Infrared and Raman spectroscopic characterisation of the sulphate mineral creedite - Ca3Al2SO4(F,OH)•2H2O - and in comparison with the alums

The mineral creedite is a fluorinated hydroxy hydrated sulphate of aluminium and calcium of formula Ca3Al2SO4(F,OH)center dot 2H(2)O. The mineral has been studied by a combination of electron probe analysis to determine the molecular formula of the mineral and the structure assessed by vibrational spectroscopy. The spectroscopy of creedite may be compared with that of the alums. The Raman spectrum of creedite is characterised by an intense sharp band at 986 cm(-1) assigned to the SO42-nu(1) (A(g)) symmetric stretching mode. Multiple bands of creedite in the antisymmetric stretching region support the concept of a reduction in symmetry of the sulphate anion. Multiple bands are also observed in the bending region with the three bands at 601, 629 and 663 cm(-1) assigned to the SO42-nu(4) (A(g)) bending modes. The observation of multiple bands at 440, 457 and 483 cm(-1) attributed to the SO42-nu(2) (B-g) bending modes supports the concept that the symmetry of the sulphate is reduced by coordination to the water bonded to the Al3+ in the creedite structure. The splitting of the nu(2), nu(3) and nu(4) modes is attributed to the reduction of symmetry of the SO4 and it is proposed that the sulphate coordinates to water in the hydrated aluminium in bidentate chelation. (C) 2013 Elsevier B.V. All rights reserved.