FMF - Friends of Minerals Forum, discussion and message board

[Pete Richards]

Yesterday I went collecting in a river which contains septarian concretions which contain various minerals. These concretions are often broken, so mud and organic detritus can filter into the interior voids, covering the crystals that line the septarian cracks. Cleaning the crystals is a challenge similar to but not the same as the oft-discussed challenge of removing iron stains or lime crusts. The worst problem is organic films, probably mostly bacteria and non-cellular organics, that coat crystals that are not protected by mud, and other organic novelties like aquatic insect egg cases.

The usual approach is probably physical - ultrasonic baths, water jets, scraping, etc. These don't do the whole job, though, at least the ones I have available.

So I am thinking of a chemical approach. In my case, the minerals I want to clean are Baryte and calcite. My candidates are chlorine bleach, oxyclean, and maybe iron out.

Does any one know whether these chemicals will attack calcite or Baryte? Any other suggestions?

[Jordi Fabre]

Usually for biological materials people use ammonia in high concentration and leaving the objects submerged in ammonia at least a few hours.

Hopefully it helps.

[Calita]

Try with concentrated and pure ammonia (25%) . It gives good results with organic material but it's not very quick. Depending the kind of material takes several days to remove.
Chlorine bleach (pure not with additives as perfume ) is another chance but it's worst for the calcite.
For some kinds of organics if there is mud aside works very well the oxygen peroxide 33% alone or combined with ultrasonic cleaning.
Regards Paco.

[Calita]

Just in/at time Jordi.

Only a warning : Never mix cleaning products as H202 + ammonia or ammonia and chlorine bleach it's very dangerous.

[Bob Harman]

PETE, Over the years I have successfully cleaned many Indiana geodes and some other field collected or otherwise acquired specimens. My approach is first to have an overall plan for what may realistically be accomplished. And then individualize to each example and first "experiment" the cleaning with the lowest end example.
In general my approach to the kind of examples you are talking about is to first get the loose mud and grit out, swooshing the example in a bucket of water. Then soak it in soapy water with a small amount of bleach for at least a day. The dilute bleach gets rid of the organics and does not seem to hurt most crystals. I personally do not use ammonia. After a thorough rinsing, I then use a hi pressure cleaning gun, start gently and rinse constantly throughout the hi pressure cleaning. Reassess the specimen and only then, if there seems to be removable iron staining, resoak it for a bit in iron out; as short a soaking as will get much of the rusty iron stains off the example. Finally then either a second hi pressure water cleaning followed by a rinse or just a followup thorough rinse without the second hi pressure treatment if felt to be unnecessary at this point.
I do not think these treatments will harm either intact calcites or Baryte. The only caveat is to individualize the cleaning for each individual specimen. Incipient cleavages in either calcite or Baryte might result in crystal breakage when any physical pressure is put on these crystals. Successful cleaning is not only a science, but an art!
Hope this is of some general help! BOB

[alfredo]

For really tough jobs, when ammonia doesn't work, I use a solution of NaOH or KOH, aka "caustic soda" etc. That will nuke any organic material.... grease, lichens, algae, hair...

PS: There are a few minerals that are destroyed by NaOH: some phosphates and arsenates (eg: vivianite, scorodite...), realgar, orpiment... but most common species should be safe.

[Pete Richards]

[Ivan Blanco (PDM)]