5 - Crystal field spectra of transition metal ions in minerals

Summary

The end products of the analysis (measurement and interpretation of mineral absorption spectra by crystal field theory) are some parameters that can be correlated with structural properties.

K. L. Keester & W. B. White, Proc. 5th IMA Meeting (Cambridge, 1966), p. 22 (1968)

Introduction

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.

Perhaps a more fundamental application of crystal field spectral measurements, and the one that heralded the re-discovery of crystal field theory by Orgel in 1952, is the evaluation of thermodynamic data for transition metal ions in minerals. Energy separations between the 3d orbital energy levels may be deduced from the positions of crystal field bands in an optical spectrum, making it potentially possible to estimate relative crystal field stabilization energies (CFSE's) of the cations in each coordination site of a mineral structure. These data, once obtained, form the basis for discussions of thermodynamic properties of minerals and interpretations of transition metal geochemistry described in later chapters.