Causes of colour and pleochroism in minerals

Inset crystal field spectrum of andradite from Val Maleneo, Italy, contains 0.995 Fe3+ ions per formula unit. 

The hue or vividness of colour may be correlated with the intensities of the absorption bands. Thus, Al-Fe epidotes, with relatively low molar extinction coefficients typical of spin-forbidden transitions within Fe3+ ions ( 3.7.2), exhibit pastel shades. The Al-Mn-Fe epidotes, however, display vivid colours correlating with high e values and originating from spin-allowed transitions within Mn3+ ions located in the very distorted acentric octahedral M3 site (fig. 

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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. 

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