FMF - Friends of Minerals Forum, discussion and message board

[Rei]

Can anyone point me to a place where I can figure out which minerals are compatible with which acids? I have hydrochloric and phosphoric on-hand and will probably at some point pick up sulfuric (I tried oxalic before but wasn't impressed versus alternatives)

[Mike Wood]

Not as such, but those sort of acids are definitely not compatible with zeolites!

[GneissWare]

Pick up a copy of Gemstone & Mineral Data Book by John Sinkankas, available on Amazon. This is really the only comprehensive source ever printed. (But it doesn't have all the tricks!)

[Rei]

[Jordi Fabre]

[vicen]

I suppose is a matter of trying different chemical available and do some experiments in your materials.
Some acids that has been used too are:
Sulfamic acid _ Padilla_and_Parra_2009
and
Sodium polytunstate_microfossils_ Mitchell&Heckert 2010
Both have been used in the preparation of fossils to remove matrix successfully.
Vicen

[Rei]

Hmm, anyone know about stilbite compatibility with Super Iron Out / Waller solution? I'm thinking about turning one of my smaller pieces into a piece of jewelry for one of my friends who's in the hospital with a blood clot (yeah, I'll warn her about its fragility and all of that :) ), so I'd like to get it as clean as realistically possible. I've already given it a good bleach soak and scrubbed as best I could with a plastic brush.

In addition to Waller solution, Another thing I was thinking about was a sandblaster with a small tip and table salt as the abrasive.

[Mike Wood]

Hi Rei, the only medium I have tried for cleaning stilbite (or any other zeolite) is chlorine bleach. This will remove or at least render organic matter colourless - for example, algae. I've never tried Iron Out or Super Iron Out on anything.
I've never tried sandblasting.
I suppose an ultrasonic bath might shake off any encrustation on the sample, but, yep - you guessed it - I haven't got one of those either!
Sorry :(
Mike.

[Mike Wood]

"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.

Happy dissolving!

Cheers, Mike :-)

[Rei]

Wow, crazy-great info there!

I've stopped using oxalic acid.. it works but I find it too likely to discolour specimens, as that book mentions is a risk. I like hydrochloric as it doesn't usually seem to discoulour things... but yeah, good to now about the time limit there ;)

I did an iron out test with a low quality piece I had. I'm not sure whether I like the before or after better... it got the iron out without damaging the crystals, no question, but the spots where the iron was still have something else left, so now instead of having iron-red blotches it has dark gray blotches. That's sort of my experience with super-iron-out in general... it's really selective to rust and only rust ;) But it's still nice to have, it's one of the only chemical means I know of to clean calcite.

The piece I'm cleaning up to make into for a sick friend, I'll probably just keep it as-is, I think that's probably best.