Colour symmetry

More complex physical properties may require the specification of three or more colours . In this case the general term colour symmetry is used, and the lattices and point groups so derived are the colour lattices and colour point groups. 

Shubnikov, A.V. Belov, N.V. (1964). "Coloured Symmetry". New York Pergamon Press. 

Fig. 2 The experimentally determined potential energy V(), expressed as a wavenumber for convenience, as a function of the angle in the hydrogen-bonded complex H20- HF. The definition of

Fig. 18 The n-pair/jt-pair model of furan together with the experimental geometries of furan- -HC1 and furan- -C1F. Furan- -HC1, which has a planar geometry of C2V symmetry with HC1 lying along the C2 axis, clearly obeys rule 3 but the observed face-on arrangement for furan- -C1F demonstrates that rule 3 is violated in this case. See Fig. 1 for key to the colour coding of atoms...

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals. 

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