Raman spectroscopic study of the multi‐anion mineral dixenite CuMn 2+ 14 Fe 3+ (AsO 3 ) 5 (SiO 4 ) 2 (AsO 4 )(OH) 6

The mixed anion mineral dixenite has been studied by Raman spectroscopy, complemented with infrared spectroscopy. The Raman spectrum of dixenite shows bands at 839 and 813 cm −1 assigned to the (AsO 3 ) 3− symmetric and antisymmetric stretching modes. The most intense Raman band of dixenite is the band at 526 cm −1 and is assigned to the ν 2 AsO 3 3− bending mode. DFT calculations enabled the calculation of the position of AsO 2 2− symmetric stretching mode at 839 cm −1 , the antisymmetric stretching mode at 813 cm −1 , and the deformation mode at 449 cm −1 . The Raman bands at 1026 and 1057 cm −1 are assigned to the SiO 4 2− symmetric stretching vibrations and those at 1349 and 1386 cm −1 to the SiO 4 2− antisymmetric stretching vibrations. Both Raman and infrared spectra indicate the presence of water in the structure of dixenite. This brings into question the commonly accepted formula of dixenite as CuMn 2+ 14 Fe 3+ (AsO 3 ) 5 (SiO 4 ) 2 (AsO 4 )(OH) 6 . The formula may be better written as CuMn 2+ 14 Fe 3+ (AsO 3 ) 5 (SiO 4 ) 2 (AsO 4 )(OH) 6 · x H 2 O. Copyright © 2009 John Wiley & Sons, Ltd.