Arsenic -Darkening over time

Hi All,

A year or so ago, I was trimming native arsenic specimens from the Port Alberni, British Columbia location and I noticed afterwards that, after exposing a nice fresh metallic lustre and colour, the trimmed areas darkened over time. They darkened differentially. Why do the specimens darken in a banded manner? I'm sure that this is not a new phenomenon in mineralogy but I was intrigued. As a result, I captured this series of nine images over time to produce a "time exposure" of darkening/tarnishing arsenic. Comments/thoughts are welcome!

Here are three more!

And the final three images!

Maybe you think that this is exciting as "watching grass grow" or "watching bacon fry" but I found it intriguing. Thoughts?

David K. Joyce

A few % Sb (single digits) seems to keep arsenic looking bright and untarnished; pure native As can tarnish dark grey in a few hours even. So perhaps you're looking at variations in purity/Sb content?

Very interesting. It would be interesting to do an analysis across the specimen before and after to see if it could be figured out what causes the darkening over time to the various portions of the specimen.

Rock Currier
Crystals not pistols.

Intriguing. What is the composition of the tarnish? The oxides are colourless, tarnishing is accelerated with humidity. Sulphide?
Regards
Rog

The tarnish dissolves rapidly in alkaline solutions like NaOH in water, so I'm assuming it's an As oxide, perhaps mixed with microscopic particles of metallic As, which would make it black, but that's just a guess on my part; no analyses.

You describe the specimen as 'native Arsenic'. By which do I assume correctly that's 'elemental Arsenic'?

The pics speak to me of a 'rock', i.e. a mix of minerals (banded structure). Fair or not?

Darkening (tarnishing) is a property common to many metals in their elemental state. Its cause is a chemical reaction between the element and 'air' which is often augmented by the presence of water vapour. 'Air' is of course a mix of gases that includes a variable range of 'pollutants', principally (for present purposes) SO₂st (forming H₂SO₃ with any water present and NO₂ which will form H₂SO₃. Both of these compounds are strong acids that react readily with most metals at room temperature. Then of course there is simple oxydisation with the O₂ in the air. Also CO₂ a larger constituent of most air, forms a weak acid with water.

It seems likely that your pieces capture some mix of such reactions over time with the native arsenic in your rock. One can't tell much from the speed of the tarnishing, since it might depend as much on the makeup of the air and its temperature as it might on the makeup of the rock.

However, what must relate to the rock alone is that there is a differential rate of tarnishing in different (layered) parts of your specimen.

I'd *guess* that the differential tarnishing results from differing percentages of elemental Arsenic being present in the different layes of your specimen. But this is simple conjecture. Only an analyses of the different layes in your sample can put any 'beef' behind this conjecture. But, was I a betting man.....

Owen,

Amongst mineralogists, mineral collectors and geologists, a traditional way of describing a naturally occurring element is to call it "native". as in: native copper, native arsenic, native silver, etc. This distinguishes it from a manufactured or processed element. It could also be called "naturally occurring" or something else but "native" is the traditional way.

The specimen is, mostly the mineral arsenic. A rock is a mixture of various minerals. This is not a "rock" but a portion of a vein, a special type of "intrusive", thus the banded structure. I think there is a difference. I don't think most geologist would call a vein, a rock, most of the time. A vein is a special aggregate of minerals that occurs, intruded into a rock mass. A "vein" or intrusive of rock is usually called a dyke(dike), sill, or other type of rock intrusive.

I concur with your chemistry comments and the possibility that one or more of those caused this darkening. Very well put. It did ocurr to me that there are many components of air, particularly in proximity to other mineral specimens (some of which "give off" molecules due to slow decomposition) which could cause reactions.

This particular specimen is available if anyone with the capability is inclined to analyse the surface of it. Could be interesting and, with the right equipment, probably fairly easy! Any takers?

Roger, thank you very much for the real "time-lapse" sequence of images. I didn't know that was possible in this venue!

Alfredo, thanks, as well, for your comments.

Any other thoughts, folks?

David K. Joyce

David,

Thanks for helping an old man over a stile

The underlying conundrum is this, it seems to me. All parts of the specimen that are pure Arsenic should tarnish art the same rate when exposed over time to the same mix of gases (and light?). In the case of this specimen this does not happen. It seems to me that the most probable explanation for this is that the specimen is not pure Arsenic but is some differential mix or chemical compounding, as broadly deliniated by the differntly tarnished areas. I'm pushed to think of any alternative explanation.

Anyway, it's a neat experiment with an interesting result.

Owen,

Agreed! The experiment isn't over until we know the various phases involved. Hopefully, that will happen some day!

David K. joyce

Pure arsenic slowly oxidises in air, especially if it's fine-grained. At high mag, one can see the tiny sharp octahedra of As2O3.

Great record of tarnishing in action! Thanks Dave for taking the time to do that.

Uwe,

When you say "high magnification" do you mean SEM-type magnification? I'd love to see the products of arsenic tarnishing up close. Do you have any images that you can show us?

David K Joyce

Hi David,
Here's a larger version which shows the differential tarnishing better. Animated gif made online free www.gickr.com

Uwe, if your observations were done optically rather than SEM, were the oxide crystals colourless? I think the first thing to pin down is the actual nature of the tarnish. Why is it black? Is Alfredo right?
Thanks for doing the experiment David,
Regards,
Rog

Thanks Dave for a very interesting sequence. I agree with Alfredo about a little Sb making it silvery. What caught my eye was two oxidation products- one brown and one black. Ultimately the black triumphed, but the brown reminds me of the scuzz on Domeykites and Algodinites. I think Arsenates are involved but the xl's small size still defeats XRD.

David: "high magnification": depends on the crystal size, the longer one waits, the larger the crystals.
If one is lucky, sharp colourless octahedra can be observed already at 40x. In other cases (more or less freshly broken arsenic surfaces), a SEM is necessary.

A similar phenomena was observed in native arsenic from the Kusa Mine near Bau, Sarawak, Borneo. A great deal of material was obtained many years ago by Forrest Cureton during a collecting trip there in search of sarabauite, most of the specimens passing to Excalibur when the Cureton inventory and business was purchased in the early 1990s. I recollect that Forrest and I unwrapped "fresh" material from his Tucson warehouse in preparation for our joint Tucson room at the Executive Inn, and brilliant, silver botryoids of native arsenic turned "pyrrhotite brown" in a matter of hours, and ultimately a dull greyish after longer exposure to air.

In looking at those specimens today, they are uniformly grey, even those with considerably intergrown stibnite, but breaking a sample exposes the silvery metallic luster on the fresh surface, only to darken rapidly with prolonged exposure. I haven't put these in the SEM to see what is there, but now that's another project for the "to do" list!

Thanks, Tony,

If you want to help complete the "loop" on the above discussion, the specimen that I featured above would be available for analysis.

Let me know?

David K. Joyce.

Washington State has a notable native arsenic location. It is in sandstone right in the bed of the Green River just above the Green River Gorge.

A few miles downstream are the famous outcrops which produced what I consider the source of the World's best realgar crystals. Not the largest, mind you, but the best. Cinnabar and metacinnabar are associted.

The arsenic occurs as rich veins and as little "BBs" included in white kaolinite. When I found the arsenic as a youngster, I presumed that I had discovered metacinnabar and proceeded to retort the mercury from some rich "ore". No mercury. Just arsenic oxide sublimate which was dark in color on the lining of the glass tubing.

I think the banding in Dave's sample is derived from the mixed texture of the arsenic and the host minerals. A quick polished section would answer that question. Note that Napolean and his interior decorators died of arsenic poisioning. Napolean loved arsenic green wallpaper.

The Washington arsenics quickly develop a black surface. As the leading proponent on Mindat of bacteria as the culprit behind mineral alteration, I suggest that the newly discovered class of arsenic metabolizing bacteria is lurking on the specimens.

Experiment with bacteriacides.

Pure arsenic crystals from Aldrich turn black within days of opening their containers.

If I were even more perverse than I am, I would notify the EPA that our precious Green River is in need of a Superfund clean up, and I would then volunteer to remove the arsenic species at no cost to the public. But of course, they don't work that way.

~Bart