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.