FMF - Friends of Minerals Forum, discussion and message board

[Jared3339]

Hey guys. I wanted to know a little more about the acids used to remove unwanted "stuff" from minerals. I know certain acids are incompatible with certain minerals. I have also heard that this can be a dangerous process from time to time. Any basic beginner tips will be greatly appreciated. Thanks.

[vic rzonca]

Jared, this is from the index.

"Actually, my understanding is that while hydrochloric is not compatible with stilbite, sulfuric is. But I'm going to wait on a proper ID before I even consider any harsher cleaning methods. :) "

"Hi Rei, I finally got around to perusing my copy of 'Zeolites of the World' by Rudy W. Tschernich, 1992. It is a fantastic book, and whilst it is currently out of print, I believe it is available as a free (PDF?) download on Mindat! I haven't tried downloading it myself, but I would think it would take a while - there are over 560 pages in the book...

Anyhow, there is plenty of information regarding the cleaning of zeolites (pages 27-32). It would appear I was a bit presumptious regarding the use of acids on zeolites; I will attempt to condense some of the info here :)
1. Some acids can be used to 'clean' certain zeolites. The reactivity of the zeolite mineral depends on it's silicon content - there is a really useful table of zeolites and their silicon content on page 32.
2. Strong and dangerous acids should NEVER be used on zeolites: for example sulphuric, nitric, hydrofluoric, aqua regia; also strong bases (alkalis) such as sodium hydroxide.
3. Hydrochloric acid may be used for removing calcite and iron stains, on very high silicon content zeolites. For zeolites such as stilbite, which has a silicon content ranging from 73 - 77% (which is quite high but not the highest), an immersion in HCl for no more than THREE MINUTES is allowed. (HCl, hydrochloric acid, muriatic acid - all the same - the muriatic acid obtainable from builders' merchants as brick cleaner is typically 28% strength and Tschernich says to dilute this to 50:50).
Any zeolite with a silicon content of <70% will very quickly become 'frosted' or affected somehow by the acid, and low-silicon zeolites such as thomsonite (down to 47%) would instantly be affected.
4. "Acetic acid (ethanoic acid, 'vinegar') is suitable for removing calcite from all zeolites, even those with low silica content" (paraphrasing here!). Apparently vinegar is to weak to be much use, so he recommends obtaining glacial acetic acid (100%) and diluting it to 50%. Heating the solution (slowly) is recommended. The neat acid is extremely pungent and can cause serious skin burns; it's quite nasty stuff.
5. Oxalic acid is preferred for removing iron stains as it is less harmful to the zeolites than hydrochloric acid, though don't use it if there is calcite on the specimen otherwise a yellowish calcium oxalate can be deposited on the specimen which is very difficult to remove...(you still need to be careful with this one). Oxalic acid can be purchased from a chemical supply company and some pharmacies (that might be difficult, as it is very poisonous!). It come in powder form and should be dissolved in hot water - one tablespoon to one cup of water. Don't leave the specimen in the solution too long, check every 15 minutes, give it a scrub and put it back in the solution if necessary. If discolouration of the solution occurs (like turning yellow) then replace the solution (same goes for all acid treatments).
6. Ascorbic acid (vitamin C) is excellent for removing black manganese oxide from zeolites, it can shift the black stain/coating very quickly. It can be purchased in powdered form from "any drugstore". Mix approximately half a tablespoon per cup of warm water. This solution is not stable, however, and will not keep more than twelve hours (turns yellow) so only prepare enough solution for your immediate needs.

The above is more or less copied straight from the Book.

A few general rules; when diluting acids ALWAYS add the acid to the water, NEVER the other way!
Test a sample of your mineral with the solution, before trying your best specimens!
When the solution containing the specimen becomes discoloured, replace the solution with fresh solution.
Rinse the specimen afterwards, very thoroughly, maybe many times... with clean water. It may have to be soaked in water several times, overnight, to remove all traces of the acid (solution) and its by-products.

Also, remember that more than one zeolite may be present so make sure the solution won't attack the zeolite with a lower silicon content if you want to keep the whole specimen intact!
Other minerals which are commonly found in association with zeolites can easily be attacked by acids - apophyllite, gyrolite, prehnite, datolite, okenite and even the clay coating which often occurs as the first mineral growth on the surface of the vesicle. (Any mineral formed on the clay coating will fall off if the clay coating is affected by the acid.)

Interestingly, R.W.Tschernich doesn't mention Iron-out or Super-iron-out in the section on cleaning zeolites! Perhaps even more interestingly I have never tried any of the above acids on MY zeolites! Not only that but I probably never will. I would just try and find a better or cleaner specimen next time." Posted by Mike Wood.

Mike' post is informative if not exactly what you had in mind. You're pretty vague on what the stuff is you're working on. Part of the game here is to know what you are dealing with all around. You should always be very careful with any solutions used for cleaning, both on the specimen and in handling such solutions.

[Jared3339]

Thanks

[NellsRocks]

Forgive me from hijacking this thread, but I have a question as well.

I have used a "dummy acid", hydrofluoric acid, aka: WHINK Rust Stain Remover on a few of my little "interesting rocks".

I've noticed that after using the product, rinsing it all off with cold water, and allowing it to dry, the specimen has a very "dry" look. Parched, if you will. Is there a way to treat the specimen to give it that "wet", "hydrated" look once again?

I hope I typed that up well enough for people to understand! :-)

[GneissWare]

The resulting luster or lack thereof is a function of what you are cleaning, and what you are using to clean with. So, there is not one simple answer to your question.

The MSDS for the Whink Rust Remover states it contains hydrofluoric acid (HF - 1.5 to 3%). This is not a benign acid, and if you have no experience with acids, you should not be using this product for cleaning minerals. Hydrochloric acid is a completely different chemical.

HF will etch most minerals and will leave behind a whitish residual on many things. Again, you should not use it, as it can cause significant injuries.

[NellsRocks]

Hmmm, ok. I figured it was far safer to use than straight acids mentioned elsewhere. I have been careful, and no injuries to report!

;-)

Well then... how else can I remove that dirty/rusty "shmoo" that is on so many samples?

[GneissWare]

If it is iron oxides, try Super Iron Out. It is much safer than acids, and actually does a very good job.

If you have specific questions about cleaning a specimen, try posting a specimen. It is impossible to give a generic answer, as different minerals and coatings require different cleaning methods.

[NellsRocks]

[Jordi Fabre]

[Reinhardt van Vuuren]

[GneissWare]

There is lots of info online about the hazards of HF. The inhalation risk is quite high, both as vapors and aerosols. There are also significant dermal exposure risks. I cannot understand how HF gets packaged in ordinary consumer cleaning products in this age of litigation.