FMF - Friends of Minerals Forum, discussion and message board

[GneissWare]

Nice.

[daviddillman]

Thanks to everyone who suggested the Super Iron Out. It took several baths in the solution but the results were very good.

David

[Barry Vincent]

Oxalic Acid still hasn't been mentioned yet or am I missing some detail here :)

[Jesse Fisher]

Oxalic acid has a high affinity for calcium and will etch most fluorite, dulling the luster. Speaking from experience, I would not recommend using it to clean fluorite (or much of anything else for that matter). It is also rather toxic.

[Barry Vincent]

[GneissWare]

Super Iron Out is much safer and just as effective (and maybe more so) as Oxalic acid. One should use the least aggressive cleaner possible when cleaning. Also, I have seen many specimens ruined by using Oxalic acid, leaving behind an insoluble stain of calcium oxalate when used with hard water.

[Barry Vincent]

[Jesse Fisher]

As I mentioned, oxalic acid likes to turn into calcium oxalate and will pull Ca from fluorite to do so. While it is good for solubilizing Fe oxides/hydroxides, it takes a lot of soaking to get it out of anything porous such as micas or feldspars with a lot of cleavage planes. If any of the solution remains in the cracks then it dries you get a nasty yellow stain on the mineral you have been trying to clean. Quartz, being without cleavages, tends to respond fairly well. Other minerals can be a problem. I always start with sodium dithionite (active ingredient in Super Iron Out) and move on to phosphoric acid if that doesn't remove all the iron stain.

[Les Presmyk]

Oxalic acid not only has an affinity for calcium, being acidic means it will attack and dull the calcite. It is the calcium in fluorite the oxalic acid is starting to dissolve. Finally, once calcium goes into solution in the presence of oxalic acid, it begins to precipitate as calcium oxalate, an insoluble compound (also one of the most common versions of kidney stones) that deposits back on the specimen.

[basti]

Oxalic acid is pretty good but also has some drawbacks:

The toxicity is quite overrated - it precipitates calcium so it can harm you if you eat few tablespoons but thats probably not going to happen. You can put your bare fingers into the oxalic acid solution and nothing happens (same for the 5-10 % HCl btw.). It doesnt produce toxic odour like super iron out and in my experience works much faster.

The bad part is that it is much more aggresive and can harm luster of micas. It should not be used on minerals with calcium as it can produce precipitates of calcium oxalate, which is sometimes difficult to remove. You should use rainwater or distilled water as there is always small amount of calcium in the water = precipitates as a fine "white dust". What I see as a biggest problem with oxalic is the production of yellowish precipitate sometimes, which is very difficult to remove.

SIO or sodium dithionite:

Much more gentle and effective and much slower then oxalic. However it is not all roses too. It is about 5x more expensive then oxalic. It produces toxic and smelly gases = no indoor at all. It is a strong reducing agent, which sometimes recults in formation of green-black coatings which must be removed by citric acid. Sodium dithionite is unstable chemical which decomposes if not sealed from air properly. Do NOT mix SIO with other chemicals except baking soda (sodium hydrogencarbonate) or citric acid!