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EducationWhat happened here?

7th May 2017 17:20 UTCEddy Vervloet Manager

I have seen a lot of things happen in my collection over the many years, but this is rather new for me!

It is a Romerite specimen from the Clara mine, Black Forest, Germany. I haven't looked at it for maybe 7-8 years and now this.

It wants to escape from the box!

So what happened here? I know these sulphates are not the most stable stuff, but can someone shed some light on the events here please? Did the water in the molecule make it flow?? It was stored under normal circumstances, in a Joussi box, as you can see... Any ideas?

Thanks,

7th May 2017 20:16 UTCReiner Mielke Expert

The sample is hydroscopic and takes water out of the air and basically dissolves itself. A common problem with may sulphates and some chlorides. The only way to prevent this is to seal it in an airtight container or coat it with something to keep the air off it.

7th May 2017 22:40 UTCSusan Robinson

Does silica gel help keep it dry enough, if put in a tray or box with a cover?

8th May 2017 00:31 UTCKeith Compton 🌟 Manager

Eddy,

As Reiner said, it soaks up the moisture in the air so you have to keep it locked up tight!!

Susan,

The gel may help a bit if it is also in an airtight container. But it may also depend on where you live and how humid. Those things like desert conditions !!

And sometimes the specimens of the same mineral may react differently in the same environment. I have a couple of Hanksite specimens (same locality) at home and have had them for many years. Both kept in airtight containers, one - has remained unchanged in 20 years, no coatings or preservative action taken, other than in a container. The other, also in a container needs to be re-oiled annually. I simply put it down to impurities etc.

Cheers

Keith

8th May 2017 00:59 UTCAlfredo Petrov Manager

Roemerite is one of the more stable Fe sulphates (not completely stable, but better than the others). Perhaps it was mis-IDed and was a different Fe sulphate? Or else you left it under a hot roof in summer?

8th May 2017 01:10 UTCReiner Mielke Expert

Hello Alfredo,

I have had trouble with roemerite as well but I can't say it was the roemerite itself it may have been something in the matrix.

8th May 2017 03:29 UTCAlfredo Petrov Manager

I may of course be completely wrong here, Reiner, but looking at the high water content of roemerite, I suspect that it would suffer more from dehydration than from humid air, so dessicants like silica gel might just be speeding up its demise? I should follow my own motto of experimentation rather than speculation ;))

In general, Fe(II)-bearing sulphates generate sulphuric acid when the Fe oxidises to Fe(III), which is what makes the Fe(II) sulphates so much more unstable than the Fe(III) sulphates like coquimbite. Roemerite has (relatively) little Fe(II), but nevertheless i suppose some oxidation could be a problem. Has anyone tried keeping Fe sulphates in humid oxygen-poor atmospheres? (sealed in a jar together with some damp steel wool should do it)

8th May 2017 05:03 UTCKeith Compton 🌟 Manager

Hi

I can only comment that my only coquimbite crumbled to dust and I presumed that it was due to absorbing moisture .. perhaps I was wrong.

Either way it "died" a slow crumbly death.

In a Mineralogical Record article on The Sulfur Hole in San Bernardino Co - in Axis, Vol 1, No.5 (2005) the following quote is pertinant in relation to roemerite and other sulphates found there:

"Exposure to either low or high humidity will allow the minerals found in the more acidic footwall zone (römerite, voltaite, melanterite) to literally dissolve in their own water of crystallization or crumble into a fine-grained powder. In extreme cases storing the specimens with desiccants has proved successful and coating the surface with acrylic resins has also proved successful, although doing so will change the luster of the crystal faces. Under normal conditions, storage in a sealed plastic box will suffice,but the survivability of the acidic footwall minerals is best guaranteed in a sealed glass jar. "

Cheers

Keith

8th May 2017 05:31 UTCSteve Hardinger 🌟 Expert

Perhaps there exists here a confusion between waters of hydration (water that is an integral part of the molecular structure and chemical formula) versus deliquescence (the continued absorption of atmospheric moisture to the point of solution). See: https://en.wikipedia.org/wiki/Hygroscopy

For example calcium chloride (CaCl2) can form a hexahydrate (CaCl2-6H2O), and this hexahydrate continues to absorb water from the air until it is complete dissolved. Decades ago, there existed a commercial product consisting of solid calcium chloride in a can, which you opened and placed in whatever space you wanted to dehydrate. When the can was full of liquid, you discarded it. (This was the course of cheap calcium chloride for my basement chemistry lab.)

So its possible that some iron salts might absorb atmospheric water to the point of deliquescence. I believe FeCl3 might be one of these salts, so no good common-sense reason why an iron sulfate mineral might also be deliquescent.

8th May 2017 15:02 UTCEddy Vervloet Manager

Thanks for your replies all,

I was and am not very familiar with romerite as a species. I must admit I added it to my Clara collection as an insignificant sample because I did not have one yet. So I cannot comment on the mis ID suggestion. Alfredo, it might have gone up to 30 degrees celcius at one point, but surely not more than that. I am just intrigued by the way it looks now, as if it flowed instead of crumbled... and it is not like I live in the tropics, we have a dry continental climate here. I am not saying a joussi box is air tight, but still!

8th May 2017 15:07 UTCEddy Vervloet Manager

One more thing occurred to me. Steve, thanks for the lesson on deliquesence (word of the day...!). I am trying to picture it. So ok, the sample takes on so much water that it becomes dissolved and fluent? That seems to have happened with my sample indeed. But now it is dry! So afterworths, can the water evaporate again? Maybe a change in temperature, as Alfredo suggested???

8th May 2017 15:09 UTCEddy Vervloet Manager

Should I try putting the box in the sun this summer and see what happens? Like Alfredo says, experimentation rather than speculation! ;)

8th May 2017 16:03 UTCReiner Mielke Expert

Seems to me that you could have a situation where you have two species on the same sample, one that dehydrates and one that deliquesces in which case you would just want a stable environment that replicates the environment in which it was found. Just sealing it in an air tight container should create that environment but not until after some deterioration as it equilibriates. The best thing I have found to stabilize things is to liberally coat the sample with mineral oil and then seal it in a plastic container. If you have to, the oil can easily be removed with an organic solvent. I failed to do this with the Roemerite I had because I though it was stable. Fortunately I noticed deterioration before it got too bad and now it is coated with oil and has stabilized. If the container is not air tight the oil will eventually evaporate and you will need to reapply. For example if you use perky boxes which are not airtight. I have found that for very unstable minerals petroleum jelly lasts much longer but only works for robust crystals. The advantage of mineral oil over petroleum jelly is that it is not noticeable under the microscope because it drains off and leaves behind only a thin coating. Note: some minerals ( not many) change color when using mineral oil; for example: sturmanite turns dark brown.

8th May 2017 16:17 UTCAlfredo Petrov Manager

Cold helps stability of Fe sulphates too. After doing the oiling and putting them in a sealed container as Reiner described, store that container in the fridge. Every systematic collector needs a sub-collection in the fridge, unless you want to make a lot of "Used to be XYZite" labels.

I collected some wonderfully gemmy green ~1cm melanterite crystals in Oruro once, a species that usually doesn't survive more than a few hours. But sealed in black plastic, in the refrigerator, the crystals survived about 5 years, taken out occasionally for a few seconds viewing, which is pretty good for crystals that normally decompose the same day you collect them. Most other Fe sulphates are much hardier than melanterite.

17th May 2017 20:24 UTCUwe Kolitsch Manager

Roemerite from the Clara mine is usually associated with other other metal/Fe sulphates that grew on a matrix containing unstable marcasite/pyrite. Usually, in central Europe, these things just grow and grow until they fill out the whole micromount box.

I have never seen them "flowing away" and suspect at one point the humidity was very high.

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