Raman spectroscopic study of the multi-anion mineral dixenite CuMn2+14Fe 3+(AsO3)5(Sio4)2 (AsO4)(OH)6

Abstract

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 (AsO3-)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 ν2 AsO33- bending mode. DFT calculations enabled the calculation of the position of AsO22- 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 SiO42- symmetric stretching vibrations and those at 1349 and 1386 cm-1 to the SiO42- 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 CuMn2+14 Fe3+(AsO3)5(SiO4)2(AsO4)(OH)6. The formula may be better written as CuMn2+14 Fe3+(AsO3)5(SiO4)2(AsO4)(OH)6·xH2O. © 2009 John Wiley & Sons, Ltd.