Mineralogy question - the weirdest thing happened...

[Quaxo76]

Hello,

This is my first post here... I've been a lurker for quite some time, but now the time has come to ask my first question! I know this is not a mineralogy forum, but a really strange thing happened and that's definitely something chemistry-related so I thought you might know! It's something I had never heard about before, and Googling about it didn't return anything.

So, basically, I have a small but quite varied minerals collection. I keep all my minerals in medium-sized transparent plastic boxes; usually 2 to 5 minerals are placed into a single box.

In one of these boxes, there was a large piece of what was told me to be antimonite (Sb2S3) together with a large crystal of pyrite (FeS2). The rocks were not touching, but close to each other (there was a gap of maybe 1cm). They were left to sit there for maybe 4-5 years (many boxes are stacked on top of each other, so it's not easy/convenient to move them often; and besides, they're sealed so no dust gets inside, so there's no need to clean/dust the minerals). But today I was moving things around (to test my specimens for radioactivity) and I noticed the weirdest thing... On an area of approx. 4cm^2 of the pyrite crystal, on the side near the antimonite rock... well... lots of medium-sized crystals of antimonite have grown. What is going on here? Is this normal and maybe expected or what? Are Sb atoms "flying" from antimonite to pyrite to combine with the sulfur there? And if so, why does this happen, and what happens to the iron atoms? Is there a cloud of free Sb atoms flying around inside that box?

 

Please forgive me if this is a stupid question but I couldn't find any info anywhere. I can try to post macro photos if needed.

Thank you in advance for any information!

 

Cristian

[hermanntrude]

i'd love to see the photos. I would assume that a displacement reaction is occuring. It makes sense that antimony would displace the iron is pyrites, since antimony is more reactive than iron.

 

As for how they got from one sample to the other I can give you two pieces of info:

 

1) solids DO have a vapor pressure, although ionic ones tend to have extremely low (negligible, usually). Some solids have unusually high ones because of the formation of small covalent units... perhaps this happens in antimonite... i dont know

2) if antimonite is water soluble to any extent then high humidity would allow some of the antimonite to traverse the gap via being dissolved in water vapor or droplets.

 

Also, of course, in the chemical world, 3-4 years is an extraordinarily long time. Most reactions occur in minutes or seconds, some even happen in picoseconds, so unlikely things can occur on such a long time scale

[iNow]

Also, of course, in the chemical world, 3-4 years is an extraordinarily long time. Most reactions occur in minutes or seconds, some even happen in picoseconds, so unlikely things can occur on such a long time scale

 

Even with crystal growth? I thought that took a while...

[Quaxo76]

Hello,

Thank you for your reply! I know that several years is a long time, but I had no idea that natural minerals could sublimate so quickly - after all that rock must have formed ages ago... But on a more careful check, I saw that it's "degenerating": it now has cracks it didn't have before, and it lost some "crumbs".

The environment isn't too humid, as it's stored in my room, in a closed box.

But I forgot to add something, I wonder if you can make something out of it: in the box there's a third specimen. It's very small. It's a 1.5 cm long electrolytic crystal of copper. I made it from a solution of CuSO4, and it's been in the box with the other specimens for all the time. It's quite farther away from antimonite than pyrite. And instead of being red (or brownish or green, like I would have expected after years) it has the top part of the top crystals (and only those parts) turned pitch black. The rest of the specimen is brown-red as expected.

On this copper crystal, I can't see any acicular crystal like I can see on the pyrite sample. What could that be? Copper oxide? Antimony? Iron? I really wonder... I would like to be able to speed up the process and set up some controlled environment experiments...

 

As for the photos: now it's night here, I'll take some tomorrow morning with the sunlight and post them ASAP. What size photos can be attached here?

 

Regards,

Cristian

[hermanntrude]

the black is probably copper impurities. copper forms sulfides, although i'm not sure what colour they are in bulk.

 

You might be right that it's a very fast process... my statements above are just guesses, really, but experienced ones. perhaps they're not sublimating but i think that's the most likely explanation, either through gas phase or via transfer in water. perhaps there was some condensation in there

 

photos work best at about 640x480 i think. I usually get my image hosting site to resize them when i upload them

[Quaxo76]

Here are the photos! In the first, you can see the whole box, and you can see how the rocks were laid out. You can also see the copper crystal, with the blackened tips.

The black over the copper crystal isn't hard, like an oxide layer: it looks like soot, and sticks to the fingers when you touch it. I had never seen copper do anything like that before!

All this thing keeps looking weirder and weirder...

 

Cristian

[hermanntrude]

beautiful photos. I think this is an excellent demonstration of the activity series.

 

My guess is that antimony is crystallising out on the copper and the pyrites. Someone who's better at inorganic chemistry could tell you for sure

[YT2095]

absolutely Fascinating!

I`d love to take a look at these for real, esp under a microscope, I can`t really offer any explanation beyond speculation either.

it looks interesting how the new formed crystals are swept back though! it suggest some sort of force acting upon the growth, as if it`s trying to get away from it`s source?

in a box it`s unlikely to be an air current, maybe a magnetic or electrical influence? (lodestone nearby?).

it would be interesting to set another one up but with a piece of Paper between them, and see what happens then.

are you 100% Sure that they weren`t touching? and did you notice any "strange" smell when you opened the box (sulphurous)?

 

I do a lot of Long-Term experiments myself (I think it appeals to my stubborn streak) that can and do last for Years (some over a Decade), I think I may try this for myself if I can get hold of some antimonite.

 

as for the copper, I don`t think it has played any part in this reaction, although it Is obviously oxidised, and may have contained trapped water (because of how it was made) that contributed to the overall humidity in the box.

[Quaxo76]

The box has absolutely no air circulation (the lid has a tight fit). I can't rule out magnetic fields totally, but I tried "scanning" the area with a sensitive compass, and it didn't move. Same for electric fields: I can't rule them out, but there are no electric appliances or hidden wires nearby.

I also tested for radioactivity (just in case) but the only slightly active rock nearby is a piece of lava, but it's really mild.

The samples shelf though is a couple of feet over a radiator. So, in the winters, there has been some thermal cycling.

 

I don't know if the copper is simply oxidized. I've seen a lot of oxidized copper, and usually the oxide "sticks" to the copper. Here instead, it's really like black soot, if you touch it ever so gently the "soot" sticks to the finger, and the copper below is exposed (and it's shiny).

 

Do you think the new crystals are antimonite, or pure elemental antimony? The photos don't show it clearly, but it has areas that look pitch black and glassy/oily. Fascinating stuff... I think I'll try to find some more samples and set up a few long-term boxes with varying conditions...

[hermanntrude]

i suspect the crystals are antimony. that'd be what to expect if there was a displacement reaction going on. I have some antimony in my lab and it's very crystalline.

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good luck with your experiments

[Quaxo76]

So you think that the antimonite's molecules are dissociating? But why? What's a "displacement" exactly? And why does then the atomic element only crystallize in a single part of the pyrite? And what happens to the sulfur, does it remain on the surface of the antimonite?

Whew... lots of questions...

[hermanntrude]

OK i see what u mean.

 

I didn't think this through very carefully. Obviously it can't be a displacement because that'd require a native metal,. rather than iron sulfide... although now i've noticed that i can propose two new hypotheses:

 

1) perhaps the "native metal" is copper. Perhaps antimony is crystallising out and sulfur is reacting with the copper... i think this is unlikely, though since it'd require more "jumps" from sample to sample

2) perhaps what's happening is what's known as a double displacement reaction. In a normal double displacement reaction the ions would swap with each other. in this case, though, the anions are identical, since they're both sulfides. However, it does mean that the cations switch positions so "antimonite" (which is usually referred to as stibnite) is forming on the pyrites. I'm not sure if you should expect to see any pyrites forming on the stibnite, though... and i'm still not sure about the growth on the copper... perhaps that's some sulfur leftover from a reaction?

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update:

 

I looked up the vapor pressures of antimony and sulfur and i think it's incredibly unlikely that antimony is going anywhere fast. it has an ultra-low vapor pressure. however, sulfur volatilises easily from many minerals. so whatever that black crystal on the pyrites is, it's formed probably from sulphur and/or iron. there are a few non-stoichiometric iron sulfides but i dont know what they look like. perhaps it's one of those

[Quaxo76]

Hmm, so you think the black stuff on the pyrite is made of iron and sulfur... so, would some sulfur have left the pyrite leaving the black stuff behind? But that's so strange, as the new stuff on the pyrite looks *exactly* like the antimonite! Same color, same crystal shape, same "glossy" look...

OR there's another possibility: I'm not an mineralogy expert, and though those minerals look very much like antimonite and pyrite, there's a chance that the sellers were (unwillingly?) wrong, and they're totally different things... Is there any way to check for sure? Or even to check if the stuff on the pyrite is the same stuff of the other sample?

[hermanntrude]

pyrites is fairly distinctive. it also smells of sulfur because sulfur slowly evaporates from it. I dont think u need to doubt the nature of your pyrites. As for the antimonite it seems ok but you;d need to do some more work to be sure.

 

However i dont think it's time to doubt your mineral's identity yet. I think we just havent figured it out right yet. One possibility is that antimonite itself is vaporising. some ionic solids can vaporise in small covalent chunks which gives them a surprisingly high vapor pressure compared to their constituent elements. If that happened, then it might crystallise on the pyrites for some reason...

[Quaxo76]

OK, I managed to find 8 small (3cm) crystals of pyrite. All of them are nice, shiny and without any "extraneous" marking. And cheap, at about $1 each.

I have been less lucky in locating antimonite. I only found a small crystal, same size as the pyrites, for $20. But I bought two crystals of galena (PbS). So I setup 2 small sealed boxes, one with galena and pyrite, and one with pyrite and antimonite. I would like to find other antimonite crystals, to experiment with adding a small piece of metallic copper, and varying humidity and temperature in the boxes... We'll see what happens. Too bad it'll be a "long-term" experiment! But I REEEALLY would like to know what's happening here...

 

Cristian