Vibrational spectroscopic characterization of the sulphate mineral khademite Al(SO4)F-5(H2O).

Abstract

Vibrational spectroscopy has been used to characterize the sulphate mineral khademite Al(SO4)F_5(H2O). Raman band at 991 cm^-1 with a shoulder at 975 cm^-1 is assigned to the m1 (SO4)2_ symmetric stretching mode. The observation of two symmetric stretching modes suggests that the sulphate units are not equivalent. Two low intensity Raman bands at 1104 and 1132 cm^-1 are assigned to the m3 (SO4)2_ antisymmetric stretching mode. The broad Raman band at 618 cm^-1 is assigned to the m4 (SO4)2_ bending modes. Raman bands at 455, 505 and 534 cm_1 are attributable to the doubly degenerate m2 (SO4)2_bending modes. Raman bands at 2991, 3146 and 3380 cm^-1 are assigned to the OH stretching bands of water. Five infrared bands are noted at 2458, 2896, 3203, 3348 and 3489 cm^-1 are also due to water stretching bands. The observation of multiple water stretching vibrations gives credence to the nonequivalence of water units in the khademite structure. Vibrational spectroscopy enables an assessment of the structure of khademite.