Lapis Lazuli: Blue For an Unexpected Reason

[exchemist]

Today I visited the minerals gallery at the London Natural History Museum. I was struck by the intense blue colour of the specimens of lapis lazuli, which was very valuable, both as a pigment and for decorative objects, in the Ancient and Medieval worlds. I recall the Arabic word for the colour blue is azraq (m) or zarqa (f.), from which we get "azure", so presumably lazuli comes from the same root. (Lapis is just Latin for stone, obvs.) 

I had assumed the colour would be due to copper and was a bit shocked to find the formula is: 

Na₇Ca(Al₆Si₆O₂₄)(SO₄)(S₃).H₂O , i.e no Cu in sight!  

Turns out the clue is in the S₃. This is present in the form of the trisulphide radical anion, S₃⁻•,  a curious species that breaks the school-level rules for stability and bonding - and so is automatically interesting to me. This radical anion apparently has an absorption band in the orange region of the visible spectrum, and thus reflects mainly blue light. I haven't managed to find a molecular orbital diagram for it on the internet but presume the odd electron may be in a relatively high energy orbital, from which it can be promoted to another one that is only slightly higher, i.e. with relatively little energy and this will be why it absorbs in the visible rather than the UV. S₃ itself is regarded as having a similar bonding scheme to ozone, i.e. the centre atom sp2 hybridised with one lone pair, but I presume the extra odd electron must go into either an sp2 hybrid antibonding orbital or else something involving participation by d orbitals, which obviously is possible in sulphur, unlike oxygen. 

I suppose one should expect this ion to be paramagnetic. From what I have found on the internet this radical anion has some applications in synthesis of organosulphur compounds. There seems to be a guy  called Tristram Chivers at Calgary (now retired and emeritus) who has done a lot of work on it. 

If anyone knows more about this anion I'd be interested to learn more about it, especially the bonding and electronic structure. 

 

Edited February 11 by exchemist

[exchemist]

I've found that the odd electron does indeed go into a relatively high energy orbital: a π* antibonding orbital which is shared across all 3 atoms. The information was buried in this interesting but very long review of polysulphide anions by Tristram Chivers and someone else:  https://pubs.rsc.org/en/content/articlelanding/2019/cs/c8cs00826d. What I have yet to find out is what it gets promoted to in the transition responsible for the colour. 

There is a comment that neutral sulphur molecules have a +ve electron affinity, such that addition of one extra electron is both exothermic and exergonic (ΔG<0), so the radical anion is stable with respect to the neutral atom. Adding a second electron, to make a 2- anion, is energetically favourable in polarisable solvents but not in the gas phase, so is more marginal.

Lapis Lazuli has a quite open zeolite-type aluminosilicate structure, with cages big enough to sequester this big anion and prevent it from reacting further, in spite of the reactivity implied by the odd electron.  It would be interesting to know how this unusual mineral is formed in the Earth's crust. The Chivers paper says it has been found that S₃•⁻ has been found to form in sulphurous hydrothermal fluids, at temperatures >200C and pressures > 1kbar.  So perhaps lapis lazuli is formed by some kind of metamorphic alteration of a pre-existing aluminosilicate mineral, involving sulphur-containing hydrothermal fluids.  

 

Edited Friday at 10:35 PM by exchemist

[studiot]

Interesting subject +1

 

It is important to distinguish between the gemstone lapis lazuli, lazulite and lazurite minerals, all of which are blue stones.

 

https://minds.wisconsin.edu/bitstream/handle/1793/11597/Lazulite.pdf?sequence=1&isAllowed=y

 

Since my return I have also dug out my 3rd ed of Wells  (1962) which has some things to say about this. I cannot trace a later edition.

I willpost what he has to say when i can.

[exchemist]

7 hours ago, studiot said:

Interesting subject +1

 

It is important to distinguish between the gemstone lapis lazuli, lazulite and lazurite minerals, all of which are blue stones.

 

https://minds.wisconsin.edu/bitstream/handle/1793/11597/Lazulite.pdf?sequence=1&isAllowed=y

 

Since my return I have also dug out my 3rd ed of Wells  (1962) which has some things to say about this. I cannot trace a later edition.

I willpost what he has to say when i can.

I’ll look forward to that. In the meantime I’ll see what I can discover about lazulite, which I see does contain a transition metal, though it is Fe which is not normally associated with blue colours, pace Prussian Blue.

[exchemist]

11 hours ago, studiot said:

Interesting subject +1

 

It is important to distinguish between the gemstone lapis lazuli, lazulite and lazurite minerals, all of which are blue stones.

 

https://minds.wisconsin.edu/bitstream/handle/1793/11597/Lazulite.pdf?sequence=1&isAllowed=y

 

Since my return I have also dug out my 3rd ed of Wells  (1962) which has some things to say about this. I cannot trace a later edition.

I willpost what he has to say when i can.

Incidentally I notice from your link that there is some variation in how the formula for lazurite, responsible for the blue of lapis lazuli, is written. Reading a bit more about this it looks as if the zeolite cages can hold either sulphate or this trisulphide radical and it qualifies as lazurite if >25% of the cages contain trisulphide.

Also, rather paradoxically, Wiki describes it as being formed from contact metamorphism (metasomatism?) of limestone. This seems very  counterintutive as it does not explain where the aluminosilicate matrix comes from, let alone the sulphate/trisulphide content of the cages.  Curiouser and curiouser. 

[studiot]

20 hours ago, exchemist said:

Incidentally I notice from your link that there is some variation in how the formula for lazurite, responsible for the blue of lapis lazuli, is written. Reading a bit more about this it looks as if the zeolite cages can hold either sulphate or this trisulphide radical and it qualifies as lazurite if >25% of the cages contain trisulphide.

Also, rather paradoxically, Wiki describes it as being formed from contact metamorphism (metasomatism?) of limestone. This seems very  counterintutive as it does not explain where the aluminosilicate matrix comes from, let alone the sulphate/trisulphide content of the cages.  Curiouser and curiouser. 

The classification depends upon where, and in which country, you look as well as the date of the classification scheme.

All very confused.

I think the sulphur comes from pyrites.

https://www.nicholaswylde.com/content/lapis-lazuli/

This discussion below is interesting.

https://www.mindat.org/mesg-326838.html

 

Meanwhile here is some scanned material that I promised.

First, Wells

 

Then here is some well presented wider information from Latimer and Hildebrand  Reference Book of Inorganic Chemistry

Edited Monday at 12:56 PM by studiot

[exchemist]

7 hours ago, studiot said:

The classification depends upon where, and in which country, you look as well as the date of the classification scheme.

All very confused.

I think the sulphur comes from pyrites.

https://www.nicholaswylde.com/content/lapis-lazuli/

This discussion below is interesting.

https://www.mindat.org/mesg-326838.html

 

Meanwhile here is some scanned material that I promised.

First, Wells

 

Then here is some well presented wider information from Latimer and Hildebrand  Reference Book of Inorganic Chemistry

Thanks for these further references. I too had stumbled across Haüyne . This mineralogists' discussion about nomenclature was interesting. Whatever the ins and out of the naming of these minerals, it seems clear than when they are bright blue it is due to polysulphide anions in the cages, replacing a proportion of what would otherwise have been sulphate. Chivers comments in his paper that in earlier work people thought the anion was S₂⁻ (your scanned pages reflect this earlier view) but it is now recognised to be this S₃•⁻,  with the odd electron in the π* antibonding orbital, that is responsible for the absorbance that creates the colour.

Pyrite is indeed found in association with lapis lazuli but my suspicion, from the Chivers paper about the reversible formation of S₃•⁻ in the lab at elevated temperature and pressure, is that the same hydrothermal fluids that create pyrite can also alter suitable rocks to create lapis lazuli, by partial replacement of sulphate with this trisulphide radical anion.

Anyway, all good and interesting stuff. I must pop back to the Natural History museum for another interesting mineral. There were some very dramatic, long, blackish crystals of stibnite, for example.......