Arsenic -Darkening over time

Here is an example where As2O3 octahedrons are visible. Arsenic from Lengenbach. FOV 0,8 mm.

Cheers Stephan

Stephan,

Over what time period did these octahedra form?

David K Joyce

I purchased an old Schneeberg native arsenic specimen when I was a kid. A few years ago, I put the piece in an ultrasonic cleaner, and when it came out I noticed it was much brighter. My first thoughts were that there must have been some arsenic oxides on the piece and I had blithely dipped my hand in the cleaning water to retrieve the specimen. I washed my hands thoroughly and hoped I had not been poisoned. I just checked and the piece is back to being dark.

Bob



Edited 1 time(s). Last edit at 03/24/2012 07:06PM by Robert Meyer.

David,

I aquired this piece in a virgin status silver grey and shiny appr. 20 years ago. The process seems being finished months after.
Since 19 years or more no visible difference.

Stephan

All,

I can understand a surficial "tarnish". I do not understand how three dimensinal arsenic oxide crystals of that size can form on arsenic in air, as on Stephan's specimen.. Does arsenic have a "vapour pressure"? Are arsenic molecules "given off' into the atmosphere to react with oxygen? How does this reaction happen, mechanically? How do the molecules of arsenic migrate to build the arsenic oxide crystals? Inquiring minds must know! (Mine must, anyway)

David K. Joyce

Arsenolite is colorless. Perhaps the fine grained texture of the alteration film produces a "false" black color.

Upon reflection, I think that the black sublimate I observed on my glass retort tube during my dangerous mineral processing experiment was most likely just sublimed metallic arsenic.

I think the following should be located somewhere else, but to the Northeast of the ASARCO arsenic smelter were some beautiful homes with gorgeous views of Puget Sound. Most of the somewhat elderly homeowners grew tomatoes in their gardens.

I sampled their yards and had little difficulty in recovering euhedral arsenolite crystals from their lawns.

I would have bought one of those homes in half a second. And eaten the hormehtic tomatoes with gusto.

Bart

Hi Bart, et al,

I can see why there might be arsenolite octahedra in their lawns near the ASARCO smelter. There was arsenic floating around in the air! My wonderment at the above specimen is why arsenolite crystals would form on native arsenic in a quiescent air environment. The arsenic molecules must either be floating off of the arsenic to react with oxygen in the air or crawl up the nucleating arsenic oxide crystal to enable the larger three dimensional crystal to grow. I've often wondered the same about other "cabinet growths" such as silver sulphide on native silver. Anyone understand this mechanism? How does the reactive silver "get off" of the element in question to form crystals?

David K. Joyce

In the case of wires growing from sulphides, they seem to get material added from the bottom, at the contact, pushed up by the roots, so to speak.

In the case of arsenolite on arsenic, just a guess, in humid climates there may intermittently be very thin films of moisture condensed on the metal, perhaps diurnally. You will notice this whenever you look at a smooth metallic crystal face under the microscope on a cold morning - water droplets condensing on the crystal from the breath of the observer. As arsenolite is slightly water soluble, the water film may provide a medium for mobilization? I guess one doesn't even need the "humid climate" part; there is condensation of humidity even in the Atacama.

Damn you, Alfredo! You've come up with a perfectly workable, logical, simple reason for the arsenolite crystal formation! I guess the air environment around such a specimen is not always as quiescent as we think or even just simple, stable air. It is , in fact, dynamic and, at times, conducive to enabling chemical reactions. (Thus careful atmospheric control in museums and galleries?!) Thanks!

See you in Rochester!

David K. Joyce

Growth of sulphides at ambient temperature: The cation diffusion of Ag+ and Cu+ ions within and on the surface of sulphides is fairly quick; just check out scientific literature on Ag- and Cu-sulphides for example.
In other cases air may also contain molecules that can act as catalysts.

Uwe, can you give some examples of "air may also contain molecules that can act as catalysts". If that were true, I'd be rich and wouldn't be spending so much time working on PGM-based catalysts

Air may contain H2S, CO, NH3, O3, H2SO4, H2CO3, HNO3, and small hydrocarbons in low concentrations, as well as O2, CO2 and H2O - all of these can be reactive if activated by a catalyst or can react homogeneously under some circumstances at ambient conditions. The minerals themselves are the catalysts, as Ag, Cu, Fe, Cr, etc... are all well known to be catalytically active, occasionally at room temperatures. So in essence, the minerals are auto-catalytically destroying themselves!



Edited 1 time(s). Last edit at 03/30/2012 05:51PM by Jeff Weissman.

I can't give you examples offhand (would have to make a detailed literature search myself).
Maybe halogen-bearing gas species? People have probably made experiments comparing (auto-?)catalytic reactions in normal air, air with impurity gas species and vacuum.

I think that most of us have seen museum drawer acanthite fuzz on our silver bearing species.

In fact, sometimes we don't even realize that a specimen contains silver minerals until we notice the fuzz at a later time.

I think this topic has been addressed rather extensively elsewhere on Mindat.

Bart

Yes see this thread: [www.mindat.org]

Hello!

I got this sample of native arsenic from Port Alberni, BC, from David in April. However, it took me three attempts to produce a polished section. The arsenic polishes like a mirror - no other ore or gangue minerals are visible in reflected ligth. SEM, however, will tell a different story. The polished surface tarnishes rather quickly, but very selectively. The tarnish can be simple removed with a paper towel, and the tarnish itself is nearly white. In reflectetd light, some nice As2O3 crystals can be seen on the tarnished surface.

Before I will put it in the SEM, I will remove the tarnish with a paper towel and alcohol, no really repolishing, and will check for mineralogy and composition along the scratch line. If the surface is not clean enough, I will repolish or even regrind it. This is also to check if the tarnishing pattern is reproduceable on a fresh surface.

Any comment or suggestions welcome!

Franz Bernhard

Thank you Franz! We are looking forward to what compositional variances you find with the SEM work.

David K. Joyce

Here are some SEM-results on the native arsenic sample from Port Alberni:

The tarnish could not be removed completely with a paper towel, so I had to repolish the sample. Even after prolonged polishing, the differential tarnishing was still visible as differential etching - the more tarnished parts more etched then the less tarnished parts. Despite this, I put the sample in the SEM.

Results: I could not find any microinclusions or zoning on BSE images, there is still only arsenic there. I made analyses of four spots in most etched (tarnished) parts (between the etch pits) and three spots in least etched (tarnished) parts. The only detectable element beside As was Sb (EDS detection limits around 0.1 wt% 1s).
The four analyses in the most etched / tarnished parts yielded 1.10 +/- 0.12 (1s) wt% Sb.
The three analyses in the least etched / tarnished parts yielded 1.31 +/- 0.09 (1s) wt% Sb.
Note, that the individual counting error on each spot is also around 0.11 (1s) wt%.

Conclusions: There is still no definitive answer, why there is differential tarnishing.

Further work will include:
- Slight regrinding followed by polishing of the sample to remove the etching completely.
- Investigation of grain size, twining etc of most and least tarnished parts by reflected light microscopy.
- Automatic analyses of 50-150 spots parallel to the scratch line (which can be seen on the scan in my previous post) on the freshly ground and polished surface. Number of spots, however, will depend on available SEM-time, which is very limited for me, and there are many other tasks. It may take another few months to do this analyses.

Any suggestions or comments are welcome!
Franz Bernhard

Thanks so much Franz. This isfacinating. So the Sb is hindering the tarnishing. Chemical attacks should begin in areas of high stress. Perhaps from the higher concentration Sb has diffused into such regions and somehow relieved the stress?

I'm a little surprised at your reflected light images, posted two messages above, The darkest bit in the central images gives the lightest image on the left and the lightest in the centre image gives the darkest image on the right. If I'm reading the 100 micron bars correctly the field of view is about half a mm, so with mm bands in the central image, the left should be dark and the right light? Is there a hint of epitaxi in the arsenolite triangles?

Dear Franz,

How interesting! I was wondering about grain size and subsequent reactivity, as well. We appreciate your efforts to gain an insight into this phenomenon and look forward to what you learn.

David K. Joyce

Rob,

thank you very much for your comments.

There COULD be subtle differences in Sb-content within the sample, but there is no statistical significant difference yet. Therefore I will measure a line of points accros the banding to have more data points.

Left and right image were made with an reflected light (ore) microscope. The middle image was made with a flat bed scanner. A freshly polished metallic surface will appear black on a scanner - try it! The tarnishing "breaks" the polish and the surface appears brighter. The more tarnished, the brighter - at least the image from a flat bed scanner. The micron bars are correct, left and right image also.

Concerning epitaxie, I would have to determine the orientation of the arsenic crystalites also - this would be a very difficult, if not impossible task. But I will see, if I can see something!

Franz Bernhard