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

► We have studied the sulphate mineral creedite. ► A comparison is made with the spectroscopy of the alums. ► Multiple bands in the antisymmetric stretching region support the concept of a reduction in symmetry of the sulphate anion. ► Sulphate is coordinated to the water bonded to the Al3+ in the creedite structure.