz, it is a lot harder than the needle. Let us know how it comes out.
Rock

One of the drawbacks of using HCl (or oxalic acid) to remove iron staining from minerals is that anything with fractures or pronounced cleavege (such as micas or feldspars) will trap and hold the acid solution. To prevent the iron-containing acidic solution from re-precipitating on the mineral when it dries you will need to soak the specimen in clean (best distilled) water for a number of days after the acid bath in order to allow it all to leach out. One way to help prevent this is to soak the specimen in water first, before putting into acid. This will saturate the cracks and fractures with water and help prevent the acid from penetrating deep into the cracks. If staining from HCl does occur, re-application of fresh acid solution should remove it. If it's oxalic, however, you may have to live with the result as many oxalates are rather insoluble.

Erica:
Sometimes the stains from HCl can be difficult to remove, especially if it works into cracks. Hydrochloric acid has a great ability to penetrate even into tight crevices. The pre-soaking process will help to prevent this, but in some cases you want the acid to penetrate cracks, especially when they are stained with iron oxides.
But the real important issue here is to properly neutralize acids after cleaniong, and this may take a while, so be patient. After rinsing in runniing water, it is best to follow with a prolonged immersion in baking soda and water. Allow to sit until no bubbles are evident.
Follow this with a soaking in sudsy ammonia solution. It is best to keep this solution covered, as the ammonia will rapidly dissipate into the air.This may take as much as a week.
If yellowing persists, follow with a bath of fresh, hot oxalic acid. This will almost always remove any traces of yellowing; then rinse & neutralize in baking soda again.
I once cleaned a specimen of cleavelandite from the Little Three Mine that had an opening (a vug within a vug) that kept "bleeding" iron stains out. This required repeated baths in oxalic acid, followed by neutralizing, and then oxalic, and so on, until all traces of staining were eliminated. I do NOT recommend, however, that repeated baths in hydrochloric be used.
Chris

If using HOT liquids, of any kind, put the specimen in room temp liquid, then heat. If you put it in hot fluid the sudden temperature change could crack/break the specimen.

Bill G



Edited 1 time(s). Last edit at 05/02/2008 04:38PM by Bill Gordon.

After treating specimens with HCl I soak them in water for 2 or 3 days with a lump or two of calcite to neutralise any residual acidity. I may change the water if it gets cloudy with precipitated hydrous iron oxide. This seems to lessen the tendency for yellow stains along cracks where acid deep inside has not been fully neutralised. If the specimen has such a stain then citric acid/sodium sulphite or oxalic acid may remove it, followed by water soaking. Soaking in water, rather than just washing, seems to be important in getting rid of acid deep inside cracks.

Thanks for all the tips! I let it soak in water/ baking soda solution for a day then ultrasonic in a amonia-dish soap solution for a while. Then I used a small pressure gun to clean out the cracks and the HCl staing seemed to dissapear-Thanksfully!. I think another soaking in either Iron out or HCl is on order though, to remove the rest of the rust stains in the in all the hard to get spots. Such a long process.. its going to take forever to clean all Crystals I found! Haha oh well. guess it will be worth it in the end! THanks again!

like someone else said about oxalates, watch out. Being from new york i suppose its herkimer type xls you are trying to clean. I would make sure there is no dolomite left otherwise the oxalic acid would more than likely bind with the calcium to form a bunch of those oxalates, ruining your piece

"A man is not finished when he is defeated. He is finished when he quits."

Oxalate precipitates are relatively easy to get rid of. Many heavy metal oxalates dissolve again when an excess of oxalic acid is applied (due to complex formation). Barium and strontium oxalates dissolve in acetic acid (!), and when a mineral acid is applied, calcium oxalate dissolves as well. Oxidants such as H2O2, dilute HNO3, or an acidic permanganate solution destroy the anion completely. Concentrated H2SO4, however, should never be applied, because it decomposes the oxylate to carbon monoxide (among other cleavage products).

Erica,
You could try packing them in a box and sendifn them to your good friend Ian (haha) in Arizona. I will do my best to get those Shawangunk beauties clean. I promise to baby them like they are my own. Once clean I will continue to treat them like my own!

Haha.

I hope they come clean. I can't wait to see them when I get back "home".

Cheers.
ISM

Haha! They are cleanin gup rather nicely, though you can never tell what you initally pull out bc they are so encrusted in rock hard clay and Fe. Don't worry Ian, when you come home I'll surely take you there!

Erica