exchemist
Valued Senior Member
I visited the minerals gallery at the London Natural History Museum a few days ago and was struck by the intense blue colour of the specimens of lapis lazuli on display. This material was very valuable in the Ancient and Medieval worlds, both as a pigment and for decorative objects. I recall from my time in Dubai that 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.) Anyway I thought I'd check up on its chemistry, this being real chemistry of the type Mr. G likes, as opposed to the quantum chemistry he affects to despise 
.
I had assumed the colour would be due to copper. cf. copper sulphate, and was surprised formula is:
Na₇Ca(Al₆Si₆O₂₄)(SO₄)(S₃).H₂O , i.e no transition metal ion is present, let alone the Cu I had been fairly confidently expecting!
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 "normal" rules for stability and bonding by having an unpaired electron. I always find rule-breaking chemistry intriguing, but in this case I was particularly interested to find the reason for the blue colour, as inorganic compounds without transition metals tend not to be strongly coloured. This radical anion, however, has a strong absorption band in the orange region of the visible spectrum, and thus reflects mainly blue light.
Now to the bit that will annoy Mr G:-
I didn't manage to find a molecular orbital diagram for it on the internet but presumed the odd electron might be in a relatively high energy orbital from which it can be promoted to another one, close enough in energy for the light absorbed to be in the visible rather than the UV. I found that S₃ itself is regarded as having a similar bonding scheme to ozone, i.e. the centre atom sp2 hybridised, with one lone pair. I also found that the extra odd electron indeed goes into a π*- antibonding orbital, which will be fairly high in energy, thus making the energy gap between it and the range of unpopulated electronic states into which it can be excited by absorption of light somewhat narrower than usual. This accounts for absorption in the visible region.
As to why this radical anion seems to be stable in lapis lazuli, perhaps there are two factors. One is that this π*-antibonding orbital extends across all 3 sulphur atoms, smearing out the "free radical" character, so it doesn't weaken the bonding that much. (In fact, neutral sulphur atoms have a +ve electron affinity, so the addition of an extra electron is exothermic and exergonic, i.e. ΔG<0). The other factor is that the aluminosilicate structure is of the zeolite type, which has large "cages", capable of accommodatng large ions like trisulphide. Each S₃⁻• ion is thus held within a cage in the crystal lattice and has little opportunity to react with anything. The formula above shows SO4 and S3 in brackets. There seems to be in practice a variable ratio of sulphate to sulphide in the mineral, the sulphate ions being alternative occupants of the cages. I presume this must involve some variation of the Na/Ca ratio to preserve electrical neutrality, though I have not found anything to confirm this.
Anyway, a nice little thing to research, for someone with an interest in chemistry.
P.S. There seems to be quite a family of these polysulphide anions. There's a guy at Calgary called Tristram Chivers, an expat Brit, who has published quite a lot on the subject. Here's a review he has co-authored for the Royal Society of Chemistry: https://pubs.rsc.org/en/content/articlelanding/2019/cs/c8cs00826d
.I had assumed the colour would be due to copper. cf. copper sulphate, and was surprised formula is:
Na₇Ca(Al₆Si₆O₂₄)(SO₄)(S₃).H₂O , i.e no transition metal ion is present, let alone the Cu I had been fairly confidently expecting!
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 "normal" rules for stability and bonding by having an unpaired electron. I always find rule-breaking chemistry intriguing, but in this case I was particularly interested to find the reason for the blue colour, as inorganic compounds without transition metals tend not to be strongly coloured. This radical anion, however, has a strong absorption band in the orange region of the visible spectrum, and thus reflects mainly blue light.
Now to the bit that will annoy Mr G:-
I didn't manage to find a molecular orbital diagram for it on the internet but presumed the odd electron might be in a relatively high energy orbital from which it can be promoted to another one, close enough in energy for the light absorbed to be in the visible rather than the UV. I found that S₃ itself is regarded as having a similar bonding scheme to ozone, i.e. the centre atom sp2 hybridised, with one lone pair. I also found that the extra odd electron indeed goes into a π*- antibonding orbital, which will be fairly high in energy, thus making the energy gap between it and the range of unpopulated electronic states into which it can be excited by absorption of light somewhat narrower than usual. This accounts for absorption in the visible region.
As to why this radical anion seems to be stable in lapis lazuli, perhaps there are two factors. One is that this π*-antibonding orbital extends across all 3 sulphur atoms, smearing out the "free radical" character, so it doesn't weaken the bonding that much. (In fact, neutral sulphur atoms have a +ve electron affinity, so the addition of an extra electron is exothermic and exergonic, i.e. ΔG<0). The other factor is that the aluminosilicate structure is of the zeolite type, which has large "cages", capable of accommodatng large ions like trisulphide. Each S₃⁻• ion is thus held within a cage in the crystal lattice and has little opportunity to react with anything. The formula above shows SO4 and S3 in brackets. There seems to be in practice a variable ratio of sulphate to sulphide in the mineral, the sulphate ions being alternative occupants of the cages. I presume this must involve some variation of the Na/Ca ratio to preserve electrical neutrality, though I have not found anything to confirm this.
Anyway, a nice little thing to research, for someone with an interest in chemistry.
P.S. There seems to be quite a family of these polysulphide anions. There's a guy at Calgary called Tristram Chivers, an expat Brit, who has published quite a lot on the subject. Here's a review he has co-authored for the Royal Society of Chemistry: https://pubs.rsc.org/en/content/articlelanding/2019/cs/c8cs00826d
