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Techniques for CollectorsA Challenge for the Chemists: removing fluorite as a soluble

22nd Feb 2008 20:32 UTCMorion

Issue: A thin white and very resistant layer of suspected fluorite precipitated when treating quartz with ammonium bifluoride. The ammonium bifluoride was used to remove secondary clay silicates that coated collected quartz specimens.

Physical removel of this coating is not a solution; maintaining the quality of the specimens is the objective.

Anyone with a sound understanding of this issue have a soloution to propose that can be implemented without lab?

I understand that there may not be a solution without an adequate and safe lab, but expendable specimens can be provided to a lab for any chemist with sincere interest.

Thank you in adavance,

Morion

23rd Feb 2008 20:29 UTCGeorg Graf

Hi Morion,

one can solute the fluorite with sulfuric acid, 50 %. But the generated HF can attac the Quartz. Because of this it is necessary to let always fresh sulfuric acid run over Your specimen. So You need much sulfuric acid. - That this treatment is quite dangerous I think I have not to state.

Greetings from Goslar

Georg

23rd Feb 2008 22:08 UTCChris Stefano Expert

HCl will do the same thing.

23rd Feb 2008 23:00 UTCAlbert Mura

Why are you assuming the coating is simple fluorite?

25th Feb 2008 18:26 UTCMorion

Sorry for my delayed response (no email at the collecting localities:) )

In response to Georg and Chris,

I will experiment with sulfuric and hydrochloric acids. I have used these in the past, and so apt to use them again. Thank you.

In response to Alberta,

There has been no definitive confirmation that it's fluorite. This has simply been the conclusion when discussing this issue several months ago on this site with a chemist. Do you suspect something other?

25th Feb 2008 20:24 UTCAlbert Mura

Well most clays contain aluminum and various Al fluorides are not very soluble. I guess I would have to sit down and work out the possible reactions but to do that I would need to know the exact clay. Your best bet are the acids but be careful.

26th Feb 2008 00:58 UTCMorion

Alberta,

For your interest,

The clay coatings originally on the specimens when collected were successfully removed using ammonium fluoride. The two locations that formed this white precipitant when treating the specimens originate from what i suspect is an alkaline pegmatite. Visibly you cannot see anything other than quartz, but variably when treated the precipitant forms. Very interestingly the precipitants forms on select faces.

Maybe there are surficial traces of Ca on select faces due to the originating environment causing precipitation of fluorite (or something)?

Thanks again,

Morion

26th Feb 2008 01:25 UTCDon Saathoff Expert

Morion....on selected faces of smokey xls found near Sierra Blanca, Lincoln Co., NM there is an epitaxial secondary quartz growth which I've had no luck removing w/ bifluoride.....only a CLOSELY monitored bath in HF has worked....great care must be taken to prevent etching!!! I'll not go into a discussion of the EXTREME hazards involved in using HF....those hazards have been well announced on this forum as well as others!!!!

I can't know if this is your problem or not, but something to consider as opposed to a precipitant from the bifluoride.

Don S.

27th Feb 2008 00:28 UTCMorion

Hello Don,

To answer your inquiry, it is a precipitant (see attached).

I am surprised you are not having much luck treating your SB specimens with bifluoride. When you say bifluoride do you mean the crystalline ammoniun bifluoride (lets call it CAB for now)? I have intentionally etched specimens (scrap pieces) before to learn my boundaries, but for the most part i have had much success using CAB with little monitoring. What i can say is that when CAB dissolves it appears to become "layered" at different concentrations in the solution with the most concentrated layer at the bottom. Unnoticed (and unstirred) this will begin etching specimens. Of course this condition is dependent upon concentration and temperature and can be avoided if appropriately adjusted.

You may want to reconsider using CAB to minimize your treatment hazards (and the nuisance of monitoring:)).

AZ Rockhound,

Morion

27th Feb 2008 00:31 UTCMorion

Correction about the attached file's title.

Its precipitant on left and etching on right.

28th Feb 2008 01:04 UTCDon Saathoff Expert

Morion....yes, I use xln ammonium bifluoride....have tried several concentrations, warmth, agitation, all w/ no success on the Sierra Blanca Material.....the HF worked beautifully well at a 1/5 dilution w/ distilled water. I use the same dilution for the quartz coating on the dark purple Burro Mts. fluorite (beautiful octahedra)

BTW, the white coating looks very similar to the coatings on the Sierra Blanca Smokey....

Don S.

28th Feb 2008 16:44 UTCMorion

Don,

Yes the coating looks very similar to the coatings on the Sierra Blanca Smokies (when partially treated), but this coating was non-existent before the treatment and becomes thicker with further treatment. I have had the same issue when treating pegmatite amethyst scepters from Santo Nino Mine, Santa Cruz Co., AZ.

Thanks again for your input.

28th Mar 2008 11:27 UTCRock Currier Expert

Morion,

Why is physical removal of the coating not an option? Can you scratch it of with the point of a knife blade?

Rock

28th Mar 2008 22:50 UTCMorion

Yes, but with a strong application of pressure. Additionally the faces are sometimes too complex with scepterization to allow easy access with a knife blade or have abrasion faces due micro quartz crystal that would rapidly wear away a knife blade.

My hopeful approach, if it can be chemically added then it can be chemically removed (with exceptions). Just waiting to acquire new acid solutions to test, so i might be back if unsuccessful.

29th Mar 2008 11:46 UTCRock Currier Expert

OK, I understand, but I would strongly recommend you try removing the coating with an air abrasive tool. This you might better understand as a sand blaster, but with glass beads rather than quartz sand. I think this might clean your specimens very well, probably almost as well as a complete chemical cleaning. Cleaning quartz with glass beads will have little or no visible effect on the surface of the quartz, but if subsequent to that treatment want to further clean them with a hydrofluoric based reagent you may expect that the quartz may be attacked much more readily by the HF than if the quartz had not been cleaned with glass beads. The glass beads apparently make some changes in the surface of the quartz even if it is not readily visible. You could also use something like dolomite powder in the air abrasive tool and this would probably lessen any non-visible damage to the quartz. We routinely use 200 mesh glass beads to clean our quartz specimens and find it works well enough for us that we seldom ever have to resort to HF based reagents. HF is not a pleasant reagent to work with and he have not used in several years to clean specimens. In Brazil, both in the pegmatite regions of Bahia and Minas Gerais and the amethyst-agate areas in Rio Grande do Sul; a commercial reagent called Chispas is routinely used to clean their quartz and amethyst specimens. One of the ingredients in this solution is HF, but in low concentrations. Several hours of immersion in this reagent does not damage the quartz at all, but it will much more readily attack any agate and leaves the agate with a 'white skin'. You can always tell when an amethyst specimen from Rio Grande do Sul have been left too long in the reagent, because the amethyst looks great but the underlying agate has a 'white skin'.

Rock

29th Mar 2008 14:00 UTCDonald Peck

Hello Morion,

George is correct that sulfuric acid will remove the fluorite. He is also correct that the generated H2F2 most probably will attack the quartz, probably etching the crystal faces. If you use sulfuric acid, be sure to take safety precautions and work out-of-doors where the generated hydrogen fluoride can escape . . . and stay up-wind of it.

Don

2nd Apr 2008 03:39 UTCHoward Heitner

There is an alternative to ammonium bifluoride for cleaning off clays and micaceous material. It is boiling in 50% sulfuric acid. I have used this to loosen muscovite from schorl crystals. The coating may not dissolve but may be loosened sufficiently to be removed with a brush or scraper.

2nd Apr 2008 12:58 UTCAlfredo Petrov Manager

I agree with Howard - but keep in mind that the gentleman has a doctorate in chemistry. I've used hot sulphuric acid too to decompose biotite and chlorite on quartz crystals from pegmatites and alpine veins. (Hmmmm... guess what happens to the F in the micas?) Be extremely careful! Hot sulphuric acid is NOT a good cosmetic. Don't do this unless you live next to a shop selling guide dogs and white canes.

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