Causes of colour

It is commonly stated that there are fifteen specific causes of colour, arising from a variety of physical and chemical mechanisms. These mechanisms may be collected into five groups. 

There are many ways in which colour can be caused to arise both by chemical and physical forces, all of which are used or have the potential to be used in technological applications. These different ways of producing colour can be grouped into five fundamental mechanisms, as shown in Table I.l. The five groupings can be further split into what Nassau has called the fifteen causes of colour . The main Nassau groupings of I, IV and V can be considered as physical phenomena, II is borderline between chemistry and physics and III covers purely chemical phenomena. 

Measurements of electronic absorption spectra in the visible region not only lead to the evaluation of CFSE s, but they also provide useful information about the crystal chemistry of transition metal ions in the crystal structures and causes of colour and pleochroism of minerals. In this chapter, techniques for measuring absorption spectra of minerals are briefly described and some general applications of the optical spectra to basic crystal chemical properties, such as colour and pleochroism, are discussed. These examples also amplify many of the features of crystal field spectra outlined in chapter 3. 

Chapter 4 outlines methods for measuring absorption spectra of minerals in the visible to near-infrared region, and describes applications of electronic spectral measurements to some fundamental crystal chemical problems, including causes of colour and pleochroism of minerals. 

The cause of colour in natural and synthetic chromium-bearing blue diop-sides has been widely debated and assignments of absorption bands in their polarized spectra remain controversial (Mao et al., 1972 Bums, 1975a,b Ikeda and Yagi 1977, 1982 Schreiber, 1977, 1978). One interpretation is that low-spin Cr3 ions in tetrahearal sites in the pyroxene structure are responsible for the colour and spectra of blue diopsides (Ikeda and Yagi, 1977, 1982). 

Very little is known of the primary cause of colour in such compounds, but our knowledge is so far advanced that colour is regarded as a characteristic property of whole classes of chemical compounds and the study of the constitution of such compounds has shown that a close relationship exists between their colour and their chemical structure. 

E24.25 The blue colour of the solution is due to the presence of anionic radical Sj , identical to that found as 83" in lapis lazuli described in Section 24.15. The cause of colour is the excitation of the unpaired electron in the electronic structure. In solution polysulfide, radical anions are very sensitive to O2 from the atmosphere upon oxidation the colour is lost. 

All these theories are now only of historical interest because developments in spectroscopic techniques and the application of the quantum theory and wave mechanics are throwing an entirely new light upon our understanding of the causes of colour. In the first place, spectroscopy has shown that all organic compounds, whether they contain chromophores or not, absorb radiation. The fact that some are coloured is purely fortuitous because it so happens that their strong absorption bands lie within the narrow range of radiation to which the human eye is sensitive. Colour, therefore, is only a special aspect of a general phenomenon. 

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