FMF - Friends of Minerals Forum, discussion and message board

[daviddillman]

I have some green fluorite I retrieved from an old tailings pile. The fluorite has been stained by what I think is iron in the soil. What are the best chemicals to remove iron stains from fluorite?

[Jordi Fabre]

I would suggest sodium dithionite

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[lluis]

I agree with Jordi.
Just that I would add some sodium citrate as complexing agent.

With best wishes

Lluís

[Peter Megaw]

Jordi is right...sold in US at most hardware stores as "Iron Out". It works best at elevated temperatures...crock pot is best. Put fluorite in water, heat it up, add IO let it run an hour or two, turn off and let it cool slowly.

[chris]

[lluis]

Hi, Peter

Just as chemist, sodium dithionite oxidizes very quick with oxygen of air, so, a long time does not help too much.
Also, it decomposes very quick in acidic solutions. Done that when oxidizing creates sulphuric acid, decomposition rate increases exponentially (another help with sodium citrate is that acts as a buffer, maintaining a decent pH).
Heating, it decomposes also very quick.

And as last remark: sodium dithionite, if gets wet, can (and many times do...) burnt....

With best wishes

Lluís

[trtlman]

I don't know about fluorite but Iheard products such as lime away and CLR can be used on quartz and other crystals. I do not know the chemical makeup of these cleaners so before trying perhaps someone on here could give a little more info on whether or not these products are sutible for fluorite.

[Jesse Fisher]

Sodium dithionite works well at room temperature. A soak of no more than 8 hours usually will do the trick. I only add citric acid if the stain is particularly heavy. If the solution becomes saturated with iron, it can redeposit back into the fluorite and acidifying the solution will help prevent that from happening.

What ever you do, do not put fluorite in anything that contains oxalic acid. It will remove the iron stain but can also etch the fluorite.

[daviddillman]

Everyone,

Thanks for the advice and the warning about not using oxalic acid.

David.

[bob kerr]

"What ever you do, do not put fluorite in anything that contains oxalic acid. It will remove the iron stain but can also etch the fluorite."

Huh? oxalic etches fluorite? no way will oxalic etch fluorite. where did you get or observe this happening? could you possibly have observed oxalic removing something from the surface of fluorite such that it LOOKED like it etched it??

bob

[Jesse Fisher]

Oxalic acid will, indeed, etch fluorite. This occurs by the removal of calcium from the surface of the fluorite to form calcium oxalate. If you don't believe me, try it yourself. Just put a lustrous fluorite crystal in warm oxalic for a few hours and observe the result.

[bob kerr]

I think there's a terminology issue here. IF the fluorite has calcium on the surface oxalic (and many other) acids will remove it and possibly leave behind a surface that appears to be "etched" - but I don't think this is the definition of etching. etching actually removes the base material (CaF) and oxalic just can't do that.

bob

[Jesse Fisher]

Call it what you will, but oxalic acid has a strong affinity for calcium. I have ruined some good fluorites trying to remove iron stain with oxalic acid and will not do it again. Should you care to try, however, be my guest.

[Pierre Joubert]

[Pierre Joubert]