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Remove malachite from magnetite

Posted by George Creighton  
Remove malachite from magnetite
July 08, 2012 06:42PM
    
Hi everyone

On a recent trip to the hamre cu mine tailings Bygland Norway I found a group of magnetite xls ( largest ~3mm ) covered in a thin layer of malachite,,, the magnetite striations and xl form are clearly visible under the malachite coating.
Question is how to remove the malachite without damage to the magnetite.

All suggestions very welcome.

Regards george
Re: Remove malachite from magnetite
July 08, 2012 06:58PM
George, most acids, even weaker ones, will damage magnetite. Vinegar (acetic acid) should be OK on magnetite, but you'll need patience as it will dissolve the malachite only slowly. I hope the magnetite isn't already etched underneath the malachite.
Re: Remove malachite from magnetite
July 08, 2012 07:55PM
    
Hi Alfredo

This solution ( Vinegar (acetic acid) was also my first thought after citric acid.

The dilemma is that in norway vinegar is normally sold/retailed in 3 strengths table vinegar 5%, pickling vinegar 7% and a concentrated vinegar 35%.

Which of the 3 above do you recommend.

Glacial vinegar is also available but think that is far to strong.

Regards george
Re: Remove malachite from magnetite
July 08, 2012 08:38PM
    
According to Sinkankas magnetite is unaffected by most acids. I have used HCl to remove calcite from magnetite with no ill effects, it should work for malachite as well. Question is why do you want to remove the malachite? Malachite coated magnetite is rather unusual and should be preserved as is. Are you sure it isn't coating cuprite? (assuming you didn't test it with a magnet).
Re: Remove malachite from magnetite
July 08, 2012 09:05PM
    
Reiner

Deffo magnetite compass reaction.

Already have numerous malachites from this locality but no magnetites,,, need to clean these magnetite xls in order to get a better photo of them to post here.

Regards george
Re: Remove malachite from magnetite
July 08, 2012 09:55PM
I think Sinkankas was mistaken there, Reiner. I've done some cleaning experiments with large quantities of Bolivia magnetite crystals and found the surfaces to get etched with dilute HCl, strong citric acid, and even with sodium dithionite ("Iron Out"). But, as always, much depends on how long one leaves it in the solution, the temperature (not talking about boiling here, just room temp variations), the exact strength of solvent, possible presence of microfractures and porosity in the magnetite, and I suppose there could be differences even due to the purity of the magnetite (how much Mg, Al, etc. it has in substitution). I suppose Sinkankas is right in so far as it won't simply disappear in front of your eyes like a sugar cube, but it's certainly sensitive enough to be attacked and suffer surface alteration.

George, you're lucky! I wish I could get as many different concentrations of vinegar as you do. All I can get is the weakest version: 5%. But even that's strong enough to dissolve calcite and malachite - just needs patience for several days, and changing to fresh acid whenever the bubbling slows down.



Edited 1 time(s). Last edit at 07/08/2012 09:59PM by Alfredo Petrov.
avatar Re: Remove malachite from magnetite
July 09, 2012 02:18AM
Try a small piece in Hydrochloric acid 10 the 20% and one in 7% Vinegar and see which result you like best.

Rock Currier
Crystals not pistols.
Re: Remove malachite from magnetite
July 09, 2012 07:11AM
    
Thx
Will try with the weakest vinegar to begin with, then if no result will work my way up to dilute HCL.

Will give feedback on progress.

Regards george
avatar Re: Remove malachite from magnetite
July 09, 2012 11:01AM
    
Citric acid will work best here, since it has a strong affinity to form a complex with copper. Here in Germany, food grade citric acid is sold by the kilo for use as a lime scale remover (e.g. in coffee machines). Perhaps in Norway as well ?
avatar Re: Remove malachite from magnetite
July 09, 2012 02:31PM
I have to agree with Peter (Mr Chemist) here. I believe citric acid (with an added dose of patience) is the best place to start.
Re: Remove malachite from magnetite
July 09, 2012 08:24PM
    
Hi update
The vinegar 7% seems to be working some malachite removed and magnetite xls becoming clearer, only worry is the rust colored parts starting to appear in the feldspar matrix ( possibly limonite or goethite ).

Will try a citric acid removal later.

Thanks to all for input.

Regards george
Re: Remove malachite from magnetite
July 10, 2012 02:52AM
    
Hello Mr Creighton,
Thought I would mention a possible assist to your project.
Is the magnetite only covered by malachite? That would be more complex, of course.
However, If there are exposed 'surfaces of interest' you wish to avoid etching damage to...

This reminded me of the process for prepping Benitoite etc from surrounding Natrolite by dissolving by dilute acid. I have had very good luck with coating clean, dry areas that I wished to protect from acid with clear fingernail polish(cfp). Acid will creep under edges of cfp after a while, so I would then remove, soak in H2O to neutralize,and let fully dry. Either apply more cfp directly OR flake off lifted cfp area with a tweezers and sharp pin, and reapply more.
To remove cfp, immerse in straight acetone** for ~20 min or so and the cfp is gone. Repeat as necessary.
The acetone vapors are flammable of course, and not good to breathe, and personally, I despise the odor, but have also had good luck with mitigating all 3 neg effects by placing the acetone container in a large ziplock freezer bag Or a sealed bucket. This also helps keep the acetone from attracting h2o vapor also.
**(do not use 'nail polish 'remover', it is weak, more expensive, and has perfumes and other neg additives you don't want on the specimen.)

Post some before and after pics too, please, regardless of procedure. Always interesting to see what folks have found.
Hope this helps!
JO:)

ps: I apologize to whoever first posted that CFP trick, I have lost track of where I read it at. John Veevaert at Trinity Minerals advocates wax, that is different process but same results.
Re: Remove malachite from magnetite
July 20, 2012 07:32PM
A while back I started a thread on the reaction of secondary copper minerals with aqueous ammonia. Perhaps this might dissolve the malachite or decompose it enough so that it could be removed by a water gun. Do you have any extra small fragments to try this first before risking a good specimen? I have no idea on how it might affect the luster of the magnetite.
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