Optical absorption spectra of Fe2+ and Fe3+ in beryl crystals

A variety of beryls, inclusive of blue aquamarine, heliodor and goshenite, spanning an iron concentration range of more than one order of magnitude, has been studied by absorption spectroscopy down to 16 K and in the energy range 3000-50000 cm(-1). The weak spin-forbidden transitions of Fe2+ as well as the allowed charge-transfer transitions, ligand-to-metal, mainly due to Fe3+, have been observed for the first time in these diluted systems. The comparison with reference spectra of aqueous solutions containing Fe2+ and Fe3+ allowed to reach, by the aid of d(6) (Fe2+) and d(5) (Fe3+) Tanabe-Sugano diagrams for octahedral symmetry, a satisfactory interpretation of the spectra. The colour centres absorption, their thermal instability and contribution to the colour of these crystals has also been considered. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.