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Author: Subject: Arsenic from Skutterudite
blogfast25
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[*] posted on 6-1-2014 at 06:07


I would read up on the dangers of As and As2O3 and would keep the rock under lock and key (display cabinet), so no humans or other animals can get their paws on it.

[Edited on 6-1-2014 by blogfast25]




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elementcollector1
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[*] posted on 6-1-2014 at 12:00


I've gotten powder of the stuff on my fingers due to carelessness, and experienced no symptoms. Still, with arsenic, you can never be too careful.




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[*] posted on 6-1-2014 at 13:48


Quote: Originally posted by elementcollector1  
I've gotten powder of the stuff on my fingers due to carelessness, and experienced no symptoms. Still, with arsenic, you can never be too careful.


I must say, I would probably be defecating myself after such an experience :D

I would think however, that if Skutterudite was largely toxic there would be a considerable amount more literature to found online regarding such toxicity and relating concerns. Perhaps the minerals crystal structure itself helps to reduce the Arsenic toxicity by 'isolating' it within its crystal structure.

Wikipedia states "The unit cell can be considered to consist of eight smaller cubes made up of the Co atoms. Six of these cubes are filled with (almost) square planar rings of As, each of which is oriented parallel to one of the unit cell edges.... The As atoms then form octahedra with Co in the centre." So perhaps the outside structure of Co, Fe, or Ni helps isolate the As. I added some 'commentary' to the Wikipedia picture, as I think it may help to explain the structure of Skutterudite somewhat better, see below.



Please note that this diagram is just my interpretation of the above statement, and please correct me if it is wrong, as I am not a mineralogist by any means.

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[*] posted on 6-1-2014 at 13:53


I'm sure its low solubility keeps its toxicity low (although I wouldn't want to leave it sitting on my hands for any length of time).



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[*] posted on 6-1-2014 at 14:10


Quote: Originally posted by BobD1001  
I would think however, that if Skutterudite was largely toxic there would be a considerable amount more literature to found online regarding such toxicity and relating concerns.
There is a huge amount of literature online regarding this toxicity, but most is not specific to just one arsenic-bearing mineral. See: <a href="https://en.wikipedia.org/wiki/Arsenic_contamination_of_groundwater" target="_blank">Arsenic contamination of groundwater</a> <img src="../scipics/_wiki.png" />



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[*] posted on 6-1-2014 at 14:28


Quote: Originally posted by elementcollector1  
I've gotten powder of the stuff on my fingers due to carelessness, and experienced no symptoms. Still, with arsenic, you can never be too careful.


Wow, EC1, do wear latex gloves at a minimum! :o




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[*] posted on 6-1-2014 at 15:24


Quote: Originally posted by bfesser  
There is a huge amount of literature online regarding this toxicity, but most is not specific to just one arsenic-bearing mineral. See: <a href="https://en.wikipedia.org/wiki/Arsenic_contamination_of_groundwater" target="_blank">Arsenic contamination of groundwater</a> <img src="../scipics/_wiki.png" />


Bfesser, I fail to see where in this link it describes the toxicity of Skutterudite. Of course Arsenic's toxic properties are hugely documented, however, I was specifically referring to this mineral, for which there appears to be a lack of toxicological information.
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[*] posted on 6-1-2014 at 18:04


It doesn't describe the toxicity of skutterudite. As I said, most of the published information is not specific to a particular arsenic-bearing mineral, because in the environment, it rarely matters much which mineral species provide the source for the groundwater contamination, only that they do. Why do you require toxicological information for one particular mineral species? For most minerals, it doesn't make sense to gather such data. Also, a mineral species can contain various impurities and can vary slightly in composition between localities and even specimens.



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[*] posted on 20-2-2014 at 20:44


Update: Took another gram or so and put it in a test tube, filling the test tube up with sulfuric acid and a little water. So far, it's exhibiting the exact same reaction as with the bleach - minor bubbling, some black powder that easily forms a suspension. I am so confused. This shouldn't be cobalt oxide - that should be in solution as cobalt sulfate.

Maybe I do have to try 'cracking' this with sodium nitrate and sodium hydroxide?

Update for the bleach tube: The cobalt oxide is now much larger-grained, coming out of suspension very quickly. All but the coarsest of the skutterudite particles seem to have dissolved - it's hard to tell by color which is CoO and which is skutterudite, but I'm fairly certain.




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[*] posted on 17-4-2014 at 21:49


2 months later, and not much has changed on the test tube scale - in either case. I'm going to see if the ore responds in any way to a blowtorch (in really small quantities!). If yes, I guess I should try sublimation under vacuum...



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[*] posted on 18-4-2014 at 04:31


No joy with the bleach extraction?



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[*] posted on 18-4-2014 at 06:47


Nope. No change at all...



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[*] posted on 18-4-2014 at 14:54


Ok, so I thought up a few possible reactions. Sorry if they don't work.
1A skutterudite+HCl+Na2S2O3
1B skutterudite+H2O2+Na2S2O3
One of these, If they react as I predict, would yield As2S3.
2 skutterudite+HCl+I2
should yield arsenous acid
3 skutterudite+H2O2+HCl
Again this should yield arsenous acid.
Again sorry if I mde mistakes.




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[*] posted on 18-4-2014 at 18:48


H2O2 and HCl is something to try, but keep in mind that this stuff was nigh-immune to concentrated sulfuric acid. Also, what happens to the cobalt in each of these cases?



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[*] posted on 19-4-2014 at 02:52


It theoreticaly would form cobalt chloride in solution.

[Edited on 19-4-2014 by bismuthate]




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[*] posted on 19-4-2014 at 05:03


I wonder if fusing and quenching may help. It's often used on minerals to render them responsive to strong acids, based or other reagents.

By strongly heating (melting, if possible), followed by quenching in ice cold water, the crystalline structure is often disrupted and becomes more 'glassy'. The glassy structure can then be ground up and tends to be more responsive to chemical attack.

Worth trying on a tiny piece, IMHO...




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[*] posted on 19-4-2014 at 07:15


Quote: Originally posted by blogfast25  
I wonder if fusing and quenching may help. It's often used on minerals to render them responsive to strong acids, based or other reagents.

By strongly heating (melting, if possible), followed by quenching in ice cold water, the crystalline structure is often disrupted and becomes more 'glassy'. The glassy structure can then be ground up and tends to be more responsive to chemical attack.

Worth trying on a tiny piece, IMHO...

Unfortunately, all of my stock is a fairly fine powder at this point. Also, wouldn't we want to avoid formation of gaseous arsenic or arsenic oxide under uncontrolled conditions?

Per the cobalt: Is there some complexing agent that can separate it and arsenic?




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[*] posted on 19-4-2014 at 11:52


Quote: Originally posted by elementcollector1  

Per the cobalt: Is there some complexing agent that can separate it and arsenic?


The mystery is that cobalt ad arsenic would normally be sooo easy to separate. So there's something about the ore's unusual structure that's making things hard.




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[*] posted on 19-4-2014 at 13:43


Found this tidbit from... answers.com?
Quote:
Skutterudite fuses on charcoal, giving an arsenic (garlic) odor and forming magnetic ball. A cobalt-rich skutterudite ball dissolves in nitric acid to form a pink solution, though iron and nickel are usually present in sufficient abundance to mask the color. Tests on analyzed samples showed that a nitric acid solution of (cobalt) skutterudite neutralized by ammonia becomes red-violet and a red-violet precipitate settles out. Nickel skutterudite under the same conditions gave a blue-violet solution and a pale green precipitate. Rarer ferroan skutterudite gives a strong brown iron-hydroxide precipitate that will mask either of the other elements.


So... I guess I need some nitric acid then.




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[*] posted on 2-5-2014 at 20:14


Per the post above, I wonder what happened to the arsenic?
Ah well, time to test with a few drops of nitric acid. Going to dilute it, so I don't blow my face off...

EDIT: I was so very wrong about how much to dilute it. The test tube overflowed almost immediately! (I quickly washed my hands, hope for the best!) On the plus side, the supernatant liquid is now a ruddy brown (indicating both presence of iron and a healthy reaction), and there is a fluffly gray precipitate that is so far insoluble (though more nitric acid should determine this). If this precipitate is elemental arsenic, I will be a very happy man.

[Edited on 5-3-2014 by elementcollector1]




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[*] posted on 3-5-2014 at 04:43


EC1:

Your insoluble residue is probably either arsenic or arsenic trioxide.

There's a really simple test for As in solution. It used to be used as a forensic screening test. To test the supernatant liquid for As, dip a piece of thick gauge copper conductor wire in it. Allow to stand overnight. Copper reduces As compounds and the wire will turn black (plated As) if As is present. It does this also with Sb for which I tried it and it works.

If the solution is very strongly acidic you may have to partly neutralise it for this test, or your copper will dissolve.

[Edited on 3-5-2014 by blogfast25]




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[*] posted on 3-5-2014 at 11:57


First, I added a bunch more skutterudite, which once again reacted (proving that there was still acid present). This gave another ruddy orange-brown solution, indicating ferric nitrate. Once this had died down, I attempted to dilute and filter this out, obtaining a yellow, cloudy solution (iron hydroxide precipitating from lower pH?). I added the copper wire whilst filtering, and more of this strange precipitate appeared. The color appears to be anything from off-white to peach, due to the color of the solution. Going to do a control test with no copper wire to determine if that's the cause...

On a side note, added the rest of my nitric acid to the leftover skutterudite particles, where it vigorously reacted (releasing lots of colorful fumes of NO2). The same red-brown solution was obtained, and the insoluble residue on the bottom was dark gray in color. Some insoluble residue appears to be floating, and white in color.

What an odd experiment!




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[*] posted on 3-5-2014 at 12:40


Quote: Originally posted by elementcollector1  
What an odd experiment!


Is your mineral gangue free? Gangue could explain some things.




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[*] posted on 3-5-2014 at 12:45


There was nearly no gangue present, apart from a small green deposit on the surface.



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[*] posted on 3-5-2014 at 12:55


The yellow cloudiness is almost certainly iron (III) hydroxy thingymejibs: not necessarily fully formed Fe(OH)3 but on its way there...



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