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

Abstract

The mineral creedite is afluorinatedhydroxy hydrated sulphate of alumini um and calcium of formula Ca3Al2SO4(F,OH) _2H2O. The mineral has been studied by acombination 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 SO 24 _ m1 (Ag) symmetric stretching mode. Multiple bands of creedite in the antisymmetric stretching region support the concept of areduc- tion in symmetry of the sulphate anion. Multiple ban ds are also observed in the bending region with the three bands at 601, 629 and 663 cm _1 assigned to the SO 24 _ m4 (Ag) bending modes. The observation of multiple bands at 440, 457 and 483 cm _1 attributed to the SO 24 _ m2 (Bg) bending modes supports the con- cept that the symmetry of the sulphate is reduced by coordination to the water bonded to the Al 3+ in the creedite structure. The splitting of the m2, m3 and m4 modes is attributed to the reduction of symmetry of the SO 4 and it is proposed that the sulphate coordinates to water in the hydrated aluminium in bidentate chelation.