More Minerals: Stibnite and Others with a Metallic Sheen

[exchemist]

Another mineral that caught my eye during my visit to the Natural History Museum was a dramatic specimen of stibnite, antimony sulphide, Sb₂S₃. I didn't take a pic of that specimen but here is a similar one:-

This stuff was apparently used in powdered form to make "kohl", a cosmetic used as eyeliner in the Arab world. It seems unclear whether it still is. Antimony, lying as it does beneath arsenic in the periodic table, has a degree of toxicity.

What struck me about this and also the beautiful specimens they had of pyrite (Fool's Gold), FeS₂, is the metallic lustre of both. This is even more apparent with pyrite than with stibnite. I've just had a productive discussion on another forum about how it is that these compounds have this metallic appearance, even though they are not metals.

It turns out both substances are semiconductors. Stibnite has a band gap ~2eV and with pyrite it is even smaller, at ~1eV. This band gap is narrow enough that a small proportion of the electrons can be excited by thermal kinetic energy of the structure, or by incident light, into the conduction band - where they are free to move in response to incident light and therefore cause metallic reflection. Voila!

I even read that pyrite is being looked at for photovoltaic applications, as an alternative to silicon, due to its widespread availability.

I also gained some insight into why these materials, and the "metalloid" elements of the Periodic Table, tend to be semiconductors. I may put that in another post but, doubtless to Mr. G 's irritation ( only kidding), it's all about quantum chemistry and solid state physics. You have been warned.

 

[parmalee]

What struck me about this and also the beautiful specimens they had of pyrite (Fool's Gold), FeS₂, is the metallic lustre of both. This is even more apparent with pyrite than with stibnite. I've just had a productive discussion on another forum about how it is that these compounds have this metallic appearance, even though they are not metals.

It turns out both substances are semiconductors. Stibnite has a band gap ~2eV and with pyrite it is even smaller, at ~1eV. This band gap is narrow enough that a small proportion of the electrons can be excited by thermal kinetic energy of the structure, or by incident light, into the conduction band - where they are free to move in response to incident light and therefore cause metallic reflection. Voila!

I even read that pyrite is being looked at for photovoltaic applications, as an alternative to silicon, due to its widespread availability.

I think that pyrite is, in fact*, already being used in the manufacture of some lithium batteries. But, yeah, it is quite abundant, apparently, so there's some potential potential.

* A preemptive "in fact", given that I haven't actually confirmed this.

 

[exchemist]

I think that pyrite is, in fact*, already being used in the manufacture of some lithium batteries. But, yeah, it is quite abundant, apparently, so there's some potential potential.

* A preemptive "in fact", given that I haven't actually confirmed this.

Would that be to make use of its semiconductor properties, or for another reason? I don't think I've read that semiconductors are needed in these batteries. Perhaps if you can dig up a reference?

 

[parmalee]

Would that be to make use of its semiconductor properties, or for another reason? I don't think I've read that semiconductors are needed in these batteries. Perhaps if you can dig up a reference?

Right now, this is the only thing I can dig up:

A newer commercial use for pyrite is as the cathode material in Energizer brand non-rechargeable lithium metal batteries.[20]

Pyrite - Wikipedia

en.wikipedia.org

Wayback Machine

But that linked document isn't, uh, very explicit or detailed, to say the least. But it does not seem that the semiconductor aspect is pertinent here.

I know that I've come across this before, I just can't recall where. As mentioned in another thread, I spend a lot of time perusing the US Patent and Trademark Office website, and related resources (Why? I honestly couldn't tell you. Patents just fascinate me.), and I'm thinking it was there. But I'm not having a lot of luck locating anything substntive at the moment.

 

[parmalee]

Would that be to make use of its semiconductor properties, or for another reason? I don't think I've read that semiconductors are needed in these batteries. Perhaps if you can dig up a reference?

What I find somewhat interesting here is that these attributes--at least with respect to pyrite--were discovered more than half a century ago. And yet... nothing. Most of the research on it's actual potential seems to all be relatively recent, within the past 10 years or so. Why? It's abundant--and more pertinently, with respect to the US, at least, it's abundant domestically. So why silicon, germanium, etc. and not pyrite?

 

[ольга]

What I find somewhat interesting here is that these attributes--at least with respect to pyrite--were discovered more than half a century ago. And yet... nothing. Most of the research on it's actual potential seems to all be relatively recent, within the past 10 years or so. Why? It's abundant--and more pertinently, with respect to the US, at least, it's abundant domestically. So why silicon, germanium, etc. and not pyrite?

Чтобы под воздействием какого-нибудь излучения замыкания в батарее не вызвал.

 

[exchemist]

Right now, this is the only thing I can dig up:

Pyrite - Wikipedia

en.wikipedia.org

Wayback Machine

But that linked document isn't, uh, very explicit or detailed, to say the least. But it does not seem that the semiconductor aspect is pertinent here.

I know that I've come across this before, I just can't recall where. As mentioned in another thread, I spend a lot of time perusing the US Patent and Trademark Office website, and related resources (Why? I honestly couldn't tell you. Patents just fascinate me.), and I'm thinking it was there. But I'm not having a lot of luck locating anything substntive at the moment.

OK, yes this is the lithium battery found in so many domestic battery-operated devices. It does not exploit the semiconductor properties of pyrite, but uses the fact that lithium ions can react with pyrite, to form Li₂FeS₂ and subsequently metallic Fe and Li₂S. So that's something else.

 

[exchemist]

What I find somewhat interesting here is that these attributes--at least with respect to pyrite--were discovered more than half a century ago. And yet... nothing. Most of the research on it's actual potential seems to all be relatively recent, within the past 10 years or so. Why? It's abundant--and more pertinently, with respect to the US, at least, it's abundant domestically. So why silicon, germanium, etc. and not pyrite?

Not sure. Maybe billvon might know. It could be (I'm just speculating) to do with microelectronics needing the semiconductor to be photochemically etched at the microscopic level, to make integrated circuits. Not sure how easy that is with pyrite.

For a photovoltaic cell, you don't need that feature, just a layer of the stuff exposed to sunlight.

P.S. I've done a bit more digging. It seems there are problems in the photovoltaic application, arising from deviations from stoichiometry (i.e. from the exact theoretical chemical formula, FeS₂) in pyrite. These, if I've read it correctly, can introduce pseudo-bands in between the valence and conduction bands, which can mess up the excitation mechanism. I don't understand the details, however: something to do with a photovoltage of 200mV vs. a band gap of ~1eV.

 

[exchemist]

Picking up the question of why these materials are semiconductors (to record for my own benefit before I forget the logic), they are semiconductors due to a relatively narrow band gap. This occurs because with the larger atoms, lower down the Periodic Table, the outermost electron orbitals (the valence orbitals) are more diffuse, due to the high principal quantum number. As a result, orbital overlap, when 2 atoms are brought together, is less efficient than with smaller atoms, resulting in bonds that are weaker. A "weaker" bond means that the lowering of energy of the bonded state, relative to the energy of the separate atoms, is not so great as with lighter atoms. By the same token, the raising in energy of the corresponding antibonding state is also less.

In an extended solid, both bonding and antibonding orbitals merge together into extended features, spanning a band of energies. The bonding states form the "valence band" and the antibonding states form the "conduction band". So one can see why the band gap in these solid materials becomes smaller when the atoms involved are from higher atomic number atoms.

This will be why sulphides tend to be more metallic than oxides and why sulphides of higher atomic number elements have the greater tendency to have some metallic character.

(The same phenomenon accounts for the metal/non-metal diagonal among the elements in the Periodic Table. Elements lower down the table are almost all metals. But it is diagonal because, for a given principal quantum number, the orbitals are pulled in by the increasing nuclear charge as one goes from left to right across a period. This to some degree mitigates the effect of the valence orbitals being more diffuse. The result is that the trend towards metallic behaviour sets in later for the elements in the Groups further to the right.)

There. Now, next time I visit the mineral gallery I'll have a better chance of understanding what I'm looking at.