Chapters Chapter 3 and Chapter 4 deal with optical absorption spectroscopy and comprise two interrelated parts. Part I, the present chapter, is intended as an introduction for the beginners, e.g. undergraduate students of geosciences, in order to help them acquire an understanding of the basic theoretical principles, focussing on the crystal field (CF) concept. After a short introductory section including some “technical” information, the reader is guided step by step through the development of the qualitative principles of the CF theory (CFT), referring to several aspects important for geosciences. The necessary concepts and tools are briefly outlined, whereas references to a selection of relevant textbooks and publications are given for further reading. Concise tables and figures help to illustrate and summarise important topics. Examples of the actual spectra are provided, mainly concerning the “many-electron systems” (of the first-row transition ions, i.e., 3d^2,3,7,8), since they cover the full diversity of the crystal field based spectroscopic aspects.

Part II (Chapter 4 in this volume – Andrut et al., 2004) deals with the quantitative aspects of the crystal field theory and its applications. It highlights the power of semi-empirical methods for the calculation of energy levels of the transition metal complexes with arbitrary low symmetry.